HMQC AND HSQC EXPERIMENTS WITH WATER FLIP-BACK OPTIMIZED FOR LARGE PROTEINS

An HMQC experiment is proposed, dubbed FHMQC, where water flip-back is achieved by a single water-selective pulse preceding the basic HMQC p ulse sequence. The scheme is demonstrated with a N-15,H-1-HMQC spectru m of uniformly N-15/H-2-labelled S. aureus DNA gyrase B with a molecul ar weight of 45 kDa for the unlabelled protein. The sensitivity of the experiment is improved compared to that of an FHSQC spectrum. It is f urther shown that the original FHSQC experiment can be shortened by th e use of bipolar gradients. Relaxation times of different N-15 magneti zations and coherences were measured. The new FHMQC scheme is implemen ted in 3D NOESY-N-15-HMQC and 3D N-15-HMQC-NOESY-N-15-HMQC pulse seque nces which are demonstrated with a 24 kDa fragment of uniformly N-15/C -13/H-2-labelled S. aureus DNA gyrase B.