Halide ions as chemical probes for NMR studies of proteins

There has been considerable recent interest in the application of nuclear magnetic resonance techniques to the study of biological macromolecules.(1) NMR is in general not sufficiently sensitive for the direct study of biomolecules in solution at realistic concentrations. Some of the most promising developments have therefore involved the use of nuclear relaxation effects to probe gross molecular structural features.(2) These techniques frequently rely on the effects of small concentrations of paramagnetic atoms on the relaxation times of solvent nuclei,(3-5) or on the binding and exchange of small molecules with proteins.(6) Although paramagnetic relaxation effects have been widely exploited to yield structural information on relatively simple inorganic complexes in solution,(7-9) these techniques usually suffer from the inherent complexity of the relaxation process.(2) The observed effects on nuclear relaxation are usually the result of several competing mechanisms of comparable significance, which makes interpretation of line shapes in terms of molecular events hazardous