Conformational analysis of α-helical polypeptide included l-proline residue by high-resolution solid-state NMR measurement and quantum chemical calculation

We challenged the problem about the stabilization mechanism of an α-helix formation for polypeptides containing l-proline (Pro) residue. We computed the optimized structure of α-helical poly(l-alanine) molecules including a Pro residue, H-(Ala)8-Pro-(Ala)9-OH, based on the molecular orbital calculation with density functional theory, B3LYP/6-31G(d) and the 13C and 15N chemical shift values based on the GIAO-CHF method with B3LYP/6-311G(d,p), respectively. It was found that two kinds of optimized structures, 'Bent structure' and 'Included α-helix structure', were preferred structures in H-(Ala)8-Pro-(Ala)9-OH. In addition, based on the precise 13C and 15N chemical shift data of the simple model, we successfully analyzed the secondary structure of well-defined synthetic polypeptide H-(Phe-Leu-Ala)3-PheC-Pro-AlaN-(Phe-Leu-Ala)2-OH (FLA-11P), the secondary structure of which was proven to the 'Included α-helix structure'.