THE EFFECT OF VISCOSITY ON THE PHOTOCYCLE OF BACTERIORHODOPSIN

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<jats:p><jats:bold>Abstract—</jats:bold> We study the effect of solvent viscosity on the kinetics of the photocycle of bacteriorhodopsin (bR) from <jats:italic>Halobacterium halobium.</jats:italic> Solvent viscosity is altered by changing the glycerol concentration from 20 to 80% glycerol by volume. The kinetics of the photocycle are observed after flash photolysis at four wavelengths at several temperatures between 240 and 315 K. Assuming a sequential model, bR → K ‐→ L → M → O → bR, Arrhenius plots of the rate coefficients determine the activation enthalpies and frequency factors for each step. Kinetic data from all solvents are considered together and studied as a function of temperature for fixed solvent viscosities. The early steps of the cycle are insensitive to solvent viscosity, →; the later steps are retarded with increasing viscosity. Activation enthalpies are independent of viscosity; the frequency factors are proportional to η<jats:sup>−K</jats:sup>, where the exponent <jats:italic>k</jats:italic> 0.25 for the transition K → L, 0.0 for L → M, 0.8 for M → O and 0.5 for O → bR.</jats:p>

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