Measurement of relaxation rates in crowded NMR spectra by selective coherence transfer

In crowded NMR spectra, it is shown how longitudinal relaxation rates (lab.-frame rates 1/T1) can be measured by selective inversion-recovery of a chosen site A followed by selective transfer of magnetization to another site X through a scalar coupling JAX, prior to observation of the signal. Relaxation rates in the presence of spin-locking (rotating frame rates 1/T1r) can be measured by first transferring transverse magnetization selectively from site X to A, followed by selective spin-locking of the magnetization of A, which can then be obsd. after partial decay. In both cases, the transfer can be achieved by a doubly-selective homonuclear Hartmann-Hahn method that uses simultaneous spin-locking of the transverse magnetization components of sites A and X by sidebands of an audio-modulated radio-frequency field. Provided the A-X cross-peak multiplet in a two-dimensional correlation ("COSY") spectrum does not suffer from overlap, there is no ambiguity in the one-dimensional spectra resulting from the novel methods. The techniques make it possible to measure accurate self-relaxation rates r or rt (diagonal elements of the Solomon matrixes), which are important for a quant. anal. of Overhauser effects in either lab. or rotating frames. The methods are applied to the protein bovine pancreatic trypsin inhibitor (BPTI) and to the cyclic undecapeptide cyclosporin A (CsA). [on SciFinder (R)]