Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 25

salmon muscles, astaxanthin was thus found to be of equal importance as -
tocopherol as antioxidant in protecting the highly unsaturated lipids. Astaxanthin
is among the least reducing carotenoids and a very poor radical scavenger as
shown in experiments establishing the ordering of carotenoids in eq. 1.24.
Further theoretical calculations confirmed that astaxanthin is positioned low in
the antioxidant hierarchy of carotenoids (Galano, 2007). Still astaxanthin has
been found to be superior to the more reducing carotenoids as an antioxidant in
protecting liposomes as models for cell membranes (Naguib, 2000).
Carotenoids should apparently not only be considered as an electron donor:
Car O 2 ·ÿ ! Car · O 2 2ÿ 1.25

but also as an electron acceptor:
Car O 2 ·ÿ ! Car ·ÿ O 2

1.26

Quantum mechanical calculations have provided a two-dimensional ranking
of carotenoids and also of antioxidants like vitamin A, C and E according to
their antiradical capacity (MartÂnez et al., 2008). Based on a combination of
ionization energy and electron affinity, an electron acceptance index, R a , and an
electron donating index, R d , were defined relative to fluor and sodium,
respectively (see Table 1.5). An electron acceptor/donor classification is shown
in Table 1.6 with examples of compounds affecting oxidative processes in
biological system. Compounds which are both good electron donors (R d low)
and good electron acceptors (R a high) are the best antiradical compounds since
they easily donate and accept an electron (corresponding to reactions of eq. 1.25
and eq. 1.26). -carotene is such a good antiradical compound. Compounds with
the opposite properties, i.e. R d high and bad electron donors and R a low and bad
electron acceptors like ascorbic acid, are poor radical scavengers. The best
antioxidants are compounds with low R a as bad electron acceptors but which are
good electron donors corresponding to a low R d . -tocopherol is such a
compound. The last group is exemplified by astaxanthin: high R a as a good
electron acceptor and high R d as a poor electron donor, and such compounds are
now termed antireductants (MartÂnez et al., 2008). Notably, R a and R d are not
inversely proportional to each other and the novelty lies accordingly in the two-
dimensional classification.
Among the carotenoids, the highly red-coloured like astaxanthin are the best
antireductants, but the worst antioxidants. In contrast, the more colourless
antioxidants are the best electron donors and the best antioxidants like -
tocopherol. This two dimensional ordering of antioxidants seems to explain the
often unexpected positive effect of the highly coloured carotenoids. Even
astaxanthin does not scavenge phenoxyl radicals (Table 1.5), astaxanthin as an
antireductant may prevent the formation of the more reactive ROS from the less
reactive. A proper combination of antioxidants like -tocopherol and anti-
reductants like astaxanthin as seen for salmon, may explain the positive effects
seen for lipid stability (Jensen et al., 1998). Ascorbic acid has in radical
scavenging assays been found to be a rather poor antioxidant compared with