CiteULike: Tag hydrophobicity

A simple method for displaying the hydropathic character of a protein

Jack Kyte — 2006-11-03T14:37:44-00:00

Journal of Molecular Biology, Vol. 157, No. 1. (5 May 1982), pp. 105-132.

A computer program that progressively evaluates the hydrophilicity and hydrophobicity of a protein along its amino acid sequence has been devised. For this purpose, a hydropathy scale has been composed wherein the hydrophilic and hydrophobic properties of each of the 20 amino acid side-chains is taken into consideration. The scale is based on an amalgam of experimental observations derived from the literature. The program uses a moving-segment approach that continuously determines the average hydropathy within a segment of predetermined length as it advances through the sequence. The consecutive scores are plotted from the amino to the carboxy terminus. At the same time, a midpoint line is printed that corresponds to the grand average of the hydropathy of the amino acid compositions found in most of the sequenced proteins. In the case of soluble, globular proteins there is a remarkable correspondence between the interior portions of their sequence and the regions appearing on the hydrophobic side of the midpoint line, as well as the exterior portions and the regions on the hydrophilic side. The correlation was demonstrated by comparisons between the plotted values and known structures determined by crystallography. In the case of membrane-bound proteins, the portions of their sequences that are located within the lipid bilayer are also clearly delineated by large uninterrupted areas on the hydrophobic side of the midpoint line. As such, the membrane-spanning segments of these proteins can be identified by this procedure. Although the method is not unique and embodies principles that have long been appreciated, its simplicity and its graphic nature make it a very useful tool for the evaluation of protein structures.

Unseen Proteome: Mining below the Tip of the Iceberg to Find Low Abundance and Membrane Proteins

SK Pedersen — 2008-01-15T17:22:57-00:00

J. Proteome Res., Vol. 2, No. 3. (2 June 2003), pp. 303-311.

Abstract: Abundant and hydrophilic nonmembrane proteins with isoelectric points below pH 8 are the predominant proteins identified in most proteomics projects. In yeast, however, low-abundance proteins make up 80% of the predicted proteome, approximately 50% have pI's above pH 8 and 30% of the yeast ORFs are predicted to encode membrane proteins with at least 1 trans-membrane span. By applying highly solubilizing reagents and isoelectric fractionation to a membrane fraction of yeast we have a purified and identified 780 protein isoforms, representing 323 gene products, including 28% low abundance proteins and 49% membrane or membrane associated proteins. More importantly, considering the frequency and importance of co- and post-translational modifications, the separation of protein isoforms is essential and two-dimensional electrophoresis remains the only technique which offers sufficient resolution to address this at a proteomic level. Keywords: two-dimensional electrophoresis membrane proteins low-abundance proteins multi-compartment electrolyzer fractionation

Hydrophobicity at Small and Large Length Scales

K Lum — 2007-12-07T15:58:47-00:00

J. Phys. Chem. B, Vol. 103, No. 22. (3 June 1999), pp. 4570-4577.

Abstract: We develop a unified and generally applicable theory of solvation of small and large apolar species in water. In the former, hydrogen bonding of water is hindered yet persists near the solutes. In the latter, hydrogen bonding is depleted, leading to drying of extended apolar surfaces, large forces of attraction, and hysteresis on mesoscopic length scales. The crossover occurs on nanometer length scales, when the local concentration of apolar units is sufficiently high, or when an apolar surface is sufficiently large. Our theory for the crossover has implications concerning the stability of protein assemblies and protein folding.

Cell hydrophobicity as a criterion of selection of bacterial producers of biosurfactants

Volchenko — 2007-03-11T05:38:35-00:00

Microbiology, Vol. 76, No. 1. (February 2007), pp. 112-114.

Direct Measurement of Hydrophobic Forces on Cell Surfaces Using AFM

D Alsteens — 2007-10-20T23:51:07-00:00

Langmuir (17 October 2007)

Abstract: Although hydrophobic forces are of great relevance in biological systems, quantifying these forces on complex biosurfaces such as cell surfaces has been difficult owing to the lack of appropriate, ultrasensitive force probes. Here, chemical force microscopy (CFM) with hydrophobic tips was used to measure local hydrophobic forces on organic surfaces and on live bacteria. On organic surfaces, we found an excellent correlation between nanoscale CFM and macroscale wettability measurements, demonstrating the sensitivity of the method toward hydrophobicity and providing novel insight into the nature of hydrophobic forces. Then, we measured hydrophobic forces associated with mycolic acids on the surface of mycobacteria, supporting the notion that these hydrophobic compounds represent an important permeation barrier to drugs.

An Artificial Mitochondrial Tail Signal/Anchor Sequence Confirms a Requirement for Moderate Hydrophobicity for Targeting

Binks Wattenberg — 2008-04-16T00:57:12-00:00

Bioscience Reports, Vol. 27, No. 6. (21 December 2007), pp. 385-401.

Abstract Tail-anchored proteins are a group of membrane proteins oriented with their amino terminus in the cytoplasm and their carboxy terminus embedded in intracellular membranes. This group includes the apoptosis-mediating proteins of the Bcl-2 family as well as the vesicle targeting proteins of the SNARE group, among others. A stretch of hydrophobic amino acids at the extreme carboxy terminus of these proteins serves both as a membrane anchor and as a targeting signal. Tail-anchored proteins are differentially targeted to either the endoplasmic reticulum or the mitochondrial outer membrane and the mechanism which accomplishes this selective targeting is poorly understood. Here we define important characteristics of the signal/anchor region which directs proteins to the mitochondrial outer membrane. We have created an artificial sequence consisting of a stretch of 16 leucines bounded by positively charged amino acids. Using this template we demonstrate that moderate hydrophobicity distinguishes the mitochondrial tail-anchor sequence from that of the endoplasmic reticulum tail-anchor sequence. A change as small as introduction of a single polar residue into a sequence that otherwise targets to the endoplasmic reticulum can substantially switch targeting to the mitochondrial outer membrane. Further we show that a mitochondrially targeted tail-anchor has a higher propensity for the formation of alpha-helical structure than a sequence directing tail-anchored proteins to the endoplasmic reticulum.

