Computation of through-space NMR shielding effects by functional groups common to peptides

The GIAO-HF method in Gaussian 03 was employed to calculate the isotropic shielding values of a covalently bonded hydrogen probe and to predict the through-space proton NMR shielding increment surfaces near models of simple structures containing functional groups common to peptides. The functional groups examined include the carboxylate anion, carboxylic acid, amide, amino, ammonium and guanidinium groups. Our previously developed methodology involving the use of diatomic hydrogen as a probe of through-space shielding effects was employed. Substantial shielding or deshielding effects were observed only in the cases of the charged (ionic) groups, each of which displayed shielding or deshielding effects of greater than 1 ppm at distances comparable to those observed in peptides. Equations for predicting the shielding increments of these groups as a function of the Cartesian coordinate position of the affected proton were determined. The validity of using simple structures as models of shielding by comparable functional groups in peptides was confirmed by computing the shielding effects at selected positions above a model of glycylglycylglycine and its hydrogen-bonded dimer. Knowledge of these through-space shielding effects should aid in the tertiary structure determination of peptides by NMR.