Chemistry Reference
In-Depth Information
12.6.
Vitamin C
12.6.1.
Chemistry and Analysis
Ascorbate, also known as ascorbic acid (AA) or vitamin C, is synthe-
sized from guanosine diphosphate (GDP)-mannose and the pathway shares
GDP-sugar intermediates with the synthesis of cell wall polysaccharides and
those glycoproteins that contain
D
-mannose,
L
-fucose and
L
-galactose
(Smirnoff, 2000). Humans and non-human primates, guinea pigs, the Indian
fruit bat, several birds and some fish have lost the ability to synthesize
ascorbate de novo as a result of a gene mutation that has rendered inactive
a key ascorbate biosynthetic enzyme,
L
-gulonolactone oxidase, which is
required to catalyse the last step in the biosynthesis of ascorbate (Woodall
and Ames, 1997). Ascorbate is quantitatively the predominant antioxidant in
plant cells; it is found in all subcellular compartments, including the apoplast,
and has an average cellular concentration of 2-25 mM or more in the
chloroplast stroma (Smirnoff, 2000).
Ascorbic acid is the enolic form of an
-ketolactone, 2,3-didehydro-
L
-
threo-hexano-1,4-lactone. The molecular structure (Figure 12.5) contains two
ionizable -OH groups at C
2
and C
3
that give the compound its acidic character,
CH
2
OH
6
CH
2
OH
5
HOCH
HOCH
O
O
4
1
-H
+
+H
+
pK = 4.1
H
H
3
2
O
-
OH
OH
OH
L-Ascorbic acid
(AscH
2
L-Ascorbic acid anion
(AscH
-
)
)
-
-e
-
+e
CH
2
OH
CH
2
OH
CH
2
OH
HOCH
HOCH
HOCH
O
O
O
-e
-
-
+e
-H
+
+H
+
pK
=
-
0.86
H
H
H
O
-
O
O
O
OH
O
Dehydroascorbic acid
(DHA)
Ascorbyl radical
(AscH
)
Tricarboxyl ascorbate radical
(Asc
-
)
Hydrolytic
ring rupture
CH
2
OH
CH
2
OH
HOCH
O
C
Further decomposition
products
HOCH
C
O
C
O
2,3-Diketogulonic acid
Figure 12.5.
L
-Ascorbic acid and its sequential oxidation.
