Sensitivity gain by simultaneous acquisition of two coherence pathways: The HNCA(+) experiment

In most multidimensional nuclear magnetic resonance experiments a single an d distinct coherence transfer pathway is selected by phase cycling or by pu lsed field gradients. It was shown that simultaneously exploiting more than one coherence transfer pathway could increase the overall sensitivity of N MR experiments, However, sensitivity enhancement schemes described to date introduce additional delays in the pulse schemes, resulting in considerable decrease of the expected sensitivity gain when applied to biomolecules due their fast transverse relaxation, A novel sensitivity enhancement principl e which increases sensitivity of an experiment by simultaneously exploiting two completely independent coherence pathways in a single NMR pulse scheme is presented in this paper. As an example an improved HNCA experiment, the HNCA(+), is presented, which combines the "out-and-back" coherence transfe r pathway used in HNCA with an "out-and-stay" experiment, analogous to HCAN H, without adding any time periods compared to the conventional HNCA pulse sequence. The applicability of the HNCA(+) was theoretically evaluated with regard to different sizes of peptides or proteins, which showed that the e xperimental time can be reduced twofold in ideal cases. The application of this novel experiment to a 7-kDa protein showed a 20% sensitivity gain of H NCA(+) when compared to conventional HNCA. (C) 2000 Academic Press.