Quantum efficiencies of bacteriorhodopsin photochemical reactions.

Determination of quantum efficiencies of bacteriorhodopsin (bR) photoreactions is an essential step toward a full understanding of its light-driven proton-pumping mechanism. The bR molecules can be photoconverted into and from a K state, which is stable at 110 K. I measured the absorption spectra of pure bR, and the photoequilibrium states of bR and K generated with 420, 460, 500, 510, 520, 540, 560, 570, 580, 590, and 600 nm illumination at 110 K. The fraction of the K population in the photoequilibrium state, fk, is determined by AbR and AK the absorbances of the bR and K states at the excitation wavelengths, and also by phi 1 and phi 2, the quantum efficiencies for the bR to K and K to bR photoconversion: fK = phi 1 AbR/(phi 1AbR + phi 2Ak). By assuming that the ratio phi 1/phi 2 is the same at two different but close wavelengths, for example 570 and 580 nm, the value of phi 1/phi 2 at 570 and 580 nm was determined to be 0.55 +/- 0.02, and the spectrum of the K state was obtained with the peak absorbance at 607 nm. The values of phi 1/phi 2 at the other excitation wavelengths were then evaluated using the known K spectrum, and show almost no dependence on the excitation wavelength within the main band. The result phi 1/phi 2 = 0.55 +/- 0.02 disagrees with those of many other groups. The advantages of this method over others are its minimal assumptions and its straightforward procedure.