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 LK7 beta 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 LK7 beta. 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-LK7 beta to the D-LK7 beta peptide, suggesting that the chirality of the N-H in the folded peptide LK7 beta is completely governed by the chirality of the C-alpha-H of the amino acids. This discovery provides a clue on why proteins in nature are composed of the alpha-amino acids rather than beta- or gamma-amino acids and may help us understand how life works.