Chi 1 angle information from a simple two-dimensional NMR experiment that identifies trans 3JNC gamma couplings in isotopically enriched proteins.

New quantitative J correlation experiments are used for measuring all two- and three-bond couplings between 15N and aliphatic side-chain carbons in proteins uniformly enriched in 13C and 15N. Results show that 3JNC beta and 2JNC 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 3JN gamma coupling substantially larger than 1 Hz, indicative of a trans arrangement between N and C gamma. This measurement therefore provides chi 1 angle information for residues with an aliphatic C gamma carbon, and thereby also aids in making stereospecific assignments of H beta resonances. Experiments are demonstrated for ubiquitin and for a complex between calmodulin and a 26-residue peptide.