DETERMINATION OF DIHEDRAL ANGLES IN PEPTIDES THROUGH EXPERIMENTAL ANDTHEORETICAL-STUDIES OF ALPHA-CARBON CHEMICAL SHIELDING TENSORS

A simple method for the determination of backbone dihedral angles in p eptides and proteins is presented. The chemical-shift anisotropies (CS A) of the central alanine alpha-carbon in powdered crystalline tripept ides, whose structures have been determined previously by X-ray crysta llography, were measured by cross-polarization magic-angle-spinning nu clear magnetic resonance. The experimental CSA values were correlated with ab initio chemical-shielding calculations over Ramanchandran phi/ psi space on an N-formyl-L-alanine amide fragment. Using this correlat ion, phi/psi probability surfaces for one of the tripeptides were calc ulated based only on the alpha-carbon CSA, allowing a prediction of ba ckbone angles. Dihedral angles predicted by these calculations fall wi thin +/-12 degrees of the values determined by crystallography. This a pproach should be useful in the determination of solid-slate protein s tructure.