196 3 Lipids
Peroxy radicals with isolatedβ,γdouble bonds
are formed as intermediary products after aut-
oxidation and photooxidation (reaction with sin-
glet O 2 ) of unsaturated fatty acids having two or
more double bonds.
For this reason the 10- and 12-peroxy radi-
cals obtained from linoleic acid readily form
hydroperoxy-epidioxides. While such radicals
are only minor products in autoxidation, in pho-
tooxidation they are generated as intermediary
products in yields similar to the 9- and 13-peroxy
radicals, which do not cyclize. Ring formation by
10- and 12-peroxy radicals decreases formation
of the corresponding monohydroperoxides
(Table 3.28; reaction with^1 O 2 ).
Among the peroxy radicals of linolenic acid
which are formed by autoxidation, the isolated
β,γ double bond system exists only for the
12- and 13-isomers, and not for the 9- and
16-isomers. Also, the tendency of the 12- and
13-peroxy radicals of linolenic acid to form
hydroperoxy-epidioxides results in the formation
of less monohydroperoxide of the corresponding
isomers as opposed to the 9- and 16-isomers
(Table 3.28).
Peroxy radicals interact rapidly with antioxidants
which may be present to give monohydroperox-
ides (cf. 3.7.3.1). Thus, it is not only the chain re-
action which is inhibited by antioxidants, but also
β-fragmentation and peroxyradical cyclization.
Fragmentation occurs when a hydroperoxidee-
pidioxide is heated, resulting in formation of
aldehydes and aldehydic acids. For example,
(3.58)hydroperoxide-epidioxide fragments derived
from the 12-peroxy radical of linoleic acid are
formed as shown in Reaction 3.58.
Peroxy radicals formed from fatty acids with
three or more double bonds can form bicy-
cloendoperoxides with an epidioxide radical as
intermediate. This is illustrated in Reaction 3.74.3.7.2.1.4 Initiation of a Radical Chain Reaction
Since autoxidation of unsaturated acyl lipids fre-
quently results in deterioration of food quality,
an effort is made to at least decrease the rate
of this deterioration process. However, pertinent
measures are only possible when more knowl-
edge is acquired about the reactions involved dur-
ing the induction period of autoxidation and how
they trigger the start of autoxidation.
In recent decades model system studies have re-
vealed that two fundamentally different groups of
reactions are involved in initiating autoxidation.
The first group is confined to the initiating reac-
tions which overcome the energy barrier required
for the reaction of molecular oxygen with an un-
saturated fatty acid. The most important is photo-
sensitized oxidation (photooxidation) which pro-
vides the “first” hydroperoxides. These hydroper-
oxides are then converted further into radicals in
the second group of reactions. Heavy metal ions
and heme(in) proteins are involved in this sec-
ond reaction group. Some enzymes which gen-
erate the superoxide radical anion can be placed
in between these two delineated reaction groups
sinceatleastH 2 O 2 is necessary as reactant for
the formation of radicals.
The following topics will be discussed here:- Photooxidation
- Effect of heavy metal ions
- Heme(in) catalysis
- Activated oxygen from enzymatic reactions.
3.7.2.1.5 Photooxidation
In order to understand photooxidation and to dif-
ferentiate it from autoxidation, the electronic con-
figuration of the molecular orbital energy levels
for oxygen should be known. As presented in
Fig. 3.23, the allowed energy levels correspond
to^3 Σ−g,^1 Δgand^1 Σ+g.
The notation for the molecular orbital of O 2 is
(σ2s)^2 (σ∗2s)^2 (σ2p)^2 (π2p)^4 (π∗2p)^2.
In the ground state, oxygen is a triplet (^3 O 2 ). As
seen from the above notation, the term (π∗2p)^2