Noncovalent chiral domino effect on one-handed helix of nonapeptide containing a midpoint L-residue

We here clarify whether noncovalent chiral domino effect characterized by the terminal interaction of a helical peptide with a chiral small molecule can alter the helical stability of N-deprotected peptides containing an L-residue covalently incorporated into the inner position. Two nonapeptides consisting of the midpoint L-leucine (1) or L-phenylalanine (2) and the achiral helix-forming residues were employed. NMR and IR spectroscopy and energy calculation indicated that both peptides adopt a 3(10)-helical conformation in chloroform. They strongly preferred a right-handed screw sense because of the presence of the midpoint L-residue. These original right-handed screw senses were retained on addition of chiral Boc-amino acid, but their helical stabilities clearly depended on its added chirality. Here, BOC-L-amino acid stabilizes the original right-handed helix, whereas the corresponding BOC-D-amino acid tends to less stabilize or destabilize it. This tendency was not observed for the corresponding N-Boc-protected peptides 1 and 2, strongly suggesting that the N-terminal amino group is required for controlling the stabilization of the original right-handed helix. Therefore, noncovalent chiral domino effect in peptides 1 and 2 can contribute even to the helical stability of a chiral peptide prevailing one-handed helix strongly through the midpoint L-residue. In addition, the N-terminal moiety of a 3(10)-helical peptide was found to generate chiral discrimination in complexation process with racemic additives.