Low‐temperature spectrophotometry of phoborhodopsin

Abstract
Photoreactions of the fourth rhodopsin‐like pigment, phoborhodopsin (pR480), ofHalobacterium halobiumwere studied by low‐temperature spectrophotometry. Upon irradiation of pR480 at ‐ 80 as well as ‐ 170°C with 436 nm light, a new intermediate having a difference absorption maximum at about 520 nm was produced. We designate this intermediate 'P520'. Prolonged irradiation caused the formation of a photosteady‐state mixture composed of P520 and pR480, indicating that P520 was photoreversible to pR480 at ‐ 80°C. P520 was thermally converted back to pR480 through P350 and P530, both of which were detected previously [(1986) Biochem. Biophys. Res. Commun. 139, 389–395]. Based on these results, a new photochemical reaction cycle of pR is presented.