Quantum efficiency of the photochemical cycle of bacteriorhodopsin

Values in the literature for the quantum efficiency of the photochemical cycle of bacteriorhodopsin (bR) range from 0.25 to 0.79 and the sum of the quantum yields of the forward and back photoreactions [Formula: see text] has been proposed to be 1. In the present work, low intensity laser flashes (532 nm) and kinetic spectroscopy were used to determine the quantum efficiency of bR photoconversion, [UNK]bR, by measuring transient bleaching of bR at 610 nm in the millisecond time scale. Bovine rhodopsin (R) in 2% ammonyx LO was used as a photon counter. We find that the ratio of the quantum yields of bacteriorhodopsin photoconversion and bleaching of rhodopsin, [UNK]bR/[UNK]R, is 0.96±0.04. Based on the quantum yield of the photobleaching of rhodopsin, 0.67, the quantum efficiency of bR photoconversion was determined to be 0.64±0.04. The quantum yield of M formation was found to be 0.65±0.06. From the transient bleaching of bR at 610 nm with a saturating laser flash (28 mJ/cm2) the maximum amount of bR cycling was estimated to be 47±3%. From this value and the spectrum of K published in the literature, the ratio of the efficiencies of the forward and back light reactions, [UNK]1/[UNK]2, was estimated to be 0.67±0.06 and so [UNK]2 ≈ 1 (0.94±0.06). The sum of [UNK]1 + [UNK]2 ≈ 1.6. It was found that repeated high-intensity laser flashes (>20 mJ/cm2) irreversibly transformed bR into two stable photoproducts. One has its absorption maximum at 605 nm and the other has a well-resolved vibronic spectrum with maxima at 342, 359 (main peak), and 379 nm. The quantum yield of the formation of the photoproducts is ≈ 10-4