Chemistry Reference
In-Depth Information
R
1
HO
4
5
3
6
CH
3
CH
3
CH
3
CH
3
7
8
2
1
O
R
2
CH
3
R
3
Compound
Formula
R
1
R
2
R
3
α
-Tocopherol
5,7,8-Trimethyl
CH
3
CH
3
CH
3
β
-Tocopherol
5,8-Dimethyl
CH
3
H
CH
3
γ
-Tocopherol
7,8-Dimethyl
H
CH
3
CH
3
δ
-Tocopherol
8-Methyl
H
H
CH
3
Figure 12.3
The four major forms of vitamin E (
-,
-,
- and
-tocopherol) differ by the
number and position of methyl groups on the chromanol ring. In
-tocopherol, the most
biologically active form, the chromanol ring is fully methylated. In
- and
-tocopherol, the
ring contains two methyl groups, while in
-tocopherol it is methylated in one position. The
corresponding tocotrienols have the same structural arrangement except for the presence of
double bonds on the isoprenoid side chain at C3
0
,C7
0
and C11
0
.
metabolism and the hydrophobic tail from phytyl-diphosphate (phytyl-DP)
or geranygeranyl diphosphate (GGDP) for tocopherols and tocotrienols,
respectively (DellaPenna and Pogson, 2006). The term 'vitamin E' is used to
describe all tocopherol and tocotrienols that qualitatively exhibit the biolo-
gical activity of
-tocopherol. Tocopherol is methylated at C-5, C-7 and C-8
on the chromonal ring, whereas the other homologues (
-,
- and
-) differ in
the number and positions of the methyl groups on the ring (Figure 12.3).
Tocopherols have a fully saturated 20-carbon phytyl side chain attached at C-
2 and have three chiral centres in the R configuration at positions C-2, C-4
1
and C-8
1
in the naturally occurring form, which are given the prefix 2R, 4
1
R
and 8
1
R (designated RRR). They are more biologically active than their
synthetic counterparts, which are mixtures of all eight possible stereoisomers
and are given the prefix all-rac. Tocotrienols differ from the corresponding
tocopherols in that the 20-carbon isoprenoid side chain is unsaturated at C-
3
1
,C-7
1
and C-11
1
and possess one chiral centre at C-2 in addition to two sites
of geometric isomerism at C-3
1
and C-7
1
. Natural tocotrienols have the 2R,
3
1
-trans,7
1
-trans configuration. The phenolic hydroxyl group is critical for
the antioxidant activity of vitamin E, as donation of hydrogen from this
