The molecular mechanism of temperature enhancement of proton magnetic relaxation rates in methaemoprotein solutions. III. The effect of sixth ligand in high-spin derivatives.

The distinctions of the solvent-proton longitudinal magnetic relaxation (PMR) mechanisms between high-spin ferric aquo and fluoro complexes of some haemoproteins are discussed here. It becomes apparent that the "transition" from the exchange-limited PMR to the fast-exchange PMR mechanism upon addition of fluoride to some of the aquocomplexes is due to a more intense solvent-dynamics in the vicinity of the paramagnetic haem in the fluoromet derivatives. The can be rationalized by a conformational change induced by the fluoride ion, an effect not observed by X-ray analysis thus far. A possible mechanism of this change is indicated here.