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The Stereoisomerization of 11-cis-Retinal

From the ¹ From the Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138

The thermodynamic parameters of the stereoisomerization of 11-cis-retinal to the all-trans configuration have been measured in n-heptane between 20 and 80° (C). F = -1100 to -1400 cal mole^-1, H = +150 cal mole^-1, and S = +4.4 e.u. The kinetic parameters have been measured in several solvents, which differ in their polarity, and in the presence of I₂ as catalyst. For the uncatalyzed isomerizations, the Arrhenius energy (E_a) varies between 22,400 and 26,200 cal mole^-1, and the Arrhenius or frequency factor (A) between about 10⁸ and 10¹¹ sec^-1. In the presence of I₂, A increases with relatively little change in E_a. In terms of the theory of absolute reaction rates, this means that chiefly the configurational parameter, S, changes; H varies only slightly. The implications of these findings for the mechanism of isomerization are discussed.

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