FROM METMYOGLOBIN TO DEOXYMYOGLOBIN - RELAXATIONS OF AN INTERMEDIATE STATE

Metmyoglobin has been reduced at low temperature (below 100 K) using x -rays or by excitation of tris(2,2',bipyridine)ruthenium(II) chloride with visible light. Upon reduction, an intermediate state is formed wh ere the structure of the protein is very similar to that of metmyoglob in with the water molecule still bound to the heme iron, but the iron is II low spin. The nature of the intermediate state has been investig ated with optical spectroscopy. The Q(o) and Q(v) bands of the interme diate state are split, suggesting that the protoporphyrin is distorted . The intermediate state undergoes a relaxation observed by a shifting of the Sorer band at temperatures above 80 K. Above 140 K, the protei n begins to relax to the deoxy conformation. The relaxation kinetics o f the protein have been monitored optically as a function of time and temperature from minutes to several hours and from 150 K to 190 K. By measuring the entire visible spectrum, we are able to distinguish betw een electron transfer processes and the protein relaxation from the in termediate state to deoxy myoglobin. The relaxation has been measured in both horse myoglobin and sperm whale myoglobin with the relaxation occurring on faster time scales in horse myoglobin. Both the reduction kinetics and the relaxation show nonexponential behavior. The reducti on kinetics can be fit well to a stretched exponential. The structural relaxation from the intermediate state to the deoxy conformation show s a more complex, dynamical behavior and the reaction is most likely a ffected by the relaxation of the protein within the intermediate state .