Proton NMR study of the influence on iron oxidation/ligation/spin state on the heme orientational preference in myoglobin

Proton nuclear magnetic resonance spectroscopy has been utilized to demonstrate that the degree of heme orientational disorder within a given myoglobin protein matrix can be a sensitive function of the oxidation/ligation/spin state of the heme iron. For sperm whale deuterohemin-reconstituted myoglobin, the equilibrium was found to strongly favor (5.7 to 7.8 kJ/mol) the X-ray characterized heme orientation in all six-coordinate states, but with a considerable reduction in preference (to 1.6 kJ/mol) in the five-coordinate deoxy state. In native yellow fin tuna myoglobin, changes in heme orientational preferences of ∼3 kJ/mol occur even between two six-coordinate ferric states differing solely in spin states.