NO - Besoin d'assistance

Composition of tomatoes and tomato products in antioxidants (WG1) page 68
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- E 0 (pH7) = 280 mV for ascorbate .- + H + + e - ➔ ascorbate -
HO
E 0 (pH7) = -174 mV for dehydroascorbic acid + e - ➔ ascorbate .-
CH2OH CH O O
- O OH
Ascorbate -
- Stability :
- Stable to air when dry
- It is extremely unstable in aqueous solution. Indeed it is easily oxidized by air
to dehydroascorbic acid (DHAA), and further compounds. The oxidation of AA to DHAA is
reversible, but oxidation beyond DHAA is irreversible and is enhanced by alkaline pH and
metals, especially copper and iron. The degradation processes of ascorbic acid are very
complex and contain a number of oxidation/reduction and intermolecular rearrangement
reactions (Deutsch 1998; Yuan and Chen 1998). Hence, procedures for stabilizing the vitamin
in biologic samples involve acidification with the addition of a reducing agent and a metal
chelator.
7.2.3 Antioxidant properties
- e-, - H+
+ e-, + H+
HO
CH2OH CH O
-.
O
O O
Ascorbate .-
Ascorbic acid is a well known reducing agent both in vivo and in vitro, thus being an
antioxidant compound. In contrast to this role, AA also appears to act as a prooxidant under
certain conditions in vitro, in particular in presence of ferric iron (Fe 3+ ). Indeed ascorbate can
reduce ferric iron to ferrous iron (Fe 2+ ) which, in turn, can enter the Fenton reaction (figure
12) to produce the very toxic hydroxyl radical.
Fe 3+ + ascorbate - ➔ Fe 2+ + ascorbate -.
Fe 2+ + H2O2 ➔ Fe 3+ + HO - + HO . (Fenton Reaction)
Figure 12 : role of ascorbic acid in the production of hydroxyl radical
- e-
+ e-
HO
CH2OH CH O O
O O
dehydroascorbic acid