Precise structural analysis of α-helical copolypeptide H-(Ala-Gly) 9-OH by quantum chemical calculation and high-resolution solid-state NMR measurement

Abstract

We computed the optimized structure of sequential 18-mer copolypeptide H-(Ala-Gly) 9-OH (C 45H 74N 18O 19) adopting an right-handed α-helix (α R-helix) conformation with the basis set of DFT/6-31G(d), and then calculated the nuclear shieldings of the optimized structure with the basis set of DFT/6-311G(d,p). As a result, we confirmed highly accurate conformational parameters characteristic to the α R-helical H-(Ala-Gly) 9-OH, which were identical with those of the individual Ala and Gly residues. Most of these parameters were fundamentally the same as those obtained for the optimized α R-helical H-(Ala) 18-OH. Furthermore, it was found that the calculated isotropic 13C and 15N chemical shifts were dependent on the nature of individual amino acid residues, which were greatly in good agreement with those of α R-helical model copolypeptides consisting of l-alanine and glycine residues measured by high-resolution solid-state NMR.