Directional mutational pressure affects the amino acid composition and hydrophobicity of proteins in bacteria

Xun Gu — 2007-01-25T05:58:10-00:00

Genetica, Vol. V102-103, No. 0. (16 March 1998), pp. 383-391.

Hydrophobicity in modified water models

Giuseppe Graziano — 2008-01-29T12:56:25-00:00

Chemical Physics Letters, Vol. In Press, Corrected Proof

Molecular level explanation of the poor solubility in water of nonpolar species is still a debated topic. Recently, Lynden-Bell and Head-Gordon [R.M. Lynden-Bell, T. Head-Gordon, Mol. Phys. 104 (2006) 3593] have compared in detail, at room temperature and atmospheric pressure, the hydrophobicity of suitably modified water models with that of the realistic SPC/E water model. It is shown that, by using the effective size of liquid molecules and the experimental density of water, scaled particle theory calculations reproduce satisfactorily the results of Lynden-Bell and Head-Gordon. Hydrophobicity is mainly determined by a trade-off between the effect of the small size of water molecules and the effect of the low value of its volume packing density.

Hydrophobicity of amino acid residues in globular proteins

Gd Rose — 2007-05-11T11:45:17-00:00

Science, Vol. 229, No. 4716. (30 August 1985), pp. 834-838.

During biosynthesis, a globular protein folds into a tight particle with an interior core that is shielded from the surrounding solvent. The hydrophobic effect is thought to play a key role in mediating this process: nonpolar residues expelled from water engender a molecular interior where they can be buried. Paradoxically, results of earlier quantitative analyses have suggested that the tendency for nonpolar residues to be buried within proteins is weak. However, such analyses merely classify residues as either "exposed" or "buried." In the experiment reported in this article proteins of known structure were used to measure the average area that each residue buries upon folding. This characteristic quantity, the average area buried, is correlated with residue hydrophobicity. 10.1126/science.4023714

Water clustering and percolation in low hydration DNA shells.

I Brovchenko — 2007-07-20T08:23:33-00:00

J Phys Chem B, Vol. 111, No. 12. (29 March 2007), pp. 3258-3266.

The hydrogen-bonded networks of water at the surface of a model DNA molecule are analyzed. At low hydrations, only small water clusters are attached to the DNA surface, whereas, at high hydrations, it is homogeneously covered by a spanning water network. The spanning water network is formed via a percolation transition at an intermediate hydration number of about 15 water molecules per nucleotide, which is very close to the midpoint of polymorphic transitions between A- and B-forms of the double helix. The percolation transition can occur in both A- and B-DNA hydration shells with nearly identical percolation thresholds. However, the mechanism of the percolation transition in A- and B-DNA is qualitatively different in regard to the roles played by the two opposite grooves of the double helix. Free ions can shift the percolation threshold by preventing some water molecules from hydrogen bond networking. The results corroborate the suggested relationship between water percolation and the low hydration polymorphism in DNA.

On the heat-capacity change of pairwise hydrophobic interactions.

G Graziano — 2007-07-20T12:08:28-00:00

J Chem Phys, Vol. 123, No. 3. (15 July 2005)

Computer simulations [S. Shimizu and H. S. Chan, J. Am. Chem. Soc. 123, 2083 (2001); D. Paschek, J. Chem. Phys. 120, 10605 (2004)] have demonstrated that the heat-capacity change associated with the interaction of two nonpolar spherical particles, at room temperature, shows a complex behavior with a significant maximum at the distance corresponding to the desolvation barrier configuration and a small minimum at the distance corresponding to the contact configuration. Taking advantage of the detailed analysis performed by Paschek, the two-state model of Muller is applied to estimate the energetic strength and the intactness of the H bonds in the hydration shell of a xenon atom and in the concave part of the joint Xe-Xe hydration shell. In both hydration shell regions the H bonds are energetically stronger but more broken than those in bulk water. In addition, those in the concave part of the joint Xe-Xe hydration shell are, in absolute, stronger and more broken. These thermodynamic features coupled to simple geometric arguments allow the calculation of heat-capacity values that are in agreement with those provided by computer simulations for the pairwise Xe-Xe interaction.

A fast method for the determination of fractional contributions to solvation in proteins.

D Talavera — 2007-07-20T13:59:22-00:00

Protein Sci, Vol. 15, No. 11. (November 2006), pp. 2525-2533.

A fast method for the calculation of residue contributions to protein solvation is presented. The approach uses the exposed polar and apolar surface of protein residues and has been parametrized from the fractional contributions to solvation determined from linear response theory coupled to molecular dynamics simulations. Application of the method to a large subset of proteins taken from the Protein Data Bank allowed us to compute the expected fractional solvation of residues. This information is used to discuss when a residue or a group of residues presents an uncommon solvation profile.

Integral equation study of hydrophobic interaction: a comparison between the simple point charge model for water and a Lennard-Jones model for solvent.

T Sumi — 2007-07-24T11:22:16-00:00

J Chem Phys, Vol. 126, No. 14. (14 April 2007)

The hydrophobic interaction that is characterized by a potential of mean force (PMF) between spherical apolar solutes immersed in the simple point charge (SPCE) model for water was studied using an interaction site model integral equation based on a density-functional theory for molecular fluids. For comparison with the PMFs for various size solutes in the SPCE model, the PMFs in a Lennard-Jones (LJ) model for a solvent whose diameter is same as the SPCE model were also studied using a hypernetted chain integral equation. It is noted in the LJ model that the hydrogen bond and its network structure are completely ignored, but the translational entropy is taken into account. Both PMFs obtained from the SPCE model and from the LJ model have a large first minimum at a contact distance of solutes. In the case that the solute size is about three times larger than water, these PMFs also have a large maximum at a longer distance than the contact position. The strong attraction is attributed to the translational entropy of the solvent, and that the large activation barrier is arising from the weak attraction between the solute and the solvent. The comparison between the SPCE model and the LJ solvent model suggests that the qualitative description of the hydrophobic interaction is possible by using the LJ solvent model. On the other hand, the dewetting tendency on the surface of the apolar solute in a room condition is observed only on the SPCE model. These results indicate that the characteristics of water such as the hydrogen bond affect rather the hydrophobic hydration than the hydrophobic interaction.

