An excellent introduction to carotenoids is provided in a review article by George Britton, FASEB J. 9, 1551-1558 (1995).
Carotenoids are not just “another group” of natural pigments” but substances with very special and remarkable properties that no other groups of substances possess. These properties form the basis of their many, varied functions and actions in all kinds of living organisms. Often traditionally thought of as plant pigments, the carotenoids have a much wider distribution and occur extensively in animals and microorganisms. The striking natural colours that are due to carotenoids – e.g., in yellow, orange and red flowers and fruit – are familiar to everyone, but other, less obvious roles make carotenoids essential components in oxygenic photosynthetic organisms. They perform important functions in nature, including light-harvesting, photoprotection, protective and sex-related coloration patterns in many animal species and as precursors of vitamin A in vertebrates. They may serve protective roles as well against age-related diseases in humans, being implicated in the prevention or protection against serious human health disorders such as cancer and heart disease.
Essentially all carotenoids possess certain common chemical features: a polyisoprenoid structure, a long conjugated chain of double bonds in the central portion of the molecule and near symmetry around the central double bond. The basic structure is modified in a variety of ways, most prominently by cyclization of the end groups and by the introduction of oxygen functions, to yield a family of more than 600 compounds.
The most striking and characteristic feature of the
carotenoid structure is the long system of alternating
double and single bonds that forms the central part of the
molecule. This constitutes a conjugated system in which
pi-electrons are effectively delocalized over the entire
length of the polyene chain. The conjugated double bond
system determines the photochemical properties and chemical
reactivity that are the basis of carotenoid biological
functions.
The highly reactive, electron rich polyene chain is
responsible for the instability of carotenoids toward
oxidation and is the important feature of the molecule in
relation to free-radical chemistry. Pure carotenoids, even
in the crystalline state, are susceptible after isolation
to oxidation and may break down rapidly if samples are
stored in the presence of oxygen. In vivo, the carotenoids
are usually stabilized to a considerable degree by proteins
and other molecules in their immediate vicinity. Even in
vivo, however, carotenoids are still susceptible to
oxidizing species or free radicals.
The Significance of Carotenoid Oxidation
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