STRATEGIES FOR ELIMINATING UNWANTED CROSS-RELAXATION AND COHERENCE-TRANSFER EFFECTS FROM 2-DIMENSIONAL CHEMICAL-EXCHANGE SPECTRA

A method for eliminating cross relaxation and for suppressing coherence-transfer effects from exchange spectra of macromolecules that permits a more rigorous study of chemical-exchange processes is demonstrated. Cross relaxation is suppressed when the effects of longitudinal and transverse relaxation are forced to compensate each other. For macromolecules undergoing slow rigid-body isotropic motion (ω0τc ≫ 1), the ratio of the longitudinal to transverse cross-relaxation rates is equal to - 1 2. If the mixing period of an exchange experiment is designed so that the magnetization components alternately cross relax along the longitudinal axis twice as long as they cross relax along the transverse axis, the direct cross-relaxation peaks cancel. Several mixing pulse sequences are proposed, based on average Hamiltonian theory taken to zeroth order, for the elimination of cross relaxation and suppression of TOCSY coherence transfer in exchange spectra. Both simulated and experimental results demonstrate the elimination of cross relaxation and suppression of TOCSY transfer by these mixing pulse sequences. Clean exchange spectra of turkey ovomucoid third domain (6015 Da) obtained by this method enabled the study of a ring flip exchange processes that is slow on the NMR time scale near 0°C. © 1991.