Aliphatics vs. aromatics hydration thermodynamics.

G Graziano — 2007-07-20T12:05:04-00:00

Biophys Chem, Vol. 110, No. 3. (1 August 2004), pp. 249-258.

By comparing the hydration thermodynamics of benzene with that of a hypothetical aliphatic hydrocarbon having the same accessible surface area (ASA) of benzene, Makhatadze and Privalov concluded that the whole difference is due to the weak H-bonds that water forms with the aromatic ring. The formation of such H-bonds would be characterized by a negative Gibbs energy change, slightly increasing in magnitude with temperature, and a positive entropy change over a large temperature range. The latter thermodynamic feature is not physically reliable for the formation of H-bonds. In the present article, by using a statistical mechanical dissection scheme of hydration, a microscopic interpretation for the numbers obtained by Makhatadze and Privalov is proposed. The difference in hydration Gibbs energy should be attributed to the different strength of van der Waals interactions that benzene can do with water, owing to the larger polarizability of the aromatic ring with respect to an aliphatic hydrocarbon of equal size. In addition, the difference in hydration entropy should account for the different extent of H-bond reorganization upon the insertion of benzene and the corresponding aliphatic hydrocarbon in water.

Molecular lipophilicity in protein modeling and drug design.

RG Efremov — 2007-07-20T11:57:14-00:00

Curr Med Chem, Vol. 14, No. 4. (2007), pp. 393-415.

Hydrophobic interactions play a key role in the folding and maintenance of the 3-dimensional structure of proteins, as well as in the binding of ligands (e.g. drugs) to protein targets. Therefore, quantitative assessment of spatial hydrophobic (lipophilic) properties of these molecules is indispensable for the development of efficient computational methods in drug design. One possible solution to the problem lies in application of a concept of the 3-dimensional molecular hydrophobicity potential (MHP). The formalism of MHP utilizes a set of atomic physicochemical parameters evaluated from octanol-water partition coefficients (log P) of numerous chemical compounds. It permits detailed assessment of the hydrophobic and/or hydrophilic properties of various parts of molecules and may be useful in analysis of protein-protein and protein-ligand interactions. This review surveys recent applications of MHP-based techniques to a number of biologically relevant tasks. Among them are: (i) Detailed assessment of hydrophobic/hydrophilic organization of proteins; (ii) Application of this data to the modeling of structure, dynamics, and function of globular and membrane proteins, membrane-active peptides, etc. (iii) Employment of the MHP-based criteria in docking simulations for ligands binding to receptors. It is demonstrated that the application of the MHP-based techniques in combination with other molecular modeling tools (e.g. Monte Carlo and molecular dynamics simulations, docking, etc.) permits significant improvement to the standard computational approaches, provides additional important insights into the intimate molecular mechanisms driving protein assembling in water and in biological membranes, and helps in the computer-aided drug discovery process.

An empirical approach to protein conformation stability and flexibility.

TE Creighton — 2007-07-24T13:09:19-00:00

Biopolymers, Vol. 22, No. 1. (January 1983), pp. 49-58.

Observation of Immobilized Water Molecules around Hydrophobic Groups

YLA Rezus — 2007-10-24T12:22:45-00:00

Physical Review Letters, Vol. 99, No. 14. (2007)

We have used femtosecond midinfrared spectroscopy to study the orientational mobility of water molecules in the hydration shells of hydrophobic groups. Our results show that hydrophobic groups are surrounded by a number of water molecules that display much slower orientational dynamics than the bulk liquid and that are therefore effectively immobilized. It turns out that each methyl group is surrounded by four immobilized water OH groups.

Submillisecond kinetics of protein folding

William Eaton — 2008-01-23T17:02:26-00:00

Current Opinion in Structural Biology, Vol. 7, No. 1. (February 1997), pp. 10-14.

New experimental methods permit observation of protein folding and unfolding on the previously inaccessible nanosecond--microsecond timescale. These studies are beginning to establish times for the elementary motions in protein folding -- secondary structure and loop formation, local hydrophobic collapse, and global collapse to the compact denatured state. They permit an estimate of about one microsecond for the shortest time in which a protein can possibly fold.

Recent progress in understanding hydrophobic interactions.

EE Meyer — 2007-11-23T03:42:48-00:00

Proc Natl Acad Sci U S A, Vol. 103, No. 43. (24 October 2006), pp. 15739-15746.

We present here a brief review of direct force measurements between hydrophobic surfaces in aqueous solutions. For almost 70 years, researchers have attempted to understand the hydrophobic effect (the low solubility of hydrophobic solutes in water) and the hydrophobic interaction or force (the unusually strong attraction of hydrophobic surfaces and groups in water). After many years of research into how hydrophobic interactions affect the thermodynamic properties of processes such as micelle formation (self-assembly) and protein folding, the results of direct force measurements between macroscopic surfaces began to appear in the 1980s. Reported ranges of the attraction between variously prepared hydrophobic surfaces in water grew from the initially reported value of 80-100 Angstrom to values as large as 3,000 Angstrom. Recent improved surface preparation techniques and the combination of surface force apparatus measurements with atomic force microscopy imaging have made it possible to explain the long-range part of this interaction (at separations >200 Angstrom) that is observed between certain surfaces. We tentatively conclude that only the short-range part of the attraction (<100 Angstrom) represents the true hydrophobic interaction, although a quantitative explanation for this interaction will require additional research. Although our force-measuring technique did not allow collection of reliable data at separations <10 Angstrom, it is clear that some stronger force must act in this regime if the measured interaction energy curve is to extrapolate to the measured adhesion energy as the surface separation approaches zero (i.e., as the surfaces come into molecular contact).

