Effects of N-terminal L-amino acid residues on helical screw sense in achiral peptides

Article abstract of DOI:10.1246/bcsj.72.55

To understand the effects of N-terminal L-residues on dominating helical screw sense in achiral peptides, we adopted six kinds of peptides Boc-X- (Aib-ΔPhe)₂-Aib-OMe (Boc, t-butoxycarbonyl; OMe, methoxy), in which the X residue is an L-amino acid of alanine (Ala), leucine (Leu), valine (Val), phenylalanine (Phe), 1-naphthylalanine (Nap), or proline (Pro). The segment - (Aib-ΔPhe)₂- was used for a backbone composed of two 'enantiomeric' (left- /right-handed) helices. Actually, this could be confirmed by ¹H NMR and CD spectroscopy on Boc-(Aib-ΔPhe)₂-Aib-OMe, which took left- and right-handed 3₁₀-helices with the same content. All peptides were also found to take 3₁₀-type helical conformations in CDCl₃ from solvent accessibility of NH resonances. Chloroform, acetonitrile, methanol, and tetrahydrofuran were used for solvents in CD measurement. All peptides in all solvents showed marked exciton couplets around 280 nm with a positive peak at longer wavelengths. Consequently, when an N-terminal L-residue, irrespective of types of L- residues, is introduced into a helical segment of achiral peptide, its main chain prefers the left-handed screw sense. The peptide with X = Ala showed the smallest amplitude of exciton couplets in each solvent, meaning that the Ala residue with the smallest side chain (methyl group) had the least effective chirality for taking a one-side helical screw sense preferentially, compared with the other residues used here.