chi(1) angle information from a simple two-dimensional NMR experiment that identifies trans (3)J(NC)gamma couplings in isotopically enriched proteins

New quantitative J correlation experiments are used for measuring all two- and three-bond couplings between N-15 and aliphatic side-chain carbons in proteins uniformly enriched in C-13 and N-15. Results show that (3)J(NC beta) and 2J(NC beta) invariably are very small. Therefore, a simple and relatively sensitive two-dimensional spin-echo difference experiment can be used to identify residues with a (3)J(NC gamma) coupling substantially larger than 1 Hz, indicative of a trans arrangement between N and C-gamma. This measurement therefore provides chi(l) angle information for residues with an aliphatic C-gamma carbon, and thereby also aids in making stereospecific assignments of HB resonances. Experiments are demonstrated for ubiquitin and for a complex between calmodulin and a 26-residue peptide.