Mechanistic Elements of Protein Cold Denaturation

CF Lopez — 2008-01-15T16:36:18-00:00

J. Phys. Chem. B (9 January 2008)

Abstract: Globular proteins undergo structural transitions to denatured states when sufficient thermodynamic state or chemical perturbations are introduced to their native environment. Cold denaturation is a somewhat counterintuitive phenomenon whereby proteins lose their compact folded structure as a result of a temperature drop. The currently accepted explanation for cold denaturation is based on an associated favorable change in the contact free energy between water and nonpolar groups at colder temperatures which would weaken the hydrophobic interaction and is thought to eventually allow polymer entropy to disrupt protein tertiary structure. In this paper we explore how this environmental perturbation leads to changes in the protein hydration and local motions in apomyoglobin. We do this by analyzing changes in protein hydration and protein motion from molecular dynamics simulation trajectories initially at 310 K, followed by a temperature drop to 278 K. We observe an increase in the number of solvent contacts around the protein and, in particular, distinctly around nonpolar atoms. Further analysis shows that the fluctuations of some protein atoms increase with decreasing temperature. This is accompanied by an observed increase in the isothermal compressibility of the protein, indicating an increase in the protein interior interstitial space. Closer inspection reveals that atoms with increased compressibility and larger-than-expected fluctuations are localized within the protein core regions. These results provide insight into a description of the mechanism of cold denaturation. That is, the lower temperature leads to solvent-induced packing defects at the protein surface, and this more favorable water-protein interaction in turn destabilizes the overall protein structure.

Cold-Denatured Ensemble of Apomyoglobin: Implications for the Early Steps of Folding

J Sabelko — 2008-01-15T16:35:17-00:00

J. Phys. Chem. B, Vol. 102, No. 10. (5 March 1998), pp. 1806-1819.

Abstract: The dynamics of protein-refolding experiments initiated by a temperature jump depend critically on the nature of the initial cold-denatured ensemble. The cold-denatured state of equine apomyoglobin has been investigated in aqueous buffers by near- and far-UV circular dichroism, fluorescence, infrared, and NMR spectroscopies at temperatures ranging from -20 to 98 C. Cold denaturation of apomyoglobin is well described by a cooperative transition below 3 C and differs in many aspects from acid-induced unfolding. As a reference system, the N-terminal A-peptide fragment of equine apomyoglobin has also been studied in aqueous and trifluoroethanol solutions. The A-peptide has a low helix-forming propensity in the absence of any stabilizing tertiary interactions. The results show that cold denaturation breaks the AGH-hydrophobic interface of equine apomyoglobin. Furthermore, at least some GH-helical structure appears to be preserved at the expense of the less stable A-helix.

Protein heat capacity: inconsistencies in the current view of cold denaturation

B Hallerbach — 2008-01-08T19:46:39-00:00

Biophysical Chemistry, Vol. 76, No. 3. (22 February 1999), pp. 219-227.

The present study shows on the basis of the thermodynamic stability criterion ([partial differential]S/[partial differential]T)p>0 that partitioning of the entropy of cold-unfolding of a protein into independent positive conformational and negative hydrational contributions is incorrect. Furthermore it provides a new microscopic interpretation of protein heat capacity that takes into account the significant fluctuations in energy and entropy which result from the small size of these macromolecules.

Alternative hydrogen bond implementations produce opposite effects on collapse cooperativity of lattice homopolypeptide models

Gustavo Fleury — 2007-12-20T21:57:44-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 5. (2007)

We use complete enumeration of self-avoiding chains of up to N=26 monomers in two-dimensional lattices to investigate the effect of alternative implementations of backbone hydrogen bonds on the cooperativity of homopolypeptide collapse. Following a recent study on protein folding models, we use the square lattice with z=3 local conformations per monomer and lattice extensions containing diagonal steps which result in z=5 or z=7 and assume that only a subset of zh<z local conformations is compatible with hydrogen bond formation. As previously observed in heteropolymeric folding, a significant increase in cooperativity, as measured by 2 values, results from the coupling between hydrogen bonds and hydrophobic interactions, in such a way that hydrophobic contacts are favorable only when contacting monomers are involved in hydrogen bond formation. For some z/zh combinations the energy distribution is bimodal at the collapse transition temperature. The situation can be regarded as if all hydrophobic contacts actually decrease the energy by the same amount, 2h, with the addition of an energetic increase, 2=h, as a penalty for each contacting monomer not satisfying the hydrogen bond condition. Cooperativity is little affected and might even decrease, however, when hydrogen bonds produce a decrease in energy by the same amount, 1=h, for each bonding monomer. For the more general situation when the hydrogen bond effect is not equal, in modulus, to the hydrophobic interaction, i.e., 2h or 1h, we observe a pronounced increase in 2 for small 2, with a maximum around 2/h1.5, followed by a gradual decrease to a limiting value at large 2. The opposite behavior is observed when 1 is varied. The observed qualitative difference is shown to arise from opposite effects on the convexity of the total density of states of the system when subdensities corresponding to different numbers of hydrogen bonds are differently favored as opposed to the case when subdensities corresponding to different numbers of contacting monomers not forming hydrogen bonds are differently unfavored. Potential implications for the cooperativity of protein folding and protein unspecific collapse are discussed.

Relationship between intramolecular hydrogen bonding and solvent accessibility of side-chain donors and acceptors in proteins

Alexander Efimov — 2007-12-20T21:54:36-00:00

FEBS Letters, Vol. 554, No. 3. (20 November 2003), pp. 389-393.

