9 Analytical Protocols in Antioxidant Capacity Measurement
207
observed in the literature as they may present specific chromophore carotenoids with
12 and 13 CDBs, such as echinenone and canthaxanthin, respectively (Rodrigues
et al. 2015; Patias et al. 2017; Nascimento et al. 2019).
In general, it seems that the antioxidant mechanism will be facilitated as the electromagnetic radiation energy of the molecule decreases, because the more linear,
more CDBs and more symmetrical the structure, the smaller the energy charge, the
more easily is the electron relocation, and therefore electron donation occurs more
easily (Poliak et al. 2018). This observation makes sense when we look at the results
of Rodrigues et al. (2012), where the opening of the β-ionone ring (lycopene—11
CDBs) and the increase of the chromophore extension (astaxanthin—13 CDBs) were
the main factors that increased the peroxyl-scavenging capacity of carotenoids. Moreover, the addition of OH and CO groups in each terminal ring of carotenoid structures
influenced the peroxyl-scavenging capacity of carotenoids depending on the number,
type of functional groups and if it is part or not of the chromophore. For example,
astaxanthin (13 CDBs, two OH, and two CO groups) was 2 times more efficient to
scavenge peroxyl radicals than β-carotene (11 CDBs, 2 β-rings and none OH or CO
groups), while β-cryptoxanthin (11 CDBs, 2 β-rings, and one OH group) exhibited the
same scavenging capacity of β-carotene. The same authors also demonstrated that cis
isomers of carotenoids have less antioxidant capacity than the corresponding transisomers. Additionally, a high concentration of extended chromophore compounds,
such as canthaxanthin (12 CDBs), myxoxanthophyll (12 CDBs), and echinenone (11
CDBs), all of them found in Chlorella vulgaris, contributed to a higher antioxidant
potential when compared to other microalgae.
Another well-known antioxidant mechanism of tetraterpenoids is the quenching
of singlet oxygen (
1 O 2 ) and the triplet state of sensitizers, in which carotenoids
physically interact with
1 O 2 by absorbing excess energy (physical quenching) from
the molecule in the chromophore region and releasing it into the environment as
heat (energy transfer mechanism) (El Agamey et al., El-Agamey et al. 2004). In
addition, whereas at a low rate, carotenoids are able to quench
1 O 2 via chemical
quenching, resulting in carotenoid oxidation products (Montenegro et al. 2004; Rios
et al. 2007). Additionally, the antioxidant action of carotenoid may also occur through
adduct formation and hydrogen atom transfer (HAT) (Poliak et al. 2018).
Chlorophyll, a green pigment, is a cyclic structure with a characteristic fivemembered isocyclic ring, which may or may not have a central magnesium atom
and a C17 phytol chain that confers hydrophobicity to the molecule (Roca et al.
2015). Figure 9.2 shows the main chlorophylls found in most microalgae, namely,
chlorophylls a, b, c, d, and f , as well as their suggested antioxidant mechanism of
action.
Chlorophylls a and b differ by the C7 (R1) carbon functional group, while chlorophyll a has a methyl group and chlorophyll b has an aldehyde (Fernandes et al. 2017).
In contrast, chlorophyll c have a D-ring unsaturated and a characteristic propionic
acid at C17 (R6) instead of a phytol group, which confers polarity to the molecule,
and chlorophyll c 1 and c 2 differ from each other by the radical at C8 (R5) carbon,
alkyl and vinyl, respectively, while c 3 is characterized by the presence of C7 (R4)
207
observed in the literature as they may present specific chromophore carotenoids with
12 and 13 CDBs, such as echinenone and canthaxanthin, respectively (Rodrigues
et al. 2015; Patias et al. 2017; Nascimento et al. 2019).
In general, it seems that the antioxidant mechanism will be facilitated as the electromagnetic radiation energy of the molecule decreases, because the more linear,
more CDBs and more symmetrical the structure, the smaller the energy charge, the
more easily is the electron relocation, and therefore electron donation occurs more
easily (Poliak et al. 2018). This observation makes sense when we look at the results
of Rodrigues et al. (2012), where the opening of the β-ionone ring (lycopene—11
CDBs) and the increase of the chromophore extension (astaxanthin—13 CDBs) were
the main factors that increased the peroxyl-scavenging capacity of carotenoids. Moreover, the addition of OH and CO groups in each terminal ring of carotenoid structures
influenced the peroxyl-scavenging capacity of carotenoids depending on the number,
type of functional groups and if it is part or not of the chromophore. For example,
astaxanthin (13 CDBs, two OH, and two CO groups) was 2 times more efficient to
scavenge peroxyl radicals than β-carotene (11 CDBs, 2 β-rings and none OH or CO
groups), while β-cryptoxanthin (11 CDBs, 2 β-rings, and one OH group) exhibited the
same scavenging capacity of β-carotene. The same authors also demonstrated that cis
isomers of carotenoids have less antioxidant capacity than the corresponding transisomers. Additionally, a high concentration of extended chromophore compounds,
such as canthaxanthin (12 CDBs), myxoxanthophyll (12 CDBs), and echinenone (11
CDBs), all of them found in Chlorella vulgaris, contributed to a higher antioxidant
potential when compared to other microalgae.
Another well-known antioxidant mechanism of tetraterpenoids is the quenching
of singlet oxygen (
1 O 2 ) and the triplet state of sensitizers, in which carotenoids
physically interact with
1 O 2 by absorbing excess energy (physical quenching) from
the molecule in the chromophore region and releasing it into the environment as
heat (energy transfer mechanism) (El Agamey et al., El-Agamey et al. 2004). In
addition, whereas at a low rate, carotenoids are able to quench
1 O 2 via chemical
quenching, resulting in carotenoid oxidation products (Montenegro et al. 2004; Rios
et al. 2007). Additionally, the antioxidant action of carotenoid may also occur through
adduct formation and hydrogen atom transfer (HAT) (Poliak et al. 2018).
Chlorophyll, a green pigment, is a cyclic structure with a characteristic fivemembered isocyclic ring, which may or may not have a central magnesium atom
and a C17 phytol chain that confers hydrophobicity to the molecule (Roca et al.
2015). Figure 9.2 shows the main chlorophylls found in most microalgae, namely,
chlorophylls a, b, c, d, and f , as well as their suggested antioxidant mechanism of
action.
Chlorophylls a and b differ by the C7 (R1) carbon functional group, while chlorophyll a has a methyl group and chlorophyll b has an aldehyde (Fernandes et al. 2017).
In contrast, chlorophyll c have a D-ring unsaturated and a characteristic propionic
acid at C17 (R6) instead of a phytol group, which confers polarity to the molecule,
and chlorophyll c 1 and c 2 differ from each other by the radical at C8 (R5) carbon,
alkyl and vinyl, respectively, while c 3 is characterized by the presence of C7 (R4)