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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′Risomer 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.

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