Carotenoids - Natural Products Chemistry: Sources, Separations, and Structures - page 122

H 3 C OH

CH 3 CH 3

H 3 C CH 3

H 3 C CH 3

CH 3 CH 3

HO CH 3

(3R,3 'R)-zeaxanthin

OH

HO

(3R,3 'S-meso)-zeaxanthin

FIGURE 11.8 Chemical structure of zeaxanthin.

11.5 ZEAXANTHIN
The Latin name of zeaxanthin is zea mays , “yellow maize corn.” It is isomeric with
lutein, and thus is classified as a xanthophyll. It is found in green plant leaves and
eggs. Zeaxanthin is also a nonphotochemical quenching agent, accumulating in
human eyes at the macula lutea, protecting the eyes from radical damages in a manner
similar to lutein. The stereoisomers of zeaxanthin in nature are shown in Figure 11.8.

11.6 ISOLATION OF ZEAXANTHIN
Supercritical CO 2 is used in the isolation of zeaxanthin from red paprika together
with other minor carotenoids. The separation is controlled by maintaining the tem-
perature at 60°C. First, the paprika sample is packed into an extraction column,
flushed with CO 2 gas to remove O 2 . Then, the liquid CO 2 (60°C) is streamed in, and
a cosolvent such as ethanol or ethyl ether is added to enhance the extraction. The CO 2
is evaporated off, and “mists” of extracted compounds are deposited in the collector,
which can be further separated via centrifugation and deposited on the glass walls.
Analysis of purity can be achieved by HPLC coupled to a UV detector. If further
saponification is needed, the material is dissolved in a mixture of methanol and
NaOH to generate free zeaxanthin chemical constituents.

11.7 LYCOPENE
The Latin name of lycopene is lycopersicum : tomato species. It is a bright, red-colored
carotene compound, which widely exists in red fruits and vegetables (e.g., tomatoes,
red carrots, red bell peppers, and watermelons).
The chemical structure of lycopene is similar to β-carotene. Both compounds are
formed entirely from hydrogen and 40 carbon atoms possess conjugated carbon-
carbon double bonds. However, lycopene is a linear (acyclic) carotene and there are
two additional carbon-carbon double bonds at both ends of the molecule (Figure 11.9).
There are several common geometric isomers of lycopene (Figure 11.10); in total,
there are 72 possible isomers.