Chiral Inversion of Amino Acids in Antiparallel β-Sheets at Interfaces Probed by Vibrational Sum Frequency Generation Spectroscopy

A parallel study of protein variants with all (L-), all (D-), or mixed (L-)/(D-) amino acids can be used to assess how backbone architecture versus side chain identity determines protein structure. Here, we investigate the secondary structure and side chain orientation dynamics of the antiparallel beta-sheet peptide LK7 beta (Ac-Leu-Lys-Leu-Lys-Leu-Lys-Leu-NH2) composed of all (L-), all (D-), or alternating (L-Leu)/(D-Lys) amino acids. Using interface-selective vibrational sum frequency generation spectroscopy (VSFG), we observe that the alternating (L-)/(D-) peptide lacks a resonant C-H stretching mode compared to the (L-) and (D-) variants and does not form antiparallel beta-sheets. We rationalize our observations on the basis of density functional theory calculations and molecular dynamics (MD) simulations of LK7 beta at the air-water interface. Irrespective of the handedness of the amino acids, leucine side chains prefer to orient toward the hydrophobic air phase while lysine side chains prefer the hydrophilic water phase. These preferences dictate the backbone configuration of LK7 beta and thereby the folding of the peptide. Our MD simulations show that the preferred side chain orientations can force the backbone of a single strand of (L-) LK7 beta at the air-water interface to adopt beta-sheet Ramachandran angles. However, denaturation of the beta-sheets at pH = 2 results in a negligible chiral VSFG amide I response. The combined computational and experimental results lend critical support to the theory that a chiral VSFG response requires macroscopic chirality, such as in beta-sheets. Our results can guide expectations about the VSFG optical responses of proteins and should improve understanding of how amino acid chirality modulates the structure and function of natural and de novo proteins at biological interfaces.