This study shows that intramolecular hydrogen bonding in proteins depends on the accessibility of donors and acceptors to water molecules. The frequency of occurrence of H-bonded side chains in proteins is inversely proportional to the solvent accessibility of their donors and acceptors. Estimates of the notional free energy of hydrogen bonding suggest that intramolecular hydrogen-bonding interactions of buried and half-buried donors and acceptors can contribute favorably to the stability of a protein, whereas those of solvent-exposed polar atoms become less favorable or unfavorable.

Compact phases of polymers with hydrogen bonding

Antonio Trovato — 2007-12-20T21:49:36-00:00

Physical Review E, Vol. 67, No. 2. (26 February 2003), 021805.

We propose an off-lattice model for a self-avoiding homopolymer chain with two different competing attractive interactions; mimicking the hydrophobic effect and the hydrogen-bond formation; respectively. By means of Monte Carlo simulations; we are able to trace out the complete phase diagram for different values of the relative strengths of the two competing interactions. For strong enough hydrogen bonding; the ground state is a helical conformation; whereas with decreasing hydrogen-bonding strength; helices get eventually destabilized at low temperature in favor of more compact conformations resembling β sheets appearing in the native structures of proteins. For weaker hydrogen bonding helices are not thermodynamically relevant anymore.

Direct Characterization of the Physicochemical Properties of Fungal Spores Using Functionalized AFM Probes

Yves Dufrene — 2007-11-01T13:53:14-00:00

Biophys. J., Vol. 78, No. 6. (1 June 2000), pp. 3286-3291.

A new method is described for characterizing the physicochemical properties of native microbial cells by using atomic force microscopy (AFM) with chemically functionalized probes. Adhesion forces were measured, under deionized water, between probes and model substrata functionalized with alkanethiol self-assembled monolayers terminated with OH and CH3 groups. These were found to be 6 +/- 2 nN (n = 1024), 0.9 +/- 0.4 nN, and ~0 nN, for CH3/CH3, CH3/OH, and OH/OH surfaces, respectively, and were not significantly influenced by changes of ionic strength (0.1 M NaCl versus deionized water). This shows that functionalized probes are very sensitive to changes of surface hydrophobicity. Using OH- and CH3-terminated probes, patterns of rodlets, ~10 nm in diameter, were visualized, under physiological conditions, at the surface of spores of Phanerochaete chrysosporium. Multiple (1024) force-distance curves recorded over 500 x 500-nm areas at the spore surface, either in deionized water or in 0.1 M NaCl solutions, always showed no adhesion for both OH- and CH3-terminated probes. Control experiments indicated that the lack of adhesion is not due to transfer of cellular material onto the probe, but to the hydrophilic nature of the spore surface.

Are membrane proteins "inside-out" proteins?

TJ Stevens — 2005-12-20T16:13:31-00:00

Proteins, Vol. 36, No. 1. (1 July 1999), pp. 135-143.

One of the central paradigms of structural biology is that membrane proteins are "inside-out" proteins, in that they have a core of polar residues surrounded by apolar residues. This is the reverse of the characteristics found in water-soluble proteins. We have decided to test this paradigm, now that sufficient numbers of transmembrane alpha-helical structures are accessible to statistical analysis. We have analyzed the correlation between accessibility and hydrophobicity of both individual residues and complete helices. Our analyses reveal that hydrophobicity of residues in a transmembrane helical bundle does not correlate with any preferred location and that the hydrophilic vector of a helix is a poor indicator of the solvent exposed face of a helix. Neither polar nor hydrophobic residues show any bias for the exterior or the interior of a transmembrane domain. As a control, analysis of water-soluble helical bundles performed in a similar manner has yielded clear correlations between hydrophobicity and accessibility. We therefore conclude that, based on the data set used, membrane proteins as "inside-out" proteins is an unfounded notion, suggesting that packing of alpha-helices in membranes is better understood by maximization of van der Waal's forces, rather than by a general segregation of hydrophobicities driven by lipid exclusion.

The structure of liquid water at an extended hydrophobic surface

Chyuan Lee — 2008-01-23T15:40:08-00:00

The Journal of Chemical Physics, Vol. 80, No. 9. (1984), pp. 4448-4455.

Affinities of amino acid side chains for solvent water.

R Wolfenden — 2008-02-08T15:48:19-00:00

Biochemistry, Vol. 20, No. 4. (17 February 1981), pp. 849-855.

Equilibria of distribution of amino acid side chains, between their dilute aqueous solutions and the vapor phase at 25 degrees C, have been determined by dynamic vapor pressure measurements. After correction to pH 7, the resulting scale of "hydration potentials", or free energies of transfer from the vapor phase to neutral aqueous solution, spans a range of approximately 22 kcal/mol. The side chain of arginine is much more hydrophilic than those of the other common amino acids, with an equilibrium constant of approximately 10(15) for transfer from the vapor phase to neutral aqueous solution. Hydration potentials are more closely correlated with the relative tendencies of the various amino acids to appear at the surface of globular proteins than had been evident from earlier distribution studies on the free amino acids. Both properties are associated with a pronounced bias in the genetic code.

Interfaces and the driving force of hydrophobic assembly

David Chandler — 2005-09-28T18:18:37-00:00

Nature, Vol. 437, No. 7059. (28 September 2005), pp. 640-647.

Hydrophobicity effects in the condensation of water films on quartz

Michelle Gee — 2008-03-17T19:43:03-00:00

Journal of Colloid and Interface Science, Vol. 140, No. 2. (December 1990), pp. 450-465.

