POLARIZATION-OPERATOR FORMALISM DESCRIPTION OF THE OFF-RESONANCE ROESY EXPERIMENT

Polarization-operator formalism was used to describe the behavior of spin-1/2 homonuclear off-resonance spin-lock effects occurring in the ROESY experiment. The weak-collision case was assumed. Direct- and cross-relaxation rate constants representing T-1 rho(off)- and T-2 rho(off)-relaxation processes were evaluated. Off-resonance effects associated with the ROESY experiment were accommodated in the formalism by the inclusion of T-1 rho(off) relaxation. ROE contributions dominate the cross-relaxation rate when the tilt angle (beta) of the effective field (relative to the static B-0 field) in the rotating frame approaches 90 degrees, whereas when beta becomes small, NOE contributions predominate. Consequently, a continuous change in the tilt angle of the effective field from 90 degrees to 0 degrees results in a smooth spectral change characteristic of a ROESY to NOESY transition and may be implemented experimentally by the O-ROESY pulse sequence [K. Kuwata and T. Schleich, J. Magn. Reson. A 111, 43 (1994)]. Applications of the formalism include the analysis of off-resonance spin-lock effects arising in the O-ROESY experiment, namely, the dependence of cross-peak intensity on RF offset frequency, thereby enabling assessment of macromolecular motional parameters and internuclear separation distance from the cross-peak intensity dispersion behavior. Simplified expressions for the cross- and direct-relaxation rate constants in the rotating frame were derived from the exact formalism using the approximation omega(e) tau(c) much less than 1, which provided virtually identical numerical results to those of the exact formalism. For the case of interacting spin-1/2 nuclei with similar chemical-shift values, the formalism demonstrates, in accordance with other studies, that particular tilt angles of the effective field may be selected to enable suppression of cross relaxation (beta = 35.3 degrees) for slowly reorienting macromolecules, or the constraining of the direct- to cross-relaxation-rate-constant ratio to 0.5 at all values of omega(0) tau(c) (beta = 54.7 degrees). However, at relatively large values of the chemical-shift difference between the interacting spins, these rules do not rigorously apply. Analogous behavior was observed for simulations of the normalized cross-peak intensity vs off-resonance irradiation frequency. (C) 1995 Academic Press, Inc.