NO - Besoin d'assistance

Composition of tomatoes and tomato products in antioxidants (WG1) page 65
__________________________________________________________________________________________
Among a series of carotenoids tested, lycopene was shown to be the most efficient
quencher of singlet oxygen (Table 24). The explanation for the somewhat enhanced reactivity
of lycopene towards singlet oxygen is not established.
Carotenoid
Lycopene
Kq (x 10 9 M -1 s -1 )
17
Source
Tomato, watermelon
All-trans β-carotene 13 Carrots
Astaxanthin 14 Salmon
Zeaxanthin 12 Maize
Lutein 2.9 Green vegetable
Violaxanthin 1 Green vegetable
Bixin 9.2 Annatto seeds
Table 24 : Singlet oxygen quenching constants, from Conn et al. (1991)
7.1.4.2 Quenching of free radicals
Radicals are present in vivo in various forms, they are activated species of oxygen like
hydroxyl radical (HO . .- .
), superoxide (O2 ) or peroxyl radical (ROO ). The unpaired electron,
characterizing the radical, gives them their instability. As molecules rich in electrons,
carotenoids can react with a radical in order to fill its gap of electron, thus acting as a radicalquencher.
Two mechanisms of radical quenching can occur. It is either a transfer of electron
from the carotenoid to the radical (1), or an addition reaction (2). In both cases, the radical
formed is delocalised along the polyunsaturated chain of the carotenoid, making it more stable
and then less reactive. It should be noted that upon physical quenching of 1 O2 the carotenoid
molecule stays intact. It is destroyed by chemical quenching of 1 O2 and by radical interactions
(figure 10).
R +. + CAR ➔ R + CAR +.
(1)
R +. + CAR ➔ [ R – CAR ] +. (2)
Figure 10 : mechanisms of radical quenching by a carotenoid