Flash photolysis study of ligand binding by modified myoglobins at low temperatures

The CO-binding kinetics of myoglobin containing proto-, meso- and deutero-hemes were studied by flash photolysis over the temperature range 50-300 K. Results recorded over a large dynamic range of time (microseconds to many seconds) reveal processes that are non-exponential in time and multiphasic. The data are explained by a model in which the CO molecule must surmount four barriers in migrating from the solvent to the heme iron. At least two of these barriers have heights that vary from one molecule to another. Varying the nature of the heme group affects mainly the innermost of these barriers and, to a lesser degree, the second-outermost barrier.