(H)N(COCA)NH AND (HN)UNDER-BAR(COCA)NH EXPERIMENTS FOR H-1-N-15 BACKBONE ASSIGNMENTS IN C-13 N-15-LABELED PROTEINS/

Triple resonance HN(COCA)NH pulse sequences for correlating H-1((i)), H-15((i)), H-1((i-1)), and N-15((i-1)) spins that utilize overlapping coherence transfer periods provide increased sensitivity relative to p ulse sequences that utilize sequential coherence transfer periods. Alt hough the overlapping sequence elements reduce the overall duration of the pulse sequences, the principal benefit derives from a reduction i n the number of 180 degrees pulses. Two versions of the technique are presented: a 3D (H)N(COCA)NH experiment that correlates N-15((i)), H-1 ((i-1)), and N-15((i-1)) spins, and a 3D <(HN)under bar (COCA)NH> expe riment that correlates H-1((i)), N-15((i)), H-1((i-1)), and N-15((i-1) ) spins by simultaneously encoding the H-1((i)) and N-15((i)) chemical shifts during the t(1) evolution period. The methods are demonstrated on a C-13/N-15-enriched sample of the protein ubiquitin and are easil y adapted for application to H-2/C-13/N-15-enriched proteins.