INFLUENCE OF MOLECULAR CORRELATION TIME ON THE HOMONUCLEAR OVERHAUSER EFFECT IN PARAMAGNETIC PROTEINS

The effect of solvent viscosity on the nuclear Overhauser effect, NOE, in paramagnetic myoglobin complexes has been investigated to determine the influence of the overall molecular correlation time on the magnitude of NOEs. We show that steady-state NOEs in these paramagnetic complexes are strongly decreased relative to isostructural diamagnetic systems but that this dramatic diminution characteristic of NOEs in paramagnetic complexes can be moderated by selectively increasing cross-relaxation, provided the electron spin-lattice relaxation time, which dominates intrinsic nuclear relaxation, is independent of the overall molecular motion of the complex. In such cases, the steady-state NOE will increase with the overall correlation time, τc, asymptotically approaching that NOE characteristic of an isostructural diamagnetic system. Steady-state NOE experiments on strongly paramagnetically relaxed and hyperfine-shifted resonances in high-spin ferric-aquo and low-spin ferric-cyano complexes of sperm whale myoglobin demonstrate that the NOE doubles upon doubling the viscosity by adding 30% ethylene glycol. The cross-relaxation rates determined via a truncated NOE experiment double, as expected with doubling the viscosity, and the increased steady-state NOE hence indicates that the iron T1e is independent of τc. Such an independence of T1e on τc in hemes in general is demonstrated for the model compound dicyano[tetrakis(4-sulfophenyl)porphine]iron(III) for which T1H (and hence Tle) is found unchanged when the solvent viscosity is varied 40-fold in mixtures of water and glycerol. The increased NOEs with τc provide a rationalization for earlier observations of vastly increased NOEs for model complexes in viscous solvents and improved NOE data in large hemoprotein systems and suggest that not only should NOEs be profitably pursued for a variety of paramagnetic proteins but such studies (in contrast to diamagnetic proteins in the slow-motion limit) may more likely be successful in larger rather than smaller proteins. © 1993, IEEE. All rights reserved. © 1990, American Chemical Society. All rights reserved.