Biophys. J.

The pressure dependence of the photocycle kinetics of bacteriorhodopsin from <i>Halobacterium salinarium</i> 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 <i>P</i>₁ to <i>P</i>₉. All states except <i>P</i>₁ and <i>P</i>₉ consist of two or more spectral components. The kinetic states <i>P</i>₂ to <i>P</i>₆ comprise only the two fast equilibrating spectral states L and M. From the pressure dependence, the volume differences ΔV⁰_LM between these two spectral states could be determined that range from ΔV⁰_LM = -11.4 ± 0.7 ml/mol (<i>P</i>₂) to ΔV⁰_LM = 14.6 ± 2.8 ml/mol (<i>P</i>₆). A model is developed that explains the dependence of ΔV⁰_LM on the kinetic state by the electrostriction effect of charges, which are formed and neutralized during the L/M transition