Viscosity dependence of the kinetics of the diffusion-controlled reaction of carbon monoxide with the separated alpha and beta chains of hemoglobin.

The kinetics of the recombination reaction of carbon monoxide with the isolated alpha and beta chains of human hemoglobin have been examined by laser flash photolysis in glycerol-water as a function of temperature and solvent viscosity. The second-order recombination rate constant is inversely proportional to viscosity raised to the 0.5 power--paralleling that predicted for the CO diffusion coefficient. This viscosity exponent is independent of the protein. These results are consistent with the reaction kinetics being essentially diffusion controlled in the high viscosity glycerol-water. For the alpha and beta chains, respectively, the diffusion-controlled rate constant is 0.003 and 0.002 of that predicted from the simple von Smoluchowski model based on the diffusion coefficients and molecular sizes of uniformly reactive spherical molecules. Several models incorporating steric requirements are used to rationalize the results. These models indicate that steric requirements for reaction in the diffusion-controlled limits are not greatly different in the alpha and beta chains and are only slightly less stringent than for myoglobin.