A simple and sensitive experiment for measurement of JCC couplings between backbone carbonyl and methyl carbons in isotopically enriched proteins

Abstract
A simple 2D difference experiment is described that allows quantitative measurement of 13C−13C J couplings between backbone carbonyl and side-chain carbons. Precise 3JCC values were measured from data recorded in just 2 h for a 1-mM solution of the 20-kD complex between the protein calmodulin and a 26-residue synthetic peptide. The J couplings aid in determining the χ1 angles of valine, isoleucine and threonine residues, and in making stereospecific assignments of the Val Cγ methyl groups. Error analysis indicates that the uncertainty in the derived J couplings is generally less than ca. 0.3 Hz.