The surface forces of thin water films condensed onto crystalline quartz plates have been investigated by ellipsometric measurements of film thickness as a function of disjoining pressure. Quartz substrates ranging from fully hydroxylated (contact angle - 0[degree sign]) to completely dehydroxylated (contact angle - 45[degree sign]) were used and the results obtained related to the theoretically predicted van der Waals and electrostatic forces present in the system. Water films on fully hydroxylated quartz are much thicker than expected, whereas films on fully dehydroxylated quartz are close to the Lifschitz prediction of dispersion forces. As the extent of dehydroxylation decreases, the adsorption isotherm approaches that obtained on fully hydroxylated quartz.

Long-range attractive force between hydrophobic surfaces observed by atomic force microscopy

YH Tsao — 2008-03-17T17:42:04-00:00

Science, Vol. 262, No. 5133. (22 October 1993), pp. 547-550.

There is evidence from atomic force microscopy for a long-range attractive force between hydrophobic surfaces that is virtually identical to that observed with the surface forces apparatus. This force is present in the nonaqueous solvent ethylene glycol. A possible molecular mechanism involves in-plane polarized domains of solid-like monolayers adsorbed on mica, and a theoretical model has been developed that accounts for many of the observations. 10.1126/science.8211182

Experimental study of phase separation in films of molecular dimensions

Hugo Christenson — 2008-03-17T19:34:41-00:00

Physical Review B, Vol. 39, No. 16. (1 June 1989), 11750.

Phase separation induced by the close proximity of two surfaces occurs in thin films of incompletely miscible liquids. This phenomenon is thermodynamically analogous to capillary condensation of liquid from vapor and leads to a discontinuity in the force between two surfaces across the liquid. We present measurements of the distance at which phase separation takes place in nonpolar liquids containing water at activities from 0.7 to 1 between two kinds of chemically different mica surfaces. The generality of the effect is established by similar results obtained with other sparingly soluble solutes. The importance of surface adsorption and kinetic effects is discussed and comparisons are made with a modified Kelvin equation. The results are relevant to recent theoretical and simulation studies of the phase behavior of liquids in narrow pores and thin films.

Interactions between water--stable hydrophobic Langmuir--Blodgett monolayers on mica

Per Claesson — 2008-03-17T17:37:49-00:00

Journal of Colloid and Interface Science, Vol. 114, No. 1. (November 1986), pp. 234-242.

A Langmuir-Blodgett technique was used for depositing monolayers of dimethyldioctadecylammonium ions on molecularly smooth muscovite mica surfaces. Direct measurements of the interaction between such surfaces were performed with the surface force apparatus of Israelachvili. A long--range attractive force considerably stronger than the expected van der Waals force was observed. Studies on the salt dependence of this force indicate that it does not have an electrostatic origin. Instead the present results support the view that perturbations of the dynamic association of water molecules outside a hydrophobic surface are propagated several nanometers.

The hydrophilic--hydrophobic transition on silica

J Laskowski — 2008-03-25T13:35:50-00:00

Journal of Colloid and Interface Science, Vol. 29, No. 4. (April 1969), pp. 670-679.

An investigation has been made of the contact angle [theta] and the zeta potential at the interface between aqueous solutions and vitreous silica which had been methylated by reaction with trimethylchlorosilane. The contact angle was found to depend on the pH of the solution and the time of contact of the silica with water. It eventually reached zero at all pH values, the rate of fall of [theta] being greatest at high pH. However, this rehydration process was not due to loss of methyl groups, for the hydrophobic surface could be restored simply by drying the specimen at 110[degree sign]C, and only physically adsorbed water is removed from silica at this temperature. These observations, combined with the fact that pure silica can be rendered temporarily hydrophobic merely by strong heating to remove silanol groups, have led to the conclusion that the work of adhesion of water to solids can be analyzed into three main terms, depending respectively on dispersion forces, hydration of non-ionic polar sites, and ionization. The magnitude of the terms has been roughly estimated. "Hydrophobicity" arises whenever the two latter terms are small, because dispersion energies are always smaller than the (exceptionally large) work of cohesion of liquid water. With silica, the hydration term controls the wettability. From the standpoint of the Frumkin-Derjaguin treatment of wetting phenomena, hydrophobicity indicates that multimolecular water layers in a certain range of thickness are unstable. This instability must be ascribed to a less favorable state of molecular association of water molecules at a certain distance from the hydrophobic surface than in normal water; but the range of hydrophobic influence is still not known.

The hydrophobic interaction is long range, decaying exponentially with distance

Jacob Israelachvili — 2008-03-17T17:25:21-00:00

Nature, Vol. 300, No. 5890. (25 November 1982), pp. 341-342.

Effect of Alcohol-Water Exchange and Surface Scanning on Nanobubbles and the Attraction between Hydrophobic Surfaces

Marc Hampton — 2008-06-06T12:18:38-00:00

Journal of Colloid and Interface Science, Vol. In Press, Accepted Manuscript

Origin of the long-range attraction between surfactant-coated surfaces

Emily Meyer — 2008-03-17T17:45:45-00:00

Proceedings of the National Academy of Sciences, Vol. 102, No. 19. (10 May 2005), pp. 6839-6842.

We compare the "long-range hydrophobic forces" measured (i) in the "symmetric" system between two mica surfaces that had been rendered hydrophobic by the adsorption of a double-chained cationic surfactant, and (ii) between one such hydrophobic surface and a hydrophilic surface of bare mica ("asymmetric" case). In both cases, the forces were purely attractive, stronger than van der Waals, and of long-range, as previously reported, with those of the asymmetric, hydrophobic-hydrophilic system being even stronger and of longer range. Atomic force microscopy images of these surfaces show that the monolayers transform into patchy bilayers when the surfaces are immersed in water, and that the resulting surfaces contain large micrometer-sized regions of positive charges (bilayer) and negative charges (bare mica) while remaining overall neutral. The natural alignment of oppositely charged domains as two such surfaces approach would result in a long-range electrostatic attraction in water, but the short-range, "truly hydrophobic" interaction is not explained by these results. 10.1073/pnas.0502110102

Submicrocavity Structure of Water between Hydrophobic and Hydrophilic Walls as Revealed by Optical Cavitation

OI Vinogradova — 2008-03-25T12:30:24-00:00

Journal of Colloid and Interface Science, Vol. 173, No. 2. (August 1995), pp. 443-447.

