Proton NMR Chemical Shift Behavior of Hydrogen-Bonded Amide Proton of Glycine-Containing Peptides and Polypeptides as Studied by ab initio MO Calculation

NMR chemical shifts of the amide proton of a supermolecule, an N-ethylacetamide hydrogen-bonded with a formamide, were calculated as functions of hydrogen-bond length RN. O and hydrogen-bond angles by FPT-GIAO method within the framework of HF/STO 6-31(++) G( d, p) ab initio MO method. The calculations explained reasonably the experimental data reported previously that the isotropic proton chemical shifts move downfield with a decrease in RN. O. Further, the behavior of proton chemical shift tensor components depending on the hydrogen-bond length and hydrogen-bond angle was discussed.