DETECTING STRESS-INDUCED REACTIVE OXYGEN SPECIES IN PLANTS UNDER UV STRESS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 143

polyunsaturated fatty acids to form carbon centred (lipid) radicals by hydrogen
abstraction. In the presence of oxygen these are converted into lipid peroxy radicals,
which are capable of hydrogen abstraction and thus of propagating oxidative stress in
the membrane. Besides the chemical nature of the molecule, the life-time of ROS
depends the environment where they are produced (e.g. lipid membrane or cytosol),
temperature and several other factors.
Fig. 1. Electron configuration on molecular orbits in ground state (triplet) and singlet oxygen and in the
superoxide anion radical. In 1 ' g the electron pair can be localised either on 2PS x * or on 2PS y * . The
unpaired electron of the superoxide radical can occupy either 2PS x * or 2PS y * .
As the example of carbon centred radicals illustrates, free radicals other than
oxygen centred ones are also important in oxidative damage. olyunsaturated lipids in
biological membranes may be converted into carbon-centred lipid radicals upon
hydrogen abstraction by x OH, hydroperoxide (HO 2 x- ) or other peroxide radicals.
Superoxide does not participate in this process directly, only through its protonated
form, HO 2 x- . The unpaired electron is usually formed on bis-allyl methyl groups. Two
carbon-centred radical groups may form cross links and stabilise as conjugated double
bonds. These lower membrane fluidity and alter hydrophobicity. Carbon-centred
radicals may also combine with oxygen, forming ROS capable of hydrogen abstraction
and thus propagating oxidative damage in the membrane (for reviews see Refs. 2,3).
Singlet oxygen may also yield hydroperoxides from lipids directly [4]. As lipids in
thylakoid membranes are ca. 90% polyunsaturated, photosynthetic organisms are prone
to oxidative damage [5-7].
ROS may also damage proteins yielding various carbon-centred (alkoxyl or
CO 2 x- anion) free radicals and bound amino acid hydroperoxides [8]. By reacting with
neighbouring amino acids, these latter may spread oxidative damage along the protein.
This promotes protein fragmentation or may also produce free radicals [9].