NMR RELAXOMETRIC STUDIES OF WATER ACCESSIBILITY TO HEME CAVITY IN HORSE HEART AND SPERM WHALE MYOGLOBIN

The measurement of longitudinal relaxation rates of water protons in s olutions containing horse heart and sperm whale metmyoglobins at vario us pH values and temperatures shows that the two species differ essent ially in the exchange lifetime, T-M of the water molecule in the haem cavity. The 1/T-1 nuclear magnetic relaxation dispersion dispersion pr ofiles support this view whereas O-17 NMR spectroscopy shows that the exchange mechanism involves the whole water molecule. The lack of say H-1/H-2 isotope effect on the exchange rate suggests that the displace ment of the coordinated water molecule from Fe(III) is not the rate-de termining step in the overall exchange process. In contrast to the par ent species, no difference is detectable from the relaxometric investi gations on the corresponding fluorometmyoglohin derivatives.