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Biophysical Journal 58: 597-608 (1990)
© 1990 the Biophysical Society

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Quantum efficiency of the photochemical cycle of bacteriorhodopsin

R. Govindjee, S. P. Balashov and T. G. Ebrey

Department of Physiology and Biophysics, University of Illinois, Urbana, Illinois 61801 USA

ABSTRACT

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 {approx} 1 (0.94 ± 0.06). The sum of [UNK]1 + [UNK]2 {approx} 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 {approx} 10-4.




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T. Fischer and N. A. Hampp
Two-Photon Absorption of Bacteriorhodopsin: Formation of a Red-Shifted Thermally Stable Photoproduct F620
Biophys. J., August 1, 2005; 89(2): 1175 - 1182.
[Abstract] [Full Text] [PDF]

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J. Wang and M. A. El-Sayed
Time-Resolved Long-Lived Infrared Emission from Bacteriorhodopsin during its Photocycle
Biophys. J., September 1, 2002; 83(3): 1589 - 1594.
[Abstract] [Full Text] [PDF]


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