Iodine-Catalyzed R/S Isomerization of Allenic Carotenoids
| Autor: | Synnφve Liaaen-Jensen‡, Guoqiang Gao, Zhangfei He, and Aase Strand, Lowell D. Kispert, Elli S. Hand |
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| Rok vydání: | 2002 |
| Předmět: | |
| Zdroj: | The Journal of Physical Chemistry A. 106:2520-2525 |
| ISSN: | 1520-5215 1089-5639 |
| DOI: | 10.1021/jp013006z |
| Popis: | 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. AM I 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. |
| Databáze: | OpenAIRE |
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