Viscosity dependence of the kinetics of the diffusion-controlled reaction of carbon monoxide and myoglobin.

The kinetics of the reaction of CO with myoglobin have been studied by laser flash photolysis in glycerol-water as a function of solvent viscosity and temperature. At high viscosities and low temperatures the second-order rate constant is inversely proportional to the viscosity raised to approximately the 0.5 power. This result parallels the behavior of the oxygen diffusion coefficient in glycerol-water. It is concluded that the reaction kinetics in high viscosity glycerol-water are largely diffusion controlled. At higher temperatures, though, the effect of simultaneous chemical activation control of the reaction is observed. The diffusion-controlled rate constant is 1.4 X 10(-3) of that predicted from simple von Smoluchowski theory based on diffusion coefficients and molecular radii. Several models with steric requirements for diffusion-controlled reactions are examined.