χ1 angle information from a simple two-dimensional NMR experiment that identifies trans 3JNCγ 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 3JNCβ and 2JNCβ invariably are very small. Therefore, a simple and relatively sensitive two-dimensional spin-echo difference experiment can be used to identify residues with a 3JNCγ coupling substantially larger than 1Hz, indicative of a trans arrangement between N and Cγ. This measurement therefore provides χ1 angle information for residues with an aliphatic Cγ carbon, and thereby also aids in making stereospecific assignments of Hβ resonances. Experiments are demonstrated for ubiquitin and for a complex between calmodulin and a 26-residue peptide