NMR studies of hemoproteins. VI. Acid-base transitions of ferric myoglobin and its imidazole complex.

220 MHz proton NMR was applied to the acid-base transition of ferric myoglobin and its imidazole complex. In horse and sperm whale ferric myoglobins: (1) pH-dependent shift of heme-ring methyl signals above p2H 10 was analyzed on the basis of rapid exchange between alkaline and acidic forms by the use of pK value 9.1 of acid-base transition in 1H20 solution; (2) limiting shifts of three methyl signals were reasonably determined for purely alkaline form. For the imidazole complex: (3) a drastic high field shift of each signal was observed above p2H 9.0, whereas N0methyl imidazole complex did not exhibit such a shift, which suggests the 2H+ dissociation from liganded imidazole greater than N2H. It is concluded thns.