Extension of the AMBER force field to cyclic [small alpha],[small alpha] dialkylated peptides

The popular biomolecular AMBER (ff99SB) force field (FF) has been extended with new parameters for the simulations of peptides containing [small alpha],[small alpha] dialkylated residues with cyclic side chains. Together with the recent set of nitroxide parameters [E. Stendardo, A. Pedone, P. Cimino, M. C. Menziani, O. Crescenzi and V. Barone, Phys. Chem. Chem. Phys., 2010, 12, 11697] this extension allows treating the TOAC residue (TOAC, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) widely used as a spin label in protein studies. All the conformational minima of the Ac-Ac6C-NMe (Ac = acetyl, Ac6C = 1-aminocyclohexaneacetic acid, NMe = methylamino) and Ac-TOAC-NMe dipeptides have been examined in terms of geometry and relative energy stability by Quantum Mechanical (QM) computations employing an hybrid density functional (PBE0) for an extended training set of conformers with various folds. A very good agreement between QM and MM (molecular mechanics) data has been obtained in most of the investigated properties, including solvent effects. Finally, the new set of parameters has been validated by comparing the conformational and dynamical behavior of TOAC-labeled polypeptides investigated by means of classical molecular dynamics (MD) simulations with QM data and experimental evidence. The new FF accurately describes the tuning of conformational and dynamical behavior of the Ac-TOAC-NMe dipeptide and double spin-labeled heptapeptide Fmoc-(Aib-Aib-TOAC)2-Aib-OMe (Fmoc, fluorenyl-9-methoxycarbonyl; Aib, [small alpha]-aminoisobutyric acid; OMe, methoxy) by solvents with different polarity. In particular, we found that the 310 helical structure of heptapeptide is the most stable one in vacuo, with a geometry very similar to the X-ray crystallographic structure, whereas a conformational equilibrium between the 310- and [small alpha]-helical structures is established in aqueous solution, in agreement with EPR data.