Executive Summary : | Oligomers of proline (Pro) can adopt either polyproline (PP) I or II helical type conformation due to the cis-trans isomerization of Pro amide bonds. PPI and PPII helix formation by oligo-Pro is highly solvent dependent, with PPI being preferred in aliphatic alcohols and nonpolar solvents like benzene, and PPII being preferred in an aqueous medium. The interconversion between PPI and PPII helices is an area of intense investigation. The role of N to C terminal CO··CO n ? ?* interactions in stabilizing PPII helix is established, but the PPI helical stability is poorly understood. Recent studies with non-proline PPI helices reveal the possibility of reverse (C to N terminal) CO··CO n ? ?* interactions in these helices. Therefore, it is possible that during the interconversion between PPI and PPII, not only do the amide bonds isomerize but the CO··CO n ? ?* interactions also reorganize and reverse their directions. This project aims to answer this question by synthesizing oligo-Pro peptides with N- or C-terminal capping groups that can lock the terminal Pro amide bonds in either cis or trans conformation. Enforcing cis or trans geometry to the terminal amides could have a profound effect on the intrachain CO·CO interaction network of oligo-Pro and their PPI and PPII helical stabilities. By controlling the C- or N-terminal amide bond geometry, the overall secondary structure of a proline oligomer can be controlled. |