Our MD simulations show that the preferred side chain orientations can force the backbone of a single strand of (l-). These preferences dictate the backbone configuration of LK7β and thereby the folding of the peptide. field orientation we conclude, with a confidence level of 99.7, that at the water-air interface of this saline solution there is an enantioselective magnetic interaction that acts upon racemic mixtures of hydrophobic chiral amino acids. 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. We rationalize our observations on the basis of density functional theory calculations and molecular dynamics (MD) simulations of LK7β at the air-water interface. 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 β-sheets. Here, we investigate the secondary structure and side chain orientation dynamics of the antiparallel β-sheet peptide LK7β (Ac-Leu-Lys-Leu-Lys-Leu-Lys-Leu-NH2) composed of all (l-), all (d-), or alternating (l-Leu)/(d-Lys) amino acids. 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.
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