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

To understand the effects of N-terminal L-residues on dominating helical sc rew sense in achiral peptides. we adopted six kinds of peptides Boc-X-(Aib- Delta Phe)(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-Delta Phe)(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-Delta Phe)(2)-Aib-OMe, which took left-and right-handed 3(10)-helices with the same content. All peptides were also f ound to take 3(10)-type helical conformations in CDCl3 from solvent accessi bility of NH resonances. Chloroform, acetonitrile, methanol, and tetrahydro furan were used for solvents in CD measurement. All peptides in all solvent s showed marked exciton couplets around 280 nm with a positive peak at long er wavelengths. Consequently, when an N-terminal L-residue, irrespective of types of L-residues, is introduced into a helical segment of achiral pepti de, its main chain prefers the left-handed screw sense. The peptide with X = Ala showed the smallest amplitude of exciton couplets in each solvent, me aning that the Ala residue with the smallest side chain (methyl group) had the least effective chirality for taking a one-side helical screw sense pre ferentially, compared with the other residues used here.