Ascorbate semiquinone anion

Almost all biological tissues contain some organic free radicals that are detectable by ESR. These radicals ( tissue radicals ) are of low reactivity in the sense that they do not react readily with molecules in the system, in particular oxygen if this is present. Thus, they tend to be radicals at the end of a radical chain. Examples are the ascorbate radical anion, melanin free radicals, and some other oxygen-insensitive species, such as some flavin semiquinones. The magnetic properties of these various radicals are sufficiently distinct that their ESR spectra can be differentiated on the basis of g value and linewidth. 

Ascorbate anion acts as a scavenger of ROS and other radicals by singleelectron transfer to yield a stable, persistent ascorbic anion-radical. Ascorbate anion reacts with most radicals at the diffusion limit and even reduces various semiquinone anions, which are usually stable and persistent, by electron transfer at a rate around 2x 10 M s at room temperature and water-soluble models for the tocopherol radical marginally more slowly (Figure 7.9). The product is a weak oxidant ascorbate radical anion oxidises semiquinone... 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

Vitamin C is another enol-based redox system like the hydroquinones and tyrosine, but it has no aromatic character. The enediol component is stabilized by a conjugated lactone group. The oxidation (Ej = +58 mV) occurs in two one-electron steps, but the ascorbate radical is not as stable as the semiquinone radical (Bielski, and Richter, 1977). The anion radical disproportionates via an initial dimerization in a similar way as semiquinone radicals (Bielski et al., 1981 Sawyer, et al., 1982). The lifetime of the radical is in the order of microseconds. The acidity of ascorbic acid (pk = 4.2) stems from the OH group in the P position to the lactone carbonyl group. It corresponds to the OH group of a vinylogous carboxylic acid (Scheme 7.2.11). Its UV maximum occurs at 260 nm (e = 1 x 1(T). 

A laser flash photolysis study of the behaviour of the lowest excited triplet state and semiquinone radical anion of hypocrellin A (HA") suggests that, in the presence of substrates such as ascorbic acid and cysteine, formation and decay of (HA") occurs by electron transfer. The production of superoxide radical anion (O2") was also confirmed, and the conclusion is drawn from the experimental results that an electron transfer (Type I) mechanism may be important in the photodynamic interaction between HA and some biological substrates. Photoelectron transfer and hydrogen abstraction in the phenothiazine/p-benzoquinone system proceeds competitively, and a series of porphyrin quinones (5 = H,... 

Oxidase mechanisms are not well understood in detail, but those presented here generally function via an electron transfer chain from the substrate to the various Cu centres through to 02, which may or may not be coordinated to the metal. Studies on models for ascorbate oxidase have invoked reactivity via a ternary metal-ascorbate-02 species, as well as oxidation by free Oi essentially, the O2 oxidizes the reduced metal and the ascorbate via its anionic, semiquinone radical ". 

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