PHOTOCHEMICAL REACTION OF 13‐<i>CIS</i>‐BACTERIORHODOPSIN STUDIED BY LOW TEMPERATURE SPECTROPHOTOMETRY

Abstract

Abstract— The photoreaction cycle of 13‐ cis ‐bacteriorhodopsin (13‐ cis ‐bR) was investigated by low temperature spectrophotometry using two different preparations; 13‐ cis ‐bR constituted from bacterioopsin and 13‐ cis ‐retinal, and dark‐adapted bacteriorhodopsin (bR D ), which is an equi‐molar mixture of 13‐ cis ‐bR and trans ‐bR. By irradiation with 500 nm light at — 190°C, 13‐ cis ‐bR was converted to its batho‐product, batho‐13‐ cis ‐bR (batho‐bR 13 ), which is different from batho‐product from trans ‐bR, batho‐bR t . On warming batho‐bR 13 to ‐5°C in the dark, it completely changed to trans ‐bR. We estimated the composition of 13‐ cis ‐bR and trans ‐bR in the warmed sample spectrophotometrically and then the absorption spectrum of batho‐bR 13 was calculated. The absorption maximum lies at 608 nm, 1250 cm −1 longer than that of 13‐ cis ‐bR; the molar extinction coefficient (ε) is about 74000 M −1 cm −1 , larger than that of 13‐ cis ‐bR (52000 M −1 cm −1 ). On the warming the sample containing batho‐bR 13 formed by irradiating 13‐ cis ‐bR or bR D at — 190°C, we could not detect other intermediates such as the lumi‐ or meta‐intermediates seen in trans‐bR system.