MEASUREMENT OF RELAXATION RATES IN CROWDED NMR-SPECTRA BY SELECTIVE COHERENCE TRANSFER

In crowded NMR spectra, it is shown how longitudinal relaxation rates (laboratory-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 J(AX), prior to observation of the signal. Relaxation rates in the presence of spin-locking (rotating frame rates 1/T1-rho) 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 observed 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-rho or rho(t) (diagonal elements of the Solomon matrices), which are important for a quantitative analysis of Overhauser effects in either laboratory or rotating frames. The methods are applied to the protein bovine pancreatic trypsin inhibitor (BPTI) and to the cyclic undecapeptide cyclosporin A (CsA).