CO_H(N)CACB experiments for assigning backbone resonances in 13C/15N-labeled proteins.

A triple resonance NMR experiment, denoted CO_H(N)CACB, correlates 1HN and 13CO spins with the 13C alpha and 13C beta spins of adjacent amino acids. The pulse sequence in an 'out-and-back' design that starts with 1HN magnetization and transfers coherence via the 15N spin simultaneously to the 13CO and 13C alpha spins, followed by transfer to the 13C beta spin. Two versions of the sequence are presented: one in which the 13CO spins are frequency labeled during an incremented t1 evolution period prior to transfer of magnetization from the 13C alpha to the 13C beta resonances, and one in which the 13CO spins are frequency labeled in a constant-time manner during the coherence transfer to and from the 13C beta resonances. Because 13CO and 15N chemical shifts are largely uncorrelated, the technique will be especially useful when degeneracy in the 1Hn-15N chemical shifts hinders resonance assignment. The CO_H(N)CACB experiment is demonstrated using uniformly 13C/15N-labeled ubiquitin.