Carbon-13 NMR Chemical Shift and Electronic Structure of Solid Polypeptides as Studied by Tight-binding MO Theory. Polyglycine and Poly(l-alanine)

A tight-binding MO-sum-over-state theory has been used to calculate the 13C NMR chemical shift of polyglycine having the forms I and II and that of poly(l-alanine) having the α-helix and β-sheet forms, in order to justify the origin of the conformation-dependent 13C NMR chemical shift previously determined by the cross polarization-magic angle spinning technique. The calculated results on the carbonyl, Cα and Cβ carbons were found to exhibit the conformation-dependent 13C NMR chemical shifts comparable with the experimental data.