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 pulse sequence. The scheme is demonstrated with a N-15,H-1-HMQC spectrum of uniformly N-15/H-2-labelled S. aureus DNA gyrase B with a molecular weight of 45 kDa for the unlabelled protein. The sensitivity of the experiment is improved compared to that of an FHSQC spectrum. It is further shown that the original FHSQC experiment can be shortened by the use of bipolar gradients. Relaxation times of different N-15 magnetizations and coherences were measured. The new FHMQC scheme is implemented in 3D NOESY-N-15-HMQC and 3D N-15-HMQC-NOESY-N-15-HMQC pulse sequences which are demonstrated with a 24 kDa fragment of uniformly N-15/C-13/H-2-labelled S. aureus DNA gyrase B.