N-H Chirality in Folded Peptide LK7β Is Governed by the Cα-H Chirality.

Recent chiral sum-frequency generation vibrational spectroscopy (SFG-VS) measurements revealed that two N-H stretching modes in the 3100-3500 cm-1 range in folded peptide LK7β exhibit chiral characteristics. Here, we report the first phase-resolved subwavenumber high-resolution broadband SFG-VS (HR-BB-SFG-VS) measurement of the folded peptide LK7β. The results show that this chiral N-H band consists of four, instead of two, distinctive peaks, and they are with two groups of opposite spectral phases. Moreover, the phases of these N-H peaks completely flip from the l-LK7β to the d-LK7β peptide, suggesting that the chirality of the N-H in the folded peptide LK7β is completely governed by the chirality of the Cα-H of the amino acids. This discovery provides a clue on why proteins in nature are composed of the α-amino acids rather than β- or γ-amino acids and may help us understand how life works.