N–H Chirality in Folded Peptide LK₇β Is Governed by the C_α–H Chirality

Abstract

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 LK 7 β 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-LK 7 β to the D-LK 7 β peptide, suggesting that the chirality of the N-H in the folded LK 7 β peptide is completely governed by the chirality of the C α-H of the amino acids. This discovery provides clue on why proteins in nature are composed of α-amino acids rather than βor γ-amino acids, and may help us understand the question on the origin of life.

Mirror-image antiparallel β-sheets organize water molecules into superstructures of opposite chirality

“…We also identified two peptide N-H stretching bands with an opposite phase at 3,247 and 3,270 cm −1 (Fig. 2A), consistent with a prior report (12), and two peptide N-D stretching bands with an opposite phase at 2,391 and 2,445 cm −1 (Fig. 2C).…”

“…2C). To explain the opposite phases of the two N-H stretching bands, Hu et al (12) proposed that adjacent backbone nitrogen atoms along the β-strand are chiral centers with opposite handedness (i.e., pro-S and pro-R). This proposal would require a substantial sp 3 -hybridized character of the nitrogen atoms.…”

“…Indeed, the LK 7 β peptide and its variants in native (L-) form have been commonly used as a model system in surface studies. (2,(8)(9)(10)(11)(12)(15)(16)(17) We obtain the phase-resolved chiral SFG vibrational spectra of (L-) and (D-) LK 7 β in the spectral regions of the O-H/N-H stretching at ∼3,200 cm −1 using H 2 O as solvent and the O-D/N-D stretching at ∼2,400 cm −1 using D 2 O. We also perform the experiments using H 2 18 O and D 2 18 O.…”

“…The spectra start to become noisier below 10 Å, but the opposite phases of the (L-) and (D-) LK 7 β spectra exhibited at all depths sampled suggest that some chiral water structures exist at least 15 Å below the interface. O Assignments because the vibrational modes originate from the chiral peptide backbone (12,30). However, the accompanying phase reversal of the two water-stretching bands has an important implication.…”

Handy water: Chiral superstructures around peptide β-sheets

“…1). It was also confirmed that mirror-image β-sheet assemblies of LK 7 β exhibit opposite orientations in both C-H and N-H modes, as they should since they are each other's chiral enantiomer (9). With this advancement a further question about the chirality of biomolecules and the hydration shell can now be answered: Do left-or right-handed chiral water superstructures exist around peptides?…”