NMR relaxometric studies of water accessibility to haem cavity in horse heart and sperm whale myoglobin

The measurement of longitudinal relaxation rates of water protons in solutions containing horse heart and sperm whale metmyoglobins at various pH values and temperatures shows that the two species differ essentially in the exchange lifetime, τ M , of the water molecule in the haem cavity. The 1/ T 1 nuclear magnetic relaxation dispersion dispersion profiles support this view whereas 17 O NMR spectroscopy shows that the exchange mechanism involves the whole water molecule. The lack of any 1 H/ 2 H isotope effect on the exchange rate suggests that the displacement of the coordinated water molecule from Fe(III) is not the rate‐determining step in the overall exchange process. In contrast to the parent species, no difference is detectable from the relaxometric investigations on the corresponding fluorometmyoglobin derivatives.