Biomedical Engineering Reference
In-Depth Information
R
1
HO
CH
3
CH
3
CH
3
O
CH
3
R
2
R
3
Tocopherol
R
1
HO
CH
3
CH
3
CH
3
O
CH
3
R
2
R
3
Tocotrienol
HO
CH
3
CH
3
[
]
7
O
CH
3
CH
3
CH
3
Plastochromanol-8
Isomer
R
1
R
2
R
3
α
β
γ
CH
3
CH
3
CH
3
CH
3
CH
3
H
CH
3
CH
3
CH
3
H
H
δ
H
FIGURE 6.6
Structure of plastochromanol and isomers of tocopherol and tocotrienol.
based on its structure, should be more effective as a hydrogen donor than
β
-,
γ
-, or
δ
-tocopherols. The oxidation reduction potentials of +0.273, +0.343, +0.348, and
+0.405 volts were reported for
-tocopherols, respectively.
51
A higher
redox potential indicates lower potency as a hydrogen donor. Based on these mea-
surements, the
α
-,
β
-,
γ
-, and
δ
isomer of tocopherol is the best radical regenerator compared to
the other three isomers of these antioxidants.
52
However, substitution in the
ortho
and
para
position hindered phenoxy radicals from further reactions and decreases
the possibility of oxidation by atmospheric oxygen.
50,53
The antioxidant activity of
tocopherols is 250 times greater than BHT mainly due to the heterocyclic ring —
hydrogen donor part and chroman moiety — responsible for fat solubility.
54
Phenols
with an
oxy
substituent in the
para
position to the hydroxy group produced more
stable phenoxyl radicals and exhibited higher activity as antioxidants.
55
More methyl
substitutes in phenolic ring improved the relative antioxidant activity of tocopherol
isomers but also made these isomers more soluble in fats/oils.
56
Tocotrienols also
α
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