Abstract
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). ΔF = -1100 to -1400 cal mole-1, ΔH = +150 cal mole-1, and ΔS = +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 (Ea) 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 Ea. In terms of the theory of absolute reaction rates, this means that chiefly the configurational parameter, ΔS‡, changes; ΔH‡ varies only slightly. The implications of these findings for the mechanism of isomerization are discussed.
Highlights
Table II, and the Arrhenius energies (E,) and other kinetic parameters shown in Fig. 4 and Table III
(11) of the resonance energies of conjugated systems of various lengths that the stereoisomerization of all-trans retinal to the 11-cis configuration would involve a loss in resonance energy of about 8 or 9 kcal mole-l, if the hindered cis linkage introduced a complete break in conjugation
The implications of these findings for the mechanism of isomerization are discussed
Summary
Retinol, and /3Xarotene-The preparation and crystallization of ll-cis- and all-lrans retinal have been described [4].11-cis-Retinol was prepared by reducing a sample of crystalline ll-cis-retinal in ethanol solution with powdered KBHl [4].Crystalline all-trans retinol and its acetate were obtained fromHoffmann-La Roche in Nutley, New Jersey. Retinol, and /3Xarotene-The preparation and crystallization of ll-cis- and all-lrans retinal have been described [4]. 11-cis-Retinol was prepared by reducing a sample of crystalline ll-cis-retinal in ethanol solution with powdered KBHl [4]. Crystalline all-trans retinol and its acetate were obtained from. The 11-cis isomer of the acetate and ll,ll’-dicis-P-carotene were gifts from Dr. Otto Isler of Hoffmann-La Roche in Base1 [7]. Reagent Grade I-propanol were used without further purification. Powdered crystalline digitonin obtained from Hoffman-La Roche was dissolved by heating in distilled water
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
