Abstract
In bacteriorhodopsin (bR) in purple membrane of Halobacterium halobium, there are two different forms: One is a light-adapted bacteriorhodopsin (hmax 570 nm) which has all-trans retinal as its chromophore (we call this trans-bR) and another is a dark-adapted bacteriorhodopsin (X,, 560 nm), the chromophore of which is a mixture of all-trans and 13cis retinals in an equal amount [l-3]. This paper deals with only trans-bR. On absorption of light, tmns-bR converts to several intermediates sequentially, e.g., batho-trans-bR, lumi-trans-bR and metatrans-bR, which finally revert to the original transbR. These intermediates have been identified by flash photolysis [4-61 and low temperature spectrophotometry [7,8]. They are quite similar in spectral properties to the intermediates appearing in the process of photo-bleaching of visual pigment. Cattle rhodopsin was irradiated [9,10] at liquid helium temperature (4-9 K) and hypsorhodopsin found. On warming, it converted to bathorhodopsin. It was confirmed [ 1 l] by a picosecond laser photolysis that squid hypsorhodopsin was an earlier intermediate than bathorhodopsin. Now a question arises whether or not there is an intermediate earlier than batho-trans-bR. In order to settle this question, we investigated the photoreaction of trans-bR at liquid helium temperature.
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