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

We computed the optimized structure of sequential 18-mer copolypeptide H-(Ala-Gly)9-OH (C45H74N18O19) adopting an right-handed α-helix (αR-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 αR-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 αR-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 αR-helical model copolypeptides consisting of l-alanine and glycine residues measured by high-resolution solid-state NMR.