| The left handed polyproline II (PPII) helix is a little-studied and yet common secondary structure. This conformation is adopted by polyproline, collagen, and many proline-rich regions of sequence, including ligands of SH3, WW, and EVH1 protein interaction domains. Notably, numerous sequences not rich in, or even devoid of proline, adopt this structure. Importantly, it has also been hypothesized that denatured states of proteins posses PPII structure. We have studied in a much closer point of view the determinants that might contribute to the stabilization of PPII. These include the effect of common post-translational modification (PTMs) in resiues like threonine, lysine, serine and methionine; the effect of Hofmeister series alanine based peptides; the effect of N- and C-capping; the effect of different blocking group in alanine- and proline-based peptides; and the effect of crowding agents in the stabilization of PPII conformation. We have studied the host-guest peptide Ac-GXGG-NH2 as a model (X = any aminoacid, except glycine), since it has demonstrated a high PPII content.;Our work on determining the influences stabilizing PPII helix have focused on interaction with solvent and conformational entropy of aminoacid side chain. Circular dichroism experimental studies suggest the PPII stabilization with the phosphorylation of serine and threonine, whereas no effect was observed with methylation of lysine or oxidation of methionine. On the other hand Hofmeister series have shown a normal tendency in the stability of PPII as observed in normal proteins. The presence of N- and C-capping groups stabilizes PPII due to electrostatic interaction of the free ends. This effect is suppressed when the charge is removed by acetylation and amidation of the N and C termini. Different blocking groups in dipeptides are a clear evidence of what is called hyperconjugation by using fluorine as an electron-withdrawing group.;Finally, crowding agents, such as Ficoll, do not enhance or disfavor PPII, concluding a minor contribution on PPII from the steric hindrance in aqueous solutions. These finding are crucial to understand the behavior of some proteins in PPII conformation that participate in diseases, such as Alzheimer and Huntington's disease. |
