Iodine-Catalyzed R/S Isomerization of Allenic Carotenoids

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2002-03, Vol.106 (11), p.2520-2525
• Main Authors: He, Zhangfei, Gao, Guoqiang, Hand, Elli S, Kispert, Lowell D, Strand, Aase, Liaaen-Jensen, Synnφve
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp013006z

A mechanism is proposed to explain the experimental observations in the iodine-catalyzed R/S isomerization of the allenic carotenoids peridinin, fucoxanthin, and neoxanthin. Attack of I . on C7‘, suggested to be the key step, breaks (or forms a partial bond with I in a transition state) the C7‘−C8‘ or C7‘−C6‘ π bond, which allows subsequent R/S isomerization. AM1 calculations show that the charge density at C7‘ of both the 6‘R-isomer and 6‘S-isomer is the greatest among the polyene and allenic carbons, and that the electron densities in the p x and/or p y orbitals of C7‘ are also higher than for most other polyene and allenic carbons. Thus, I . can preferentially attack C7‘, and significant R/S isomerization occurs. The calculation results can also explain the experimental observation that the ease of R to S isomerization of allenic carotenoids is peridinin > fucoxanthin > neoxanthin. Electrochemical experiments show that the radical cation and dication of fucoxanthin are unstable, which suggests that R/S isomerization of allenic carotenoids is not via their radical cations or dications. Furthermore, AM1 calculations show that R/S isomerization via the radical cations or dications is energetically unfavorable.