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Title: Cis-Trans Isomerizations of β-Carotene and Lycopene: A Theoretical Study
Authors: Guo, Wen-Hsin;Tu, Cheng-Yi;Hu, Ching-Han
Contributors: 化學系
Date: 2008-09
Issue Date: 2012-05-03T06:18:06Z
Publisher: American Chemical Society
Abstract: The all-trans to mono-cis isomerizations of polyenes and two C40H56 carotenes, β-carotene and lycopene, at the ground singlet (S0) and triplet (T1) states are studied by means of quantum chemistry computations. At the S0 state of polyenes containing n acetylene units (Pn), we find that the energy barrier of the central CC rotation decreases with n. In contrast, however, at the T1 state, the rotational barrier increases with n. For the C40H56 carotenes, the rotational barriers of lycopene are lower than those of their β-carotene counterparts. This difference renders the rotational rates of lycopene to be 1−2 orders of magnitude higher than those of β-carotene at room temperature. For both these carotenes, the barrier is lowest for the rotation toward the 13-cis isomer. The relative abundances are in the following order: all-trans > 9-cis > 13-cis > 15-cis. Although the 5-cis isomer of lycopene has the lowest energy among the cis isomers, its formation from the all-trans form is restricted, owing to a very large rotational barrier. The possible physiological implications of this study are discussed.
Relation: J. Phys. Chem. B, 112(38): 12158-12167
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