(H)N(COCA)NH and HN(COCA)NH experiments for 1H-15N backbone assignments in 13C/15N-labeled proteins.
Triple resonance HN(COCA)NH pulse sequences for correlating 1H(i), 15N(i), 1H(i-1), and 15N(i-1) spins that utilize overlapping coherence transfer periods provide increased sensitivity relative to pulse sequences that utilize sequential coherence transfer periods. Although the overlapping sequence elements reduce the overall duration of the pulse sequences, the principal benefit derives from a reduction in the number of 180 degrees pulses. Two versions of the technique are presented: a 3D (H)N(COCA)NH experiment that correlates 15N(i), 1H(i-1), and 15N(i-1) spins, and a 3D HN(COCA)NH experiment that correlates 1H(i), 15N(i), 1H(i-1), and 15N(i-1) spins by simultaneously encoding the 1H(i) and 15N(i) chemical shifts during the t1 evolution period. The methods are demonstrated on a 13C/15N-enriched sample of the protein ubiquitin and are easily adapted for application to 2H/13C/15N-enriched proteins.