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

Abstract

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. (1) pH-dependent shift of heme-ring methyl signals above p 2H 10 was analyzed on the basis of rapid exchange between alkaline and acidic forms by the use of p K value 9.1 of acid-base transition in 2H 2O solution; 2. (2) limiting shifts of three methyl signals were reasonably determined for purely alkaline form. For the imidazole complex: 3. (3) a drastic high field shift of each signal was observed above p 2H 9.0, whereas N-methyl imidazole complex did not exhibit such a shift, which suggests the 2H + dissociation from liganded imidazole > N 2 H. It is concluded that one 2H + dissociation and association is involved in the p 2H-dependent transitions.