Energy storage in the primary step of the photocycle of bacteriorhodopsin.

A pulsed-dye laser low temperature photocalorimeter is used to study the enthalpy differences between light-adapted bacteriorhodopsin (bR568) and its primary photoproduct (K) at 77 K. A key feature of our experimental method is the use of the laser-induced photostationary state as an internal reference. Analyses of the forward (bR leads to K), reverse (K leads to bR), and mixed (bR in equilibrium K) photoreactions were carried out to measure delta H12 = EK - EbR. All three experiments yielded identical values of delta H12 within experimental error (delta Have12 = 15.8 +/- 2.5 kcal mol-1). Accordingly, the primary event in the photocycle of light-adapted bacteriorhodopsin stores approximately 30% of the absorbed photon energy at the 568-nm absorption maximum. We observe that the quantum yields phi f1(bR leads to K) and phi r2(K leads to bR) add up to unity within experimental error: phi f1 + phi r2 = 1.02 +/- 0.19 for phi f1 in the range 0.28-0.33. A theoretical analysis of energy storage in K suggests that at least one-half of the enthalpy difference between K and bR is associated with charge separation accompanying chromophore isomerization.