Low-temperature photoreactions of halorhodopsin. 2. Description of the photocycle and its intermediates.

Photostationary states of halorhodopsin (HR, a retinal protein in the halobacterial membrane) and their further thermal conversions were investigated at 140-230 K by absorption spectroscopy in the visible. The difference spectra confirm several steps of the all-trans-HR photocycle, in the presence of chloride, proposed earlier on the basis of room temperature flash spectroscopy. Thus, at 140 K, the spectra reveal the HR600----HR520 reaction, and at 170-230 K the HR640----HR578 and the HR520----HR578 reactions can be seen. No evidence for the expected HR520 in equilibrium HR640 process was found, however. From the difference spectra at various temperatures, exact absorption spectra of HR600 and HR520 were calculated, and an estimate of the HR640 spectrum in a mixture also containing HR520 was obtained. The low-temperature absorption maxima of HR578 and its photointermediates relate to the room temperature maxima differently from what is expected from the spectra of the corresponding intermediates in the bacteriorhodopsin photocycle.