Synthesis of Diacetylene-Containing Peptide Building Blocks and Amphiphiles, Their Self-Assembly and Topochemical Polymerization in Organic Solvents
A series of functional iodoacetylenes was prepared and converted into the corresponding diacetylene-substituted amino acids and peptides via Pd/Cu-promoted sp–sp carbon cross-coupling reactions. The unsymmetrically substituted diacetylenes can be incorporated into oligopeptides without a change in the oligopeptide strand's directionality. Thus, a series of oligopeptide-based, amphiphilic diacetylene model compounds was synthesized, and their self-organization as well as their UV-induced topochemical polymerizability was investigated in comparison to related polymer-substituted macromonomers. Solution-phase IR spectroscopy, gelation experiments, and UV spectroscopy helped to confirm that a minimum of five N-H⋅⋅⋅OC hydrogen-bonding sites was required in order to obtain reliable aggregation into stable β-sheet-type secondary structures in organic solvents. Furthermore, the non-equidistant spacing of these hydrogen-bonding sites was proven to invariably lead to β-sheets with a parallel β-strand orientation, and the characteristic IR-spectroscopic signatures of the latter in organic solution was identified. Scanning force micrographs of the organogels revealed that compounds with six hydrogen-bonding sites gave rise to high aspect ratio nanoscopic fibrils with helical superstructures but, in contrast to the related macromonomers, did not lead to uniform supramolecular polymers. The UV-induced topochemical polymerization within the β-sheet aggregates was successful, proving parallel β-strand orientation and highlighting the effect of the number and pattern of N-H⋅⋅⋅OC hydrogen-bonding sites as well as the hydrophobic residue in the molecular structure on the formation of higher structures and reactivity.