Laser-induced cavitation in a thin water layer bounded by hydrophobic and hydrophilic surfaces has been explored. For hydrophobic surfaces, the cavitation probability is enhanced and its threshold lowered. For hydrophilic walls, the formation of a macroscopic gas bubble during the laser pulse is consistent with earlier work on the existence of submicrobubble (bubston) clusters in bulk water (Bunkin et al., submitted for publication; Bunkin and Lobeyev, JETP Lett. Engl. Transl. 58, 91 (1993)). The increased cavitation probability observed with hydrophobic surfaces can be linked to an enhanced concentration of gas-filled submicrocavities close to the surface as compared to those in bulk water. The phenomenon appears to be closely related to the long-range hydrophobic interactions between surfaces and to the slippage of water over hydrophobic surfaces.

Contact Angle Studies of Galena Particles

Clive Prestidge — 2007-10-19T19:50:19-00:00

Journal of Colloid and Interface Science, Vol. 172, No. 2. (15 June 1995), pp. 302-310.

The wettability of galena particles has been determined from measurements of the equilibrium capillary pressure across a packed bed. Both the macroscopic advancing liquid contact angle, [theta]p, and the specific wettable surface area, Awet, of the galena particles have been obtained. The influence of particle size and surface preparation method on [theta]p has been investigated. The hydrophobic nature of galena is at least partly controlled by hydrodynamics during particle conditioning. The high shear environment experienced during cyclosizing results in galena particles of greater hydrophobicity compared with those prepared by sieving; these findings are in agreement with the measured flotation recoveries. A mechanism has been proposed to explain the relationship between hydrodynamic conditions, surface chemistry, and surface hydrophobicity. X-ray photoelectron spectroscopy studies support this mechanism.

Contact angle studies of particulate sulphide minerals

CA Prestidge — 2007-10-19T19:45:43-00:00

Minerals Engineering, Vol. 9, No. 1. (January 1996), pp. 85-102.

The flotation separation of sulphide minerals is achieved by controlling particle hydrophobicity, which is often described in terms of the liquid-vapour contact angle determined from wettability studies on single surfaces. These may not be representative of the true hydrophobicity of mineral particles in a pulp and there is a requirement to determine "particle contact angles". We report an experimental approach, based on equilibrium capillary pressure measurements across a packed bed of particles, which reliably and routinely determines the powder contact angle of mineral particles in the size range 10 to 200 pm. The influence of particle size and surface treatment on the powder contact angle and flotation response of galena particles are reported. The natural hydrophobic nature of galena was shown to depend on the hydrodynamic conditions during particle conditioning. The high shear environment experienced during cyclosizing resulted in galena particles of greater hydrophobicity than those prepared by sieving. A mechanism is proposed to explain the relationship between hydrodynamic conditions, surface chemistry and hydrophobicity; this may give insight into the role of high intensity conditioning and sonication in sulphide mineral flotation. The correlation between collector coverage, particle wettability and flotation recovery was also investigated. Ethyl xanthate at surface coverages in excess of 0.1 equivalent monolayers significantly increased the particle contact angle and particle floatability. The influence of other reagents on galena particle contact angles is discussed.

The wetting of angular quartz particles: Capillary pressure and contact angles

David Diggins — 2008-03-20T14:54:11-00:00

Colloids and Surfaces, Vol. 44 (1990), pp. 299-313.

A new experimental apparatus for the determination of powder wettability is described. The principle of the technique is based on the measurement of the equilibrium capillary pressure in a packed bed of particles. Using White's thermodynamic approach (J. Colloid Interface Sci., 90 (1982) 536), both the macroscopic solid-solution-vapour contact angle, [theta], and the specific surface area, A (m2 g-1), of the powder, can be obtained. Test experiments on angular quartz particles and glass spheres have been performed. A wide range of situations have been covered: hydrophilic and hydrophobic particles wetted with water and a variety of organic liquids. The results show that reliable and reproducible values for [theta] and A for angular and relatively coarse powders can be obtained with the proposed equipment.

Effect of Gas Nuclei on Hydrophobic Coagulation

ZA Zhou — 2008-04-09T18:31:31-00:00

Journal of Colloid and Interface Science, Vol. 179, No. 1. (15 April 1996), pp. 311-314.

Experimental results on the coagulation of fine coal particles in water (a case of natural hydrophobicity) and silica in dodecylamine solution (chemically induced hydrophobicity) are presented. Different behavior was observed depending whether or not the slurry was degassed. After degassing, the apparent size of both coal and silica flocs became smaller, which was attributed to removal of gas nuclei and consequent reduction in the interparticle attractive force. The effect on settling, however, was different for both: the settling rate of silica decreased while for coal it increased. This was modeled in terms of the relative effect of size and density of coal and silica particles with and without gas nuclei. Apart from practical consequences in fine particle processing, a fundamental one is that to evaluate true hydrophobic coagulation or to measure the true hydrophobic force, gas nuclei have to be eliminated.

Interactions between a positively charged hydrophobic surface and a negatively charged bare mica surface

Per Claesson — 2008-03-17T19:15:12-00:00

Journal of Colloid and Interface Science, Vol. 118, No. 1. (July 1987), pp. 68-79.

