N–H Chirality in Folded Peptide LK7β is Governed by the Cα–H Chirality

Recent chiral sum-frequency generation vibrational spectroscopy (cSFG-VS) measurements revealed that the two N-H stretching modes in the 3000-3500 cm-1 range in folded protein and peptide exhibit chiral characteristics. Here we report the first phase-resolved sub-wavenumber high-resolution broadband SFG-VS (HR-BB-SFG-VS) measurement of the LK7β peptide. 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 LK7β peptide is completely governed by the chirality of the Cα–H of the amino acids. This discovery provides clue on why proteins in nature are composed of α-amino acids rather than β- or γ-amino acids, and may help us understand the question on the origin of life.