A NEW ANALYSIS OF PROTON CHEMICAL-SHIFTS IN PROTEINS

We present an empirical analysis of proton chemical shifts from 17 proteins whose X-ray crystal structures have been determined. The crystal structures are used to estimate the conformation-dependent part of the shift, that is, the difference between the observed shift and that of a "random-coil" linear peptide. The results indicate that a significant improvement over ring-current theories can be made by including the effects of the magnetic anisotropy of the peptide group and estimates of backbone electrostatic contributions. For 5678 protons bonded to carbon, we find a linear correlation coefficient of 0.88 between calculated and observed structural shifts, with a root-mean-square error of 0.23 ppm; contributions from the peptide group are especially noticeable for protons at the C-alpha position. If we consider only side-chain protons in non-heme proteins, the rms error is 0.18 ppm, and methyl protons show an rms error of 0.13 ppm. New estimates of intensity factors for various ring-current contributions are given (including those arising from the heme group) which suggest more nearly equal contributions from various rings than found in earlier studies. Predictions for protons bonded to nitrogen are much poorer than for protons bonded to carbon, but significant qualitative insights can be obtained. Prospects for using calculated chemical shifts in the final refinement of protein solution structures are discussed.