HOMONUCLEAR PROTON CROSS RELAXATION IN THE PRESENCE OF PARAMAGNETIC METAL-IONS

Homonuclear cross relaxation for a proton pair with external relaxation is evaluated theoretically. Results for nuclear Overhauser effect experiments and for two-dimensional NOESY spectra are given. The effects of internal motion are treated. The calculations give insight into experimental results obtained from cross-relaxation studies of proteins containing paramagnetic metals. When external relaxation is present, a much longer correlation time is required for the spins to be in the pure spin-diffusion limit. The value of the NOE enhancement becomes dependent on the interproton distance when external relaxation is significant. The initial slope of cross-peak evolution in two-dimensional spectra is not significantly changed by external relaxation. The slope is, however, very sensitive to the rotational correlation time. The implications of the theoretical analysis for comparing interproton distances for pairs of protons near and distant from the paramagnetic metal are summarized. © 1990.