Geometrical Isomers of Retinene by Ruth Hubbard,
نویسنده
چکیده
We have described experiments involving the use of five crystalline isomers of retinene: the ordinary crystalline substance known previously from the work of Ball, Goodwin, and Morton (1948); neoretinenes a and b, first iso. lated in our laboratory; and isoretinenes a and b, isolated in the Organic Re. search Laboratory of Distillation Products Industries (Hubbard and Wald 1952; 1952-53). By all indications these substances are cis-trans isomers of one another. Ordinary crystalline retinene apparently is the all-trans isomer; while neoretinenes a and b and isoretinene a appear to be mono-cis forms, and isoretinene b a di-cis isomer. This series of retinenes offers an unusual opportunity to examine interrelationships within a compact stereoisomeric "set." I t has, however, also a special interest. According to the present theory of cis-trans isomerism in such isoprenoid structures as the carotenoids, the cis configuration occurs readily only at the double bonds adjacent to methyl groups (Pauling, 1939; Zech. meister, 1944). Retinene has two such bonds, numbered 3 and 5 in Fig. 1. Consequently only four stereoisomers of retinene are expected: the all-trans, 3-cis, 5-cis, and 3,5-di-cis. We have, however, five isomers. Specifically, we have three apparently mono-cis isomers where only two were expected; but if all these are genuine, one must 10ok also for three di-cis forms and one tri-cis retinene--eight isomers in all where four were expected. These substances therefore pose a dilemma. Either they are not all geometrical isomers of one another; or the theory of cis-trans isomerization in this class of compounds must be expanded. In this paper we examine the properties and interconversions of the isomeric retinenes with an eye to this problem.
منابع مشابه
Geometrical Isomers of Retinene
Five crystalline retinenes have been isolated, which have every appearance of being cis-trans isomers of one another. They are all-trans retinene; three apparently mono-cis isomers: neoretinenes a and b and isoretinene a; and isoretinene b, an apparently di-cis isomer. The absorption spectra of these substances display the relations expected of cis-trans isomers. The main absorption band is dis...
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Rhodopsin is formed by the condensation of opsin with a cis isomer of retinene, called neo-b. The bleaching of rhodopsin releases all-trans retinene which must be isomerized back to neo-b in order for rhodopsin to regenerate. Both retinene isomers are in equilibrium with the corresponding isomers of vitamin A, through the alcohol dehydrogenase system. An enzyme is found in cattle retinas and fr...
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Vitamin A and retinene, the carotenoid precursors of rhodopsin, occur in a variety of molecular shapes, cis-trans isomers of one another. For the synthesis of rhodopsin a specific cis isomer of vitamin A is needed. Ordinary crystalline vitamin A, as also the commercial synthetic product, both primarily all-trans, are ineffective. The main site of isomer specificity is the coupling of retinene w...
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We have recently described the synthesis of rhodopsin in a solution containing four components: vitamin A, the precursor of the rhodopsin chromophore; opsin, the protein of rhodopsin; and liver alcohol dehydrogenase and cozymase, the enzyme and coenzyme which oxidize vitamin A to retinene (Hubbard and Wald, 1951). 1 This experiment was first performed with a fish liver 0il concentrate as the so...
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Among the geometrical isomers of vitamin A and retinene, the neo-b isomer occupies a special position. As the precursor of rhodopsin and iodopsin, it plays a central r81e in rod and cone vision (1, 2). It is stored almost exclusively in the eyes of certain Crustacea (3) and is equally distinctive chemically. The four unhindered geometrical isomers of vitamin A have been identified (4, 5); neo-b...
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تاریخ انتشار 2003