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Carotenoids
For further information,
see Carotenoids Advanced page
For help understanding chemical structures, see
Phenolics Intermediate and the
Glossary
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Basic
structure of the carotenoids:
lycopene The lycopene molecule represents the basic
structure of the carotenoids. It is composed of forty carbon atoms (and
their associated hydrogens, not depicted in this diagram) arranged in a long
structure with alternating single and double bonds:
A series of alternating single and double bonds are referred to as 'conjugated';
in carotenoids they make up the 'chromophore' (highlighted) which is the
part of the molecule that absorbs and gives off certain wavelengths of
light, generating the color we perceive. This chromophore not only gives
carotenoids their rich colors, but it also contributes to their antioxidant
properties. You'll notice that the lycopene
molecule is virtually symmetrical (with one half upside down relative to the
other); this is characteristic of the carotenoids
in general. Because of this symmetry, the carbons in the molecule are
numbered as follows:
Carbon 1 is on the far left and carbon 1' (pronounced 'one prime') is on the
far right. The left side of the molecule contains carbons 1 through 20 and
the right side, like an upside-down mirror image, contains carbons 1'
through 20'. There are forty carbons all together. This total of forty
carbons is why the carotenoids are also known as 'tetraterpenes': in terpene
nomenclature, one group of ten carbons = 'mono' as in 'monoterpene';
two groups of ten carbons is 'di' (diterpenes); three groups of ten
carbons is called 'tri' and four groups of ten carbons - as in
lycopene - is known as 'tetra'; hence, carotenoids are tetraterpenes.
Accumulating evidence indicates that lycopene, the red pigment in ripe
tomatoes, is a strong antioxidant and can help
prevent prostate cancer, lung cancer, stomach cancer, and possibly other
cancers including bladder, breast, cervix, pancreas, colon & rectum, and
oral cavity.1,2 |
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The
Carotenes: alpha-, beta-, delta-, and gamma- The ends of the
lycopene molecule can 'cyclize,' or curl up to form various ring structures. As a result, there are several carotenes which resemble
lycopene in the middle of the molecule, but have different ring structures on
the ends. The names given for these molecules are the 'common' or 'trivial'
names; see the advanced page for further details
of nomenclature.
beta-Carotene consists of the lycopene 'backbone'
with two rings on each end:
 The two
rings are structurally identical, but one of them is upside down and
backwards relative to the other. Beta-carotene is the most common of the
carotenes and is important as a precursor for vitamin A.
gamma-Carotene is
a precursor of beta-carotene. It has a ring like beta-carotene's rings on
one end, and no ring on the other:
 gamma-Carotene is
also a vitamin A precursor, but it has approximately 40 - 50 per cent of the
pro-A activity of beta-carotene.3
alpha-Carotene
has the same kind of ring on the left end of the molecule, and a
slightly different kind of ring on the right end of the molecule. It is an
example of a non-symmetrical carotene molecule:
This carotene also has pro-vitamin A activity, about 50 to 55 per cent of
beta-carotene's. It has been associated with reduced risk of lung cancer.4
delta-Carotene is
a precursor of alpha-carotene. It has the same type of ring as
alpha-carotene on the right end, but the left end is not cyclized:
In comparing these structures you can see that although the shapes at the
ends of the molecules change, in the middle they retain the same long chain
of alternating single and double bonds from the basic lycopene structure.
This middle structure is sometimes called the 'polyene chain,' poly- meaning
'many' and -ene referring to an organic molecule containing double bonds. As
mentioned earlier, this part of the molecule is also called the 'chromophore,'
or 'color-bearer.' |
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Oxygenated carotenoids: the xanthophylls
Carotenoids which contain oxygen as part of the ring
structures on the end(s) of the molecules are known as 'xanthophylls.'
Lutein, an antioxidant xanthophyll used to prevent macular degeneration,
cataracts, and colon cancer, has two -OH groups on its rings:
Zeaxanthin is a stereoisomer of
lutein. It differs only it the spatial orientation of the two highlighted
bonds on the right-hand ring:
 Zeaxanthin is
found along with lutein in the retina of the eye, where they are thought to
protect against oxidative damage from the shorter wavelengths of light.5,6 |
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Xanthophylls from red peppers (Capsicum spp) Capsanthin and
capsorubin are two distinctive xanthophylls from red peppers (cayenne, bell
peppers,
paprika). They contain a different kind of ring called a 'cyclopentane' ring (cyclo-
= ring; -pent- = five carbons; -ane = single bonds). These xanthophylls are
responsible for the brilliant red colors of the ripe peppers.
Capsanthin is the most abundant xanthophyll (~ 30 - 60% of total carotenoids), but it is accompanied in
ripe peppers
by some 30 - 40 other carotenoids including capsorubin. Notice that capsorubin contains two of
the oxygenated cyclopentane rings, one upside down and backwards in relation to
the other in typical carotenoid fashion.
 These two
xanthophylls, unique to ripe Capsicum fruits, have demonstrated significant
antioxidant activity; one recent study found that capsanthin's
radical-scavenging action was as strong as that of beta-carotene, lutein, and
zeaxanthin. It was also more resistant to decomposition, and retained its
antioxidant properties longer than the other carotenoids. About 70 - 80% of the
capsanthin was found to be 'esterified' (chemically bonded to) with various
fatty acids; these complex molecules were determined to be antioxidants as
well.7
A precursor of capsanthin is violaxanthin, a xanthophyll commonly found in
green leaves and many vegetables (including unripe - green - peppers). It
features a special oxygen
structure called an 'epoxide,' where the oxygen molecule is bonded to two
different carbons simultaneously. The special chemical properties of this
epoxide are what allows the six-membered rings at the ends of violaxanthin to
contract into the five-membered rings found in capsanthin and capsorubin.
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Astaxanthin
Another carotenoid that has recently been gaining attention as a powerful
antioxidant is astaxanthin, a xanthophyll found naturally in the feathers of
some birds and in marine creatures such as crustaceans, shellfish, and the
salmonids (trout and salmon). In these animals it provides a red (or
blue-purple when complexed with proteins) coloration. Synthetic astaxanthin
is added to commercial chicken feed to give the egg yolks a richer color,
and to the food of farm-raised fish to color their flesh.
For use in quality dietary supplements, astaxanthin is extracted from the
fresh water microalga Haematococcus pluvialis, which produces large
amounts of the pigment when under environmental stress. Astaxanthin contains
two 'conjugated keto groups' (highlighted), which are believed to enhance
its antioxidant power.8 Capsanthin and capsorubin, above, also
contain conjugated keto groups.
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A 3D representation of astaxanthin. Carbon atoms are grey,
hydrogens yellow, and oxygens red.
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References 1. Giovannucci et al., J Natl Cancer Inst,
87(23), 1767-76, 1995.
2. Giovannucci, E. J Natl Cancer Inst, 91(4), 317-31, 1999.
4. Michaud et al., Am J Clin Nutr, 72(4), 990-7, 2000.
3. Murray, M. Encyclopedia of Nutritional Supplements. Roseville, CA: Prima
Publishing, 1996.
5. Khachik F, Bernstein PS, Garland DL. Invest Ophthalmol Vis Sci, 38(9),
1802-11, 1997.
6. Krinsky, L. J Nutr, 132(3), 540S-542S, 2002.
7. Matsufuji et al., J Agric Food Chem, 46(9), 3468-3472, 1998.
8. Kobayashi et al., Appl Microbiol, Biotechnol 48(3), 351-6, 1997. For further information, see
Carotenoids
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