76
N. R. Jena et al.
PCM. The SET mechanism being extremely endothermic has been ruled out. It
was found that scavenging of OH
•
was possible by the HAT and RAF mechanisms.
Hyrogen abstraction reactions from the O9, O10, O11 and C4 sites (Fig. 3.7h) were
found to be barrierless. Thus vitamin C is shown to be an excellent scavenger of
OH
•
. Its most preferred site for OH
•
addition was shown to be C2 (Fig. 3.7h) for
which the Gibbs barrier energy at the B3LYP/6-311++G(3df,2pd) level was found
to be ~ 1.25 kcal/mol in solution.
A theoretical study of the OH
•
scavenging ability of vitamin E (α-Tocopherol)
(Fig. 3.7i) has revealed the following information [167]. The reaction was found to
proceed through the HAT and RAF mechanisms. Hydrogen abstraction reactions
from the phenolic OH group and methyl groups and OH
•
addition reactions at several positions of the aromatic ring were studied at the BHandHLYP/6-311 + + G(2d,2p)
level of theory. The most probable site for hydrogen abstraction with a low barrier
energy (0.8 kcal/mol) was found to be C2 while OH
•
addition was found to take
place at the C6 site in a barrierless manner. Reaction rate constants for the HAT and
RAF mechanisms were found to be 2.2 × 10
8
M
−1
s
−1
and 5.6 × 10
7
M
−1
s
−1
respectively [167]. Thus vitamin E is also shown to be an effective OH
•
scavenger like
vitamin C.
3.4.3 OOH Radical Scavengers
Garlic is a well known for its medicinal properties. It has been shown to be an excellent anti-oxidant. It effectively scavenges free radicals due to its active ingredients
allicin (2-propenyl 2-propenethiosulfinate) and 2-propenesulfenic acid (Fig. 3.7k, l)
[168–170]. Both of these molecules can effectively scavenge OOH
•
. In addition, carotenoids are also well known OOH
•
scavengers. In recent theoretical studies [171,
172], reactions of OOH
•
with several carotenoids have been studied where these
molecules have shown to be potent scavengers of OOH
•
. The OOH
•
scavenging
ability of allicin has been found to be 1000 times less than that of 2-propenesulfenic
acid [170]. A theoretical study of both the ingredients of garlic was carried out at the
BHandHLYP/6-311++G(d, p) level of theory followed by CBS-QB3 calculations to
overcome limitations arising due to the truncation of basis set and spin contamination. Solvent effect in water was treated by the IEF formalism of the PCM [141].
All the three reaction mechanisms mentioned earlier were taken into account. It was
found that the SET mechanims would not contribute significantly to the reactions of
OOH
•
with allicin and 2-propenesulfenic acid. Therefore, the HAT and RAF mechanisms are important in these reactions. Hydrogen abstraction was considered from
the α1 and α2 sites of allicin and α1 and δ sites of 2-propenesulfenic acid (Fig. 3.7k,
l). It was found that 2-propenesulfenic acid is much more reactive towards OOH
•
by
the HAT mechanism than allicin. However, allicin was found to be somewhat more
reactive towards OOH
•
by the RAF mechanism than 2-propenesulfenic acid. On
the whole, the 2-propenesulfenic acid ingredient of garlic was shown to be a better
scavenger of OOH
•
than the other ingredient allicin.
N. R. Jena et al.
PCM. The SET mechanism being extremely endothermic has been ruled out. It
was found that scavenging of OH
•
was possible by the HAT and RAF mechanisms.
Hyrogen abstraction reactions from the O9, O10, O11 and C4 sites (Fig. 3.7h) were
found to be barrierless. Thus vitamin C is shown to be an excellent scavenger of
OH
•
. Its most preferred site for OH
•
addition was shown to be C2 (Fig. 3.7h) for
which the Gibbs barrier energy at the B3LYP/6-311++G(3df,2pd) level was found
to be ~ 1.25 kcal/mol in solution.
A theoretical study of the OH
•
scavenging ability of vitamin E (α-Tocopherol)
(Fig. 3.7i) has revealed the following information [167]. The reaction was found to
proceed through the HAT and RAF mechanisms. Hydrogen abstraction reactions
from the phenolic OH group and methyl groups and OH
•
addition reactions at several positions of the aromatic ring were studied at the BHandHLYP/6-311 + + G(2d,2p)
level of theory. The most probable site for hydrogen abstraction with a low barrier
energy (0.8 kcal/mol) was found to be C2 while OH
•
addition was found to take
place at the C6 site in a barrierless manner. Reaction rate constants for the HAT and
RAF mechanisms were found to be 2.2 × 10
8
M
−1
s
−1
and 5.6 × 10
7
M
−1
s
−1
respectively [167]. Thus vitamin E is also shown to be an effective OH
•
scavenger like
vitamin C.
3.4.3 OOH Radical Scavengers
Garlic is a well known for its medicinal properties. It has been shown to be an excellent anti-oxidant. It effectively scavenges free radicals due to its active ingredients
allicin (2-propenyl 2-propenethiosulfinate) and 2-propenesulfenic acid (Fig. 3.7k, l)
[168–170]. Both of these molecules can effectively scavenge OOH
•
. In addition, carotenoids are also well known OOH
•
scavengers. In recent theoretical studies [171,
172], reactions of OOH
•
with several carotenoids have been studied where these
molecules have shown to be potent scavengers of OOH
•
. The OOH
•
scavenging
ability of allicin has been found to be 1000 times less than that of 2-propenesulfenic
acid [170]. A theoretical study of both the ingredients of garlic was carried out at the
BHandHLYP/6-311++G(d, p) level of theory followed by CBS-QB3 calculations to
overcome limitations arising due to the truncation of basis set and spin contamination. Solvent effect in water was treated by the IEF formalism of the PCM [141].
All the three reaction mechanisms mentioned earlier were taken into account. It was
found that the SET mechanims would not contribute significantly to the reactions of
OOH
•
with allicin and 2-propenesulfenic acid. Therefore, the HAT and RAF mechanisms are important in these reactions. Hydrogen abstraction was considered from
the α1 and α2 sites of allicin and α1 and δ sites of 2-propenesulfenic acid (Fig. 3.7k,
l). It was found that 2-propenesulfenic acid is much more reactive towards OOH
•
by
the HAT mechanism than allicin. However, allicin was found to be somewhat more
reactive towards OOH
•
by the RAF mechanism than 2-propenesulfenic acid. On
the whole, the 2-propenesulfenic acid ingredient of garlic was shown to be a better
scavenger of OOH
•
than the other ingredient allicin.