Some recent applications of the use of paramagnetic centres to probe biological systems using nuclear magnetic resonance

The explosion of the use of paramagnetic probes to study biological problems by nuclear magnetic resonance (n.m.r.) stems from the large and easily quantifiable perturbations they produce. The main reason for these perturbations is that the magnetic moments of unpaired electrons are about 103times greater than nuclear magnetic moments and consequently they generate much greater local fields.Such fields can give rise to large shifts in nuclear resonances; but if these fields fluctuate at the appropriate frequencies they will also cause very efficientrelaxationin the nuclear resonances. It is possible to select probes that result mainly in one of these parameters being perturbed so that the terms shift and relaxation probe are now well established in the literature.