Selective transient heteronuclear cross relaxation in a selectively C-13(alpha)-labeled peptide

A new pulse sequence for the direct measurement of heteronuclear cross-relaxation rates is presented. The pulse sequence uses proton detection of transient carbon magnetization with sensitivity-enhanced transfer of magnetization and pulsed held gradients for coherence selection and water suppression. The heteronuclear cross-relaxation rate is measured using either nonselective or selective inversion of all or particular protons. A technique for the suppression of spin diffusion is also applied. The pulse sequence is tested on a peptide with a selectively C-13-labeled alpha-carbon. The directly measured rate is compared with and agrees well with the cross-relaxation rates as traditionally calculated from the steady-state NOE and carbon longitudinal-relaxation rate. (C) 1997 Academic Press.