Chemistry Reference
In-Depth Information
14
Properties of Carotenoid
Radicals and Excited States
and Their Potential Role
in Biological Systems
Ruth Edge and George Truscott
CONTENTS
14.1 Introduction .......................................................................................................................... 283
14.2 Reactions between Carotenoids and Singlet Oxygen ........................................................... 284
14.3 Interactions of Carotenoids with Free Radicals ................................................................... 291
14.3.1 Sulfur-Containing Radicals ...................................................................................... 291
14.3.2 NO
x
............................................................................................................................
292
14.3.3 Peroxyl Radicals........................................................................................................294
14.3.3.1 Arylperoxyl Radicals .................................................................................. 294
14.3.3.2 Chlorinated Peroxyl Radicals ..................................................................... 295
14.3.3.3 Acylperoxyl Radicals.................................................................................. 296
14.3.4 Reducing Radicals .................................................................................................... 296
14.4 Reactivity of Carotenoid Radicals ........................................................................................ 297
14.4.1 Interaction with Oxygen............................................................................................ 297
14.4.2 Interaction with Other Carotenoids........................................................................... 297
14.4.2.1 Radical Anions ........................................................................................... 297
14.4.2.2 Radical Cations........................................................................................... 299
14.4.3 Interaction with Biological Substrates ...................................................................... 301
14.4.3.1 Water-Soluble Antioxidants ........................................................................ 301
14.4.3.2 Amino Acids............................................................................................... 302
14.5 Biomedical Consequences .................................................................................................... 303
References ......................................................................................................................................304
14.1 INTRODUCTION
The C
40
carotenoids (CARs) and their oxygenated derivatives xanthophylls (XANs) are one of
nature's major antioxidant pigments and they efi ciently quench singlet oxygen [
1
O
2
] and interact
with damaging free radicals. Indeed, carotenoids protect bacterial and green plant photosynthetic
systems and the skin from
1
O
2
damage. XANs protect the macula of the eye and the interaction/
quenching of free radicals can be observed in photosynthetic systems and are also believed to be
linked to the protective role of CARs against the initiation of chronic disease.
The overall process of
1
O
2
quenching simply converts the excess energy of singlet oxygen to heat
via the carotenoid [CAR] lowest excited triplet state [
3
CAR].
283
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