Biophys. J.

The pressure dependence of the photocycle kinetics of bacteriorhodopsin from Halobacterium salinarium was investigated at pressures up to 4 kbar at 25°C and 40°C. The kinetics can be adequately modeled by nine apparent rate constants, which are assigned to irreversible transitions of a single relaxation chain of nine kinetically distinguishable states P1 to P9. All states except P1 and P9 consist of two or more spectral components. The kinetic states P2 to P6 comprise only the two fast equilibrating spectral states L and M. From the pressure dependence, the volume differences ΔV0LM between these two spectral states could be determined that range from ΔV0LM = -11.4 ± 0.7 ml/mol (P2) to ΔV0LM = 14.6 ± 2.8 ml/mol (P6). A model is developed that explains the dependence of ΔV0LM on the kinetic state by the electrostriction effect of charges, which are formed and neutralized during the L/M transition