Direct measurements of interactions between one negatively charged mica surface and one positively charged modified mica surface immersed in electrolyte solutions are reported. The modified mica surface was obtained by Langmuir-Blodgett deposition of dimethyldioctadecylammonium ions which made the surface hydrophobic and gave it a small positive charge. The interactions between these dissimilar surfaces are dominated by attractive double-layer forces. However, in concentrated electrolyte solutions (C > 10-2 M) and/or at short surface separations (D < 20 nm) the measured attraction is stronger than that expected from conventional DLVO theory. This is suggested to be due to either a breakdown of the Poisson-Boltzmann approximation or the presence of an additional force related to the attractive force previously observed between two hydrophobic surfaces in aqueous solutions.

Water adsorption kinetics and contact angles of silica particles

Tim Muster — 2007-06-04T21:55:16-00:00

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 176, No. 2-3. (30 January 2001), pp. 253-266.

Water adsorption was used to characterise the wettability of non-porous colloidal silica spheres with varying degrees of dehydration. In particular, water adsorption kinetics at saturated vapour pressure were correlated with advancing water contact angles determined by capillary penetration and the surface coverage of hydroxyl groups determined by diffuse reflectance infrared spectroscopy. Water uptake was found to be controlled by (1) the hydroxylation state of the silica particle surface and (2) the rate of water condensation to form multilayers. The processes that control water adsorption kinetics were fitted with first-order rate equations, thus enabling the concentration and reactivity of surface hydroxyl groups to be estimated. A Cassie approach was used to estimate the contact angle from water adsorption and infrared data; these were compared with contact angles determined by liquid penetration. A good correlation was observed between the hydroxylation-state of silica and the contact angle. An improved understanding of the interplay between surface chemistry, water adsorption and particle wettability has resulted.

The critical rupture thickness of thin water films on hydrophobic surfaces

Yoon — 2008-03-17T19:11:42-00:00

Journal of Colloid and Interface Science, Vol. 146, No. 2. (15 October 1991), pp. 565-572.

From the Newton rings formed between an air bubble and a plate immersed in aqueous solutions, the thickness of the intervening water film can be determined. Using this technique, the critical thickness at which the film ruptures has been measured. The results obtained using methylated fused-silica as a model hydrophobic solid show that the critical rupture thickness increases with increasing degree of methylation. In solutions of dodecylamine hydrochloride, the rupture thickness of the water film on both fused-silica and mica plates increases with increasing concentration. The rupture thickness also increases with increasing pH of the amine solution, reaching a maximum at pH 9-9.5, where the adsorption is most favored. The rupture thickness measured in the present work show a reasonable correlation with the work of adhesion determined from contact angle measurements.

Atomic Force Microscopy Imaging of Thin Films Formed by Hydrophobing Reagents

Simon Biggs — 2008-03-25T13:12:04-00:00

Journal of Colloid and Interface Science, Vol. 165, No. 2. (July 1994), pp. 425-430.

The hydrophobic films produced on silica plates by a common silylating reagent, trimethylchlorosilane, at the solid/liquid and solid/gas interfaces have been imaged directly using an atomic force microscope. The results obtained here support previous findings that such reactants produce relatively thick inhomogeneous films. For the first time we are able to present direct images of the spatial inhomogeneity of such films. The thin film produced by esterification of the silica plates with 1-dodecanol has also been imaged. In this case, a good, post-reaction, cleanup procedure was determined to be vital in obtaining a uniform thin hydrophobic film. Comparison with a previous ellipsometric study from this laboratory shows broad agreement on the observed film thicknesses and homogeneities for all samples.

Microsphere tensiometry to measure advancing and receding contact angles on individual particles

S Ecke — 2008-02-02T21:27:42-00:00

Journal of Adhesion Science and Technology, Vol. 13, No. 10. (1999), pp. 1181-1191.

In this paper, a method to measure the advancing and receding contact angles on individual colloidal spheres is described. For this purpose, the microspheres were attached to atomic force microscope cantilevers. Then the distance to which the microsphere jumps into its equilibrium position at the air-liquid interface of a drop or an air bubble was measured. From these distances the contact angles were calculated. To test the method, experiments were done with silanized silica spheres (4.1 μm in diameter). From the experiments with drops, an advancing contact angle of 101 ± 4° was determined. A receding contact angle of 101 ± 2° was calculated from the jump-in distance into a bubble. Both experimental techniques gave the same contact angle. In contrast, on similarly prepared planar silica surfaces, a clear hysteresis was measured with the sessile drop method; contact angles of 104.5 ± 1° and 93.8 ± 1° were determined for the advancing and receding contact angles, respectively.

Cavitation and the Interaction Between Macroscopic Hydrophobic Surfaces

Hugo Christenson — 2008-03-25T12:10:30-00:00

Science, Vol. 239, No. 4838. (22 January 1988), pp. 390-392.

The interaction in water of neutral hydrocarbon and fluorocarbon surfaces, prepared by Langmuir-Blodgett deposition of surfactant monolayers, has been investigated. The attraction between these hydrophobic surfaces can be measured at separations of 70 to 90 nanometers and thus is of considerably greater range than previously found. Spontaneous cavitation occurred as soon as the fluorocarbon surfaces were brought into contact but occurred between the hydrocarbon surfaces only after separation from contact. The very long range forces measured are a consequence of the metastability of water films between macroscopic hydrophobic surfaces. Thus the hydrophobic interaction between macroscopic surfaces may not be related to water structure in the same way that the hydrophobic effect between nonpolar molecules is related to water structure. 10.1126/science.239.4838.390

A possible hydrodynamic origin of the forces of hydrophobic attraction

E Ruckenstein — 2008-03-17T20:13:22-00:00

Journal of Colloid and Interface Science, Vol. 147, No. 2. (December 1991), pp. 535-538.

It is suggested that the large attractive force between two hydrophobized surfaces immersed in water may have a hydrodynamic origin. This force may be due (1) to the instability of the planar interfaces between the water layer and the hydrophobic surfaces caused by the incompatibility between the two, and (2) to the fact that the fluctuations of the two water interfaces generated by the instability are hydrodynamically correlated. A simple, approximate model is suggested, on the basis of which an equation for the force is derived.