We have calculated chemical shifts for a range of diastereotopic protons in proteins (i.e. methylene protons, and the methyl groups of valine and leucine residues), using a recently optimised method for chemical shift calculation. The calculations are based on crystal structure coordinates, and ave been compared with experimental stereospecific assignments. The results indicate that chemical shifts can be used to suggest stereospecific assignments with about 80% probability of being correct, in cases where both the experimental and the calculated chemical shift differences between a pair of diastereotopic protons are greater than 0.3 ppm. Inaccurate calculations are shown to be caused in most cases by differences between crystal and solution structures. Furthermore, chemical shift calculations based on NMR structures are shown to be capable of acting as a further constraint on structure, by limiting the range of side-chain conformations adopted in structures calculated from NMR data.
