Chemical shifts and three-dimensional protein structures

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Abstract
During the past three years it has become possible to compute ab initio the 13C, 15N and 19F NMR chemical shifts of many sites in native proteins. Chemical shifts are beginning to become a useful supplement to more established methods of solution structure determination, and may find utility in solid-state analysis as well. From 13C NMR, information on ϕ, Ψ and χ torsions can be obtained, permitting both assignment verification, and structure refinement and prediction. For 15N, both torsional and hydrogen-bonding effects are important, while for 19F, chemical shifts are primarily indicators of the local charge field. Chemical shift calculations are still slow, but shielding hypersurfaces — the shift as a function of the dihedral angles that define the molecular conformation — are becoming accessible. Over the next few years, theoretical and computer hardware improvements will enable more routine use of chemical shifts in structural studies, including the study of metal-ligand interactions, the analysis of drug and substrate binding and catalysis, the study of folding/unfolding pathways, as well as the characterization of conformational substates. Rather than simply being a necessary prerequisite for multidimensional NMR, chemical shifts and chemical shift non-equivalence due to folding are now beginning to be useful for structural characterization.