77
3 Formation of DNA Lesions, its Prevention and Repair
Carotenoids have also been found to scavenge OOH
•
. Their reactions with OOH
•
can take place along more than one pathways. Certain theoretical studies have suggested that the SET mechanism is highly unfavourable for the reaction between
carotenoids and OOH
•
[173]. A theoretical study of reactions between carotenoids
and OOH
•
was performed at the BPW91/6-31G(d, p) level of theory [171]. Solvent
effect of water and benzene on the reactions was studied employing the IEF formalism of the PCM [141]. A comprehensive study of HAT and RAF mechanisms
was carried out considering three different carotenoids, namely, β-carotene (BC),
lycopene (LYC) and torulene (TOR). For the HAT mechanism, in the cases of BC
and LYC, lowest Gibbs barrier energies were found for hydrogen abstraction from
the sites 5a and 4 while in the case of TOR, the lowest Gibbs barrier energy sites
were 30 and 4 (Fig. 3.7a–c) in both water and benzene solvents. Among the three
carotenoids, TOR was found to be the most efficient as an OOH
•
scavenger through
the HAT mechanism [171]. For RAF mechanism [171] in non-polar media, the C5
site corresponds to lowest barrier energy for both BC and LYC, while in the case
of TOR, the lowest barrier energy corresponds to addition at the C30 site followed
by that at the C5 site. However, polar media alters the order of reactivities of the
different sites. For BC, Gibbs barrier energies for additions at C5, C7 and C9 sites
were found to be comparable, and in the case of LYC, the barrier energy for addition at C15 was also found to be similar to that for addition at C5. In the case of
TOR, the lowest barrier energy corresponds to the C5 site, and it was followed by
the C30 site. Therefore, there would be a wider product distribution in polar media
than in non-polar ones. The calculated reaction rate constants suggested that TOR
is appreciably more reactive than BC through the RAF and HAT mechanisms in
non-polar media. The RAF mechanism seems to be much less important in the context of reactions of carotenoids with OOH
•
than HAT. On the whole, reactivities of
carotenoids towards OOH
•
are predicted to follow the order [171]: LYC > TOR > BC
in non-polar media and TOR > LYC > BC in polar media. Adducts of OOH
•
.
are predicted to be formed mainly at the terminal C5 site of the conjugated polyene chains.
3.4.4 NO 2 Radical Scavengers
Carotenoids are also highly reactive towards NO 2
•
. Among all carotenoids,
β-carotene scavenges NO 2
•
most effectively, preventing cardiovascular diseases
[174]. Several experimental studies had been carried out for the reaction between
β-carotene and NO 2
•
in different environments but the favourable reaction mechanism and solvent effects could not be conclusively established [175–178]. Certain
pulse radiolysis experiments had suggested that the reaction would proceed through
electron transfer [177] while in other experiments, it was suggested to take place
through the RAF mechanism [178]. All the three mechanisms (SET, HAT, RAF) for
the reaction between β-carotene and NO 2
•
were studied theoretically at the B3LYP/
6-31G(d) level [174]. Solvent effect was treated employing the PCM in the solvents
heptane, methanol and water having low, medium and high polarities respectively.
3 Formation of DNA Lesions, its Prevention and Repair
Carotenoids have also been found to scavenge OOH
•
. Their reactions with OOH
•
can take place along more than one pathways. Certain theoretical studies have suggested that the SET mechanism is highly unfavourable for the reaction between
carotenoids and OOH
•
[173]. A theoretical study of reactions between carotenoids
and OOH
•
was performed at the BPW91/6-31G(d, p) level of theory [171]. Solvent
effect of water and benzene on the reactions was studied employing the IEF formalism of the PCM [141]. A comprehensive study of HAT and RAF mechanisms
was carried out considering three different carotenoids, namely, β-carotene (BC),
lycopene (LYC) and torulene (TOR). For the HAT mechanism, in the cases of BC
and LYC, lowest Gibbs barrier energies were found for hydrogen abstraction from
the sites 5a and 4 while in the case of TOR, the lowest Gibbs barrier energy sites
were 30 and 4 (Fig. 3.7a–c) in both water and benzene solvents. Among the three
carotenoids, TOR was found to be the most efficient as an OOH
•
scavenger through
the HAT mechanism [171]. For RAF mechanism [171] in non-polar media, the C5
site corresponds to lowest barrier energy for both BC and LYC, while in the case
of TOR, the lowest barrier energy corresponds to addition at the C30 site followed
by that at the C5 site. However, polar media alters the order of reactivities of the
different sites. For BC, Gibbs barrier energies for additions at C5, C7 and C9 sites
were found to be comparable, and in the case of LYC, the barrier energy for addition at C15 was also found to be similar to that for addition at C5. In the case of
TOR, the lowest barrier energy corresponds to the C5 site, and it was followed by
the C30 site. Therefore, there would be a wider product distribution in polar media
than in non-polar ones. The calculated reaction rate constants suggested that TOR
is appreciably more reactive than BC through the RAF and HAT mechanisms in
non-polar media. The RAF mechanism seems to be much less important in the context of reactions of carotenoids with OOH
•
than HAT. On the whole, reactivities of
carotenoids towards OOH
•
are predicted to follow the order [171]: LYC > TOR > BC
in non-polar media and TOR > LYC > BC in polar media. Adducts of OOH
•
.
are predicted to be formed mainly at the terminal C5 site of the conjugated polyene chains.
3.4.4 NO 2 Radical Scavengers
Carotenoids are also highly reactive towards NO 2
•
. Among all carotenoids,
β-carotene scavenges NO 2
•
most effectively, preventing cardiovascular diseases
[174]. Several experimental studies had been carried out for the reaction between
β-carotene and NO 2
•
in different environments but the favourable reaction mechanism and solvent effects could not be conclusively established [175–178]. Certain
pulse radiolysis experiments had suggested that the reaction would proceed through
electron transfer [177] while in other experiments, it was suggested to take place
through the RAF mechanism [178]. All the three mechanisms (SET, HAT, RAF) for
the reaction between β-carotene and NO 2
•
were studied theoretically at the B3LYP/
6-31G(d) level [174]. Solvent effect was treated employing the PCM in the solvents
heptane, methanol and water having low, medium and high polarities respectively.