χ1 angle information from a simple two-dimensional NMR experiment that identifies trans 3JNCγ couplings in isotopically enriched proteins
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New quantitative J correlation experiments are used for measuring all two- and three-bondcouplings between 15N and aliphatic side-chain carbons in proteins uniformly enriched in 13Cand 15N. Results show that 3JNCβ and 2JNCβ invariably are very small.Therefore, a simple and relatively sensitive two-dimensional spin-echo difference experimentcan be used to identify residues with a 3JNCγ coupling substantially larger than 1Hz, indicative of a trans arrangement between N and Cγ. This measurement thereforeprovides χ1 angle information for residues with an aliphatic Cγ carbon, andthereby also aids in making stereospecific assignments of Hβ resonances. Experimentsare demonstrated for ubiquitin and for a complex between calmodulin and a 26-residuepeptide.
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