VISCOSITY DEPENDENCE OF O-2 ESCAPE FROM RESPIRATORY PROTEINS AS A FUNCTION OF COSOLVENT MOLECULAR-WEIGHT

Laser photodissociation of respiratory proteins is followed by fast ge minate recombination competing with escape of the oxygen molecule into the solvent. The escape rate from myoglobin or hemerythrin has been s hown previously to exhibit a reciprocal power-law dependence on viscos ity. We have reinvestigated oxygen escape from hemerythrin using a num ber of viscous cosolvents of varying molecular weight, from glycerol t o dextrans up to 500 kDa. In isoviscous solutions, the strong viscosit y dependence observed with small cosolvents is progressively reduced u pon increasing the cosolvent's molecular weight and disappears at mole cular weights greater than about 100 kDa. Thus, viscosity is not a sui table independent parameter to describe the data. The power of the vis cosity dependence of the rate coefficient is shown here to be a functi on of the cosolvent's molecular weight, suggesting that local protein- solvent interactions rather than bulk viscosity are affecting protein dynamics.