Rhodopsin, a red visual pigment of vertebrate rods, is composed of the yellow carotenoid derivative, retinene, joined to the colorless protein, opsin. It is synthesized by the combination of opsin with a specific hindered cis isomer of retinene, neo-b (11-cis) and bleaches in the light to a mixture of opsin and all-trans retinene (cf. Fig. 1). Another cis isomer of retinene-iso-a (9-cis)-combines with opsin to form a second photosensitive pigment, isorhodopsin. Isorhodopsin differs spectroscopically from rhodopsin but bleaches in the light to the identical mixture of alltrans retinene and opsin. The synthesis and bleaching of these pigments therefore involve cycles of stereoisomerization of retinene. ' This seems to be a general situation that involves all the known visual pigments-rhodopsin, iodopsin, porphyropsin, and cyanopsin-and the corresponding isopigments.2' 3 If rhodopsin or isorhodopsin is bleached with heat instead of light, the retinene is released in its original cis configuration.4 Stereoisomerization to the all-trans configuration is therefore associated not with all bleaching but specifically with bleaching by light., It has been known for some years that, in the light, rhodopsin bleaches in stages, over intermediates which are short-lived at room temperature, yet stable at low temperatures or in the absence of water.6'7 Light alone does not bleach (i.e., decolorize) rhodopsin but converts it to the red photoproduct, lumi-rhodopsin. Lumirhodopsin is stable below about-45°C. Above this temperature, in light or darkness, it goes over to a second orange-red compound, called meta-rhodopsin. In lumiand meta-rhodopsin the chromophore is still attached to opsin. The major color shift-i.e., bleaching-accompanies the hydrolysis of meta-rhodopsin to retinene and opsin. In light or darkness, vertebrate meta-rhodopsins hydrolyze above about-150 C.,7 squid and lobster meta-rhodopsins above about +20°C.8, 9 The slow course of this latter reaction strongly suggests that visual excitation precedes the release of retinene and depends upon the light reaction itself or, at most, on the conversion of lumi-rhodopsin to meta-rhodopsin. 10 On warming meta-rhodopsin in the dark, one obtains a mixture containing roughly one part of retinene and opsin and one part of rhodopsin and isorhodopsin.7' 11 It was assumed earlier that all these products were formed in the dark, i.e., that meta-rhodopsin in the dark was converted back to rhodopsin and isorhodopsin, in addition to being degraded to retinene and opsin.7' 1' Recent experiments, however, have clarified the mechanism of the light reaction and necessitate a reinterpretation of these observations. Squid rhodopsin.-Squid rhodopsin does not usually bleach in the light, owing to the relative stability of squid meta-rhodopsin (see above). This enables one to examine squid meta-rhodopsin under more favorable conditions than vertebrate meta-rhodopsins. Thus we have found that, whereas the chromophore of squid rhodopsin, like vertebrate rhodopsin, has the neo-b (1-cis) configura...
Abstract— –Quantum efficiencies have been measured for the photoisomerization of four stereoisomers of retinal (all‐trans, 13‐cis, 11 cis, and 9‐cis) in two solvents at different wavelengths of irradiation and at various temperatures. In heane at 25°C the quantum efficiencies for isomerization at 365 nm are: 9‐cis to trans, 0.5; 13‐cis to trans, 0.4; 11‐cis to trans, 0.2; all‐trans to monocis isomers, 0.2‐0.06, depending upon assumptions made regarding the stereo‐isomeric composition of the product. These values vary somewhat with the wavelength of the irradiating light. The quantum efficiency for the photoisomerization of all‐trans retinal in hexane decreases by a factor of 30 when the temperature is lowered from 25° to – 65°C; the activation energy for this photoisomerization is about 5 kcal/mole. The quantum efficiencies for the isomerization of the monocis isomers to all‐trans retinal in hexane are virtually independent of temperature. In ethanol the rates of photoisomerization from trans to cis or cis to trans depend only slightly on the temperature between 25° and – 65°C. The photosensitivities of the stereoisomers of retinal are of the same order of magnitude as those of the retinylidene chromophores of rhodopsin (11 ‐cis), metarhodopsin I (all‐trans), and isorhodopsin (9‐cis); but it is not yet possible to derive the photochemistry of rhodopsin uniquely and quantitatively from that of retinal.
Abstract. The preparation and properties of three geometric isomers of 13-desmethyl retinal (13-dmr) are described. They are analogous to the all-trans, 11-cis, and 9-cis isomers of retinal since two of the cis isomers combine with cattle opsin to form pigments which are spectrally indistinguishable from rhodopsin and isorhodopsin and the all-trans isomer is unreactive.The pigment which resembles rhodopsin, 13-desmethyl (13-dm) rhodopsin, is formed at about one ninth the rate at which 11-cis retinal reacts with opsin at 200C. The reaction with 13-dmr does not go to completion; and 0.05 M hydroxylamine, to which rhodopsin is stable, decomposes 13-dm rhodopsin. Irradiation of 13-dm rhodopsin results in a cis trans isomerization of the chromophore; but the photosensitivity of 13-dm rhodopsin is only 40 per cent that of rhodopsin.13-dm isorhodopsin, the 13-desmethyl analogue of isorhodopsin, is formed at approximately the same specific rate as 13-dm rhodopsin. The reaction goes to completion and the pigment is not decomposed by 0.03 M hydroxylamine. 13-dm isorhodopsin can also be photolyzed to the all-trans chromophore plus opsin.Introduction. The remarkable discrimination for geometrical shape shown by the visual protein, opsin, in its reaction with certain cis isomers of retinal is one of the more spectacular examples of the stereospecificity of proteins. Of five geometric isomers of retinal (vitamin A aldehyde) only the 11-cis and 9-cis compounds react with opsin to form photopigments. Three other isomers that have been studied (all-trans, 13-cis, and 9,13-dicis) are completely unreactive.1 In addition to the selectivity shown by opsin in the synthesis of visual pigments, there is also a stereospecificity shown during photolysis. When either rhodopsin (11-cis) or isorhodopsin (9-cis) is irradiated, the all-trans isomer of retinal is formed exclusively.'
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