Computation of the ~3C' ~ chemical shifts (or shieldings) of glycine, alanine and valine residues in bovine and Drosophila calmodulins and Staphylococcal nuclease, and comparison with experimental values, is reported using a gauge-including atomic orbital quantum-chemical approach. The full ~ 24 ppm shielding range is reproduced (overall r.m.s.d. = 1.4 ppm) using 'optimized' protein structures, corrected for bondlength/bond-angle errors, and rovibrational effects.
Multidimensional nuclear magnetic resonance (Oschkinat et al., 1988) provides a powerful route to analyzing the three-dimensional (3D) structures of proteins in solution (Bax, 1989), similar to that provided by X-ray diffraction studies of crystalline solids (Billeter et al., 1992). However, the origins of the chemical-shift nonequivalencies observed in proteins due to folding --without which NMR structural studies would not be possible --have been poorly understood, especially for the heavier elements. We show in this communication that the full ~ 24 ppm range of 13Ca chemical shifts in glycine, alanine and valine residues in two proteins, Drosophila calmodulin and Staphylococcal nuclease, can now be reproduced by using quantum-chemical methods (de Dios et al., 1993). However, good agreement (r.m.s.d. ~ 1.4 ppm) between theory and experiment is achieved only when highly relaxed structures are used, due to the extreme sensitivity of 13C chemical shifts to bond-length errors. These findings should open up new avenues to structure refinement and determination, as well as providing a route for spectral assignment verification.
The C a sites of glycine, alanine and valine residues in proteins are known to display a very large chemical-shift range, with glycine being most shielded, followed by alanine, while valine is most deshielded (Wishart et al., 1991). Analysis of glycine, alanine and valine C a shieldings thus provides a stringent test of our ability to predict chemical shifts in proteins, because the shift range is so large --over four times the C a range previously investigated (de Dios et al., 1993). We *To whom correspondence should be addressed.