Sensitivity enhanced NMR spectroscopy by quenching scalar coupling mediated relaxation: application to the direct observation of hydrogen bonds in 13C/15N-labeled proteins.

In this paper, we demonstrate that the sensitivity of triple-resonance NMR experiments can be enhanced significantly through quenching scalar coupling mediated relaxation by using composite-pulse decoupling (CPD) or.an adiabatic decoupling sequence on aliphatic, in particular alpha-carbons in 13C/15N-labeled proteins. The CPD-HNCO experiment renders 50% sensitivity enhancement over the conventional CT-HNCO experiment performed on a 12 kDa FK506 binding protein, when a total of 266 ms of amide nitrogen-carbonyl carbon defocusing and refocusing periods is employed. This is a typical time period for the direct detection of hydrogen bonds in proteins via trans-hydrogen bond 3hJ(NC') couplings. The experimental data fit theoretical analysis well. The significant enhancement in sensitivity makes the experiment more applicable to larger-sized proteins without resorting to perdeuteration.