Formation of Carotenoid Oxygenated Cleavage Products - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 222

9 13 15

O

O

O

O

O

O

O

O

O

O

FIGURE 11.4 Hypothesis of the sequence of events when lycopene is oxidized by molecular oxygen in the
presence of ruthenium tetraphenylporphyrin.

system similar to the one we used (Groves and Quinn 1985). These results allowed us to hypothesize
a mechanism in which the Z -isomers would be the i rst products to appear, which then would be
transformed into “in-chain” epoxides, which, in turn, would undergo an oxidative cleavage to give
apolycopenals. Moreover, long apolycopenals could possibly be converted into shorter ones by a
similar sequence of events (Figure 11.4).
A similar catalytic system, but with a more hindered porphyrin (tetramesitylporphyrin =
tetraphenylporphyrin bearing three methyl substituents in ortho and para positions on each phenyl
group), was tested for -carotene oxidation by molecular oxygen (Caris-Veyrat et al. 2001). This

system was chosen to slow down the oxidation process and thus make it possible to identify pos-
sible intermediates by HPLC-DAD-MS analysis. After just 1 h of reaction, the i rst products of
the reaction could be seen, mainly Z -isomers. After 6 h, the chromatogram became more complex
(Figure 11.5), and we could tentatively identify three families of compounds: Z -isomers, epoxides,
and apocarotenals. After 24 h of reaction,

β

-carotene almost completely disappeared, but many
reaction products were still visible. A detailed analysis of the chromatograms revealed the presence
of a series of monooxygenated cleavage compounds, i.e., apocarotenals and also some epoxides of
these apocarotenals. Moreover, diapocarotendials were also detected and tentatively identii ed. It is
important to note that these last compounds were not detected in the similar model with lycopene.
The oxidation mechanism thus appears more complex in this setup. In Figure 11.6, we propose a
sequence of events that could occur in the reaction mixture. As we have observed with lycopene
(Caris-Veyrat, Schmid et al. 2003), we hypothesize that

β

-carotene may be i rst isomerized and
then oxidized and cleaved to form apocarotenals, which themselves may either undergo a second
cleavage to produce diapocarotendials or which may be oxidized into 5,6-epoxide. This latter prod-
uct could either isomerize to give an apocarotenal with a 5,8-furanoxide function, which could, in
turn, be cleaved into diapocarotendials, or it may be directly cleaved to produce a diapocarotendial.
Apocarotenals bearing epoxide or furanoxide functions may also be formed by the cleavage of the
corresponding epoxide/furanoxide

β

β -carotene.