Control of hairpin formation via proline configuration in parallel β-sheet model systems

The simplest strategy for creation of parallel beta-sheet model systems is to link adjacent peptide strands via their N-termini or via their C-termini. This connectivity requires unnatural linking segments. We describe dipeptide mimics that can serve as N-to-N or C-to-C: linkers, and we demonstrate their efficacy by conformational analysis of tetrapeptide analogues in chloroform. The tetrapeptide analogues can adopt strand-loop-strand ("hairpin") conformations in which the residues at each end, L-valine and L-leucine, engage in parallel sheet hydrogen bonding interactions. Our linkers contain proline residues, to impart a preferred local twist. We show that linkers containing D-proline promote parallel sheet interactions between the strand L-residues, while linkers containing L-proline do not promote parallel sheet interactions. The preference for one linker twist is presumably related to the right-handed twist displayed by strands in protein beta-sheets (parallel and antiparallel); analogous linker twist preferences have been observed in antiparallel beta-sheet model systems.