Viscosity dependence of O2 escape from respiratory proteins as a function of cosolvent molecular weight

Laser photodissociation of respiratory proteins is followed by fast geminate recombination competing with escape of the oxygen molecule into the solvent. The escape rate from myoglobin or hemerythrin has been shown previously to exhibit a reciprocal power-law dependence on viscosity. We have reinvestigated oxygen escape from hemerythrin using a number of viscous cosolvents of varying molecular weight, from glycerol to dextrans up to 500 kDa. In isoviscous solutions, the strong viscosity dependence observed with small cosolvents is progressively reduced upon increasing the cosolvent's molecular weight and disappears at molecular weights greater than about 100 kDa. Thus, viscosity is not a suitable independent parameter to describe the data. The power of the viscosity dependence of the rate coefficient is shown here to be a function of the cosolvent's molecular weight, suggesting that local protein-solvent interactions rather than bulky viscosity are affecting protein dynamics.