Chemistry Reference
In-Depth Information
available for their growth and accordingly production of oxygen (Godfrey and
Falkowski, 2009). Oxygen is a potent oxidant and changed the conditions for life
dramatically initially in the oceans and in two spikes separated by a lower level
period also in the atmosphere. The evolution of oxygenic photosynthesis caused
the greatest selective pressure on primordial life (Benzie, 2003), since life
dependent on one-electron Fe(III)/Fe(II) cycling and oxygen invariably will
create reactive oxygen species (ROS), which need to be controlled by scaveng-
ing or through further reaction. During early evolution endogenous protection
systems had accordingly to be developed for protection against oxidative stress
together with development of chelators for tuning of Fe(III)/Fe(II) reduction
potentials in electron transport systems and for prevention of precipitation of
Fe(III) as hydroxide in the increasingly oxidizing atmosphere. It is the paradox
of aerobic life, that oxidative damage occurs to key metabolic sites, and this
continuing threat prompted the development of customized antioxidants
concomitant with the appearance of various aerobic life forms.
1.2 Reactive oxygen and nitrogen species
Due to spin restrictions, atmospheric oxygen is rather unreactive, since ground
state oxygen is a biradical and as such a triplet, whereas organic compounds are
singlets. However, in a series of one-electron reductions as part of respiration,
the reactive forms of oxygen such as superoxide, hydrogen peroxide, and the
hydroxyl radical, have transient appearance during formation of water:
e
ÿ
e
ÿ
e
ÿ
e
ÿ
O
2
ÿ
/
·
O
2
H ÿ!
·
H
2
O 1.1
Especially the hydroxyl radical,
·
OH, is highly reactive with rates of reaction
with most organic compounds like lipids approaching the diffusion limit. Such
high reactivity is evident, for example, for wine and other alcoholic beverages in
which any hydroxyl radicals formed during maturation or oxidative deterioration
are converted to the l-hydroxyethyl radical by reaction with ethanol prior to any
further reaction (Elias et al., 2009). The spin restriction for the initial electron
transfer to yield the superoxide radical as in the reaction sequence of eq. 1.1, is
revoked by reaction of oxygen with transition metal ions either in enzymes like
lipoxygenases (iron-based) or in simple hydrated ions under some conditions:
Fe
2
O
2
ÿ!
H
2
O
2
ÿ!
OH ÿ!
aq
O
2
! Fe
3
aq
O
2
ÿ
1.2
Likewise, the superoxide radical may leak from the mitochondria during
respiration. The spin restriction, i.e. singlet/triplet reactions are forbidden, is also
circumvented through reaction of organic material with singlet oxygen,
1
O
2
,
which is electronically excited oxygen with spin pairing or with the allotropic
form, ozone, O
3
. Singlet oxygen is formed in some highly exergonic reactions
but more importantm in photosensitized reactions like:
1
hm
1
Riboflavin ÿ!
Riboflavin*
1.3
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