Mechanism for the noncovalent chiral domino effect:: New paradigm for the chiral role of the N-terminal segment in a 310-helix

Recently, novel chiral interactions on 3(10)-helical peptides, of which the helicity is controlled by external chiral stimulus operating on the N-terminus, were proposed as a noncovalent chiral domino effect (NCDE) (Inai, Y.; et al. J. Am. Chem. Soc. 2000, 122,11731. Inai, Y.; et al. J. Am. Chem. Soc. 2002, 124, 2466). The present study clarifies the mechanism for generating the NCDE. For this purpose, achiral nonapeptide (1), H-beta-Ala-(Delta(z)Phe-Aib)(4)-OMe [Delta(z)Phe = (Z)-didehydrophenylalanine, Aib = alpha-aminoisobutyric acid], was synthesized. Peptide 1 alone adopts a 310-helical conformation in chloroform. On the basis of the induced CD signals of peptide 1 with chiral additives, chiral acid enabling the predominant formation of a one-handed helix was shown to need at least both carboxyl and urethane groups; that is, Boc-L-amino acid (Boc = tert-butoxycarbonyl) strongly induces a right-handed helix. NMR studies (NH resonance variations, low-temperature measurement, and NOESY) were performed for a CDCl3 solution of peptide 1 and chiral additive, supporting the view that the N-terminal H-beta-Ala-Delta(z)Phe-Aib, including the two free amide NH's, captures effectively a Boc-amino acid molecule through three-point interactions. The H-P-Ala's amino group binds to the carboxyl group to form a salt bridge, while the Aib(3) NH is hydrogen-bonded to either oxygen of the carboxylate group. Subsequently, the free Delta(z)Phe(2) NH forms a hydrogen bond to the urethane carbonyl oxygen. A semiempirical molecular orbital computation explicitly demonstrated that the dynamic looping complexation is energetically permitted and that the N-terminal segment of a right-handed 3(10)-helix binds more favorably to a BOC-L-amino acid than to the corresponding D-species. In conclusion, the N-terminal segment of a 3(10)-helix, ubiquitous in natural proteins and peptides, possesses the potency of chiral recognition in the backbone itself, furthermore enabling the conversion of the terminally acquired chiral sign and power into a dynamic control of the original helicity and helical stability.