Singlet molecular oxygen Reactive species

The protective effects of carotenoids against chronic diseases appear to be correlated to their antioxidant capacities. Indeed, oxidative stress and reactive oxygen species (ROS) formation are at the basis of oxidative processes occurring in cardiovascular incidents, cancers, and ocular diseases. Carotenoids are then able to scavenge free radicals such as singlet molecular oxygen ( O2) and peroxyl radicals particularly, and protect cellular systems from oxidation. 

Also, reactive oxygen species such as hydroxyl radicals ( OH), organoperoxyl radicals (R02 ), hydroperoxyl-superoxide radicals, and singlet molecular oxygen ( 02) can oxidize halocarbons. Dehalogenation often is not a major pathway in indirect photooxidations, however. 

Oxidase activation is the pivotal event with respect to microbicidal metabolism in that its products, 02 and H, serve as reactants in a variety of disproportionation and radical reactions yielding H2O2 and highly reactive oxidizing species such as hydroxyl radical (-OH), and either singlet molecular oxygen ( O2) or an oxidant having the reactive po-... 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

A photooxidative reaction in which molecular oxygen is incorporated into the reaction products(s). Three mechanisms appear to be common for such processes (a) reaction of triplet O2 with free radicals that have been generated photochemically (b) reaction of photochemically produced singlet oxygen with a molecular species and (c) the production of superoxide anion which then acts as the reactive species. See also Photooxidation... 

Silylenes 1 are highly reactive homologues of the carbenes, and we have been interested to compare the reactivity and primary products of the oxidation of these divalent species. In principal one can expect two different primary adducts of a silylene and molecular oxygen the formal "end-on" adducts silanone 0-oxide 2 or "side-on" adducts dioxasilirane 3 (Scheme 1). It was shown by numerous matrix studies [8-12], experiments in solution using time resolved spectroscopy [13-16], and a preparative scale synthesis in solution [17] that triplet as well as singlet carbenes yield carbonyl 0-oxides as the primary oxidation products, while dioxiranes are products of secondary photolysis. Ando et al. reported on the synthesis of the silanone 0-oxide 2e by the reaction of dimesitylsilylene le and O2 in solid argon [1]. This is so far the only experimental evidence for a silanone O-oxide. 

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