Quinone production, ascorbic acid

The most common natural antioxidants are tocopherols, ascorbic acid and P-carotene (more often synthetic nature-identical compounds than natural products). Their changes were studied in detail in model systems, fats and oils, but experimental evidence is mainly lacking on more complicated systems, such as natural foods and ready dishes. Still less is known on different antioxidants from spices and from essential oils. These data will probably be obtained gradually. Very little is known about synergism of antioxidants in food products other than edible fats and oils or their regeneration from the respective free radicals and quinones. In mixtures, some antioxidants are preferentially destroyed and others are saved. Some data have already been published, but these complex changes should be studied in more detail. 

Organic acids are added to some foods in order to prevent sedimentation or darkening thus, citric, malic, phosphoric, and ascorbic acid are used to reduce or delay enzymatic browning. Melanin formation following production of a quinone is a pH-dependent process (the rate of the former process increases with increasing pH) (8). Polyhydroxyacids such as citric and malic acid... 

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

One of the most useful methods involves the use of L-ascorbic acid as a reducing agent. This is practiced extensively in the commercial production of fruit juices and purees. The ascorbic acid reacts with the o-quinones and changes them back into o-diphenols (Figure 10-14). 

Cu2(L )(NCS)2] (66), which has been shown (c.f. Fig. 8) [43] to contain two tetrahedrally coordinated Cu(I) ions held 2.796(8) A apart and linked inter-molecularly via the sulfur atoms of the thiocyanate ions. For the complexes 63 and 64, the reduction product is the diamagnetic complex [Cu2(L )(MeCN)2](Y)2 (67) (Y = CIO4 or BPh4) in which each three coordinate Cu(I) ion is bonded to two of the four macrocyclic nitrogen atoms and to the nitrogen of one of the two MeCN molecules (Fig, 9) [43]. In the presence of certain substrates the reduction of 63 or 64 is accompanied by substrate oxidation. For example, PhSH, PhC=CH, hydrazobenzene, catechols, hydroquinone, and ascorbic acid afford PhSSPh, PhC=CCsPh, azobenzene, o-quinones, p-quinone, and dehydroascorbic acid, respectively, together with the reduced species 67 and/or other copper complexes... 

Ascorbic acid may have a protective effect with regard to anthocyanins, since it reduces the o-quinones formed before their polymerization. However, ascorbic acid, as well as products of its degradation, increases anthocyanins degradation rate. 

Amplihcation factors of 8 to 12 were claimed for the determination of phenol in a FLA system by a cyclic process depicted in equations 5 (Section VI.A. 1). Phenol is converted to o-benzoquinone in contact with immobilized tyrosinase held in a fixed bed reactor the quinone reacts with ascorbic acid (91) to yield catechol and dehydroascorbic acid (136) catechol can be enzymatically oxidized again to o-benzoquinone and so forth. The accumulated dehydroascorbic acid forms with o-phenylenediamine (137) a highly fluorescent product (kex 345 nm, kfl 410 nm). LOD was ca 0.02 (xM for phenol and catechol the linear range for phenol was 0.1 to 2 p,M and for catechol 0.02 to 2... 

L-ascorbic acid (AA) and its isomer D-erythorbic acid (EA) (also called D-isoascorbic acid) have been used as inhibitors of enzymatic browning in fruit and vegetable products for at least 50 years, (15-17). These compounds prevent quinone accumulation and subsequent pigment formation by reducing the 0-quinones generated from the phenolic substrates of PPO back to O-dihydroxyphenolic compounds (17-18). AA also can act as a PPO inhibitor (19-20). AA and EA are used interchangeably although there are indications that AA is more effective in some systems (21-22). 

The phenolic ring in estrone can be readily oxidized. Reaction of estrone with Fremy salts (peroxylamine disulfonate) affords a mixture of the two isomeric catechols. In a more controlled manner, treatment of estrone with 2-iodoxybenzoic acid (22-1) leads intially to a mixture of the 2,3-quinone (22-2) and its 3,4-isomer (22-3) (Scheme 3.22). These products are then reduced in situ with ascorbic acid to afford 2-hydroxyestrone (22-4) and 4-hydroxyestrone (22-5). 

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). 

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,... 

Still more recent work by the same authors has suggested an alternative possibility. It has been generally assumed that L-ascorbic acid has no effect on the polyphenolase system other than its effect as a reducing agent for the o-quinone formed by the oxidation of the phenols. It has now been shown that ascorbic acid itself has an inhibitory action on the polyphenolase enzyme. When polyphenolase prepared from potato was treated with ascorbic acid under anaerobic conditions, and the ascorbic acid subsequently removed by dialysis, the activity of the enzyme was very considerably reduced. The enzyme after such treatment could not be reactivated by the addition of cupric salts and appeared to bo irreversibly inactivated. It was also shown that neither dehydroascorbic acid nor the further oxidation products of dehydroascorbic acid were responsible for this result. There is at present no explanation of the mechanism of this inhibitory action of ascorbic acid, but it is quite clear that, if these results are confirmed, other explanations are possible of why these enzymes do not exert their full potential effect in vivo. 

Natural fats possess a certain degree of resistance to oxidation, owing to the presence of compounds termed antioxidants. These prevent the oxidation of unsaturated fats until they themselves have been transformed into inert products. A number of compounds have this antioxidant property, including phenols, quinones, tocopherols, gallic acid and gallates. In the European Union, propyl, octyl or dodecyl-gallate, butylated hydroxyanisole, butylated hydroxytoluene and ethoxyquin may be added to edible oils as antioxidants in amounts specified in the EC Community Register of Feed Additives 2009. Other substances such as synthetic a-, 7- and 8-tocopherols and various derivatives of ascorbic acid may be used without limit. 

The major pathway leading to the formation of carbonyls is the Strecker degradation. This reaction occurs between dicarbonyls and free amino acids. The dicarbonyls involved have vicinal carbonyls (carbonyl groups separated by one double bond) or conjugated double bonds [41], While these carbonyls typically are intermediates in the Maillard reaction, they may also be normal constituents of the food (e.g., ascorbic acid), be end products of enzymatic browning (e.g., quinones), or be products of lipid oxidation[42]. 

Phenolic compounds are substrates for polyphenol oxidases. These enzymes hydroxylate monophenols to o-diphenols and also oxidize o-diphenols to o-quinones (cf. 2.3.3.2). o-Quinones can enter into a number of other reactions, thus giving the undesired brown discoloration of fruits and fruit products. Protective measures against discoloration include inactivation of enzymes by heat treatment, use of reductive agents such as SO2 or ascorbic acid, or removal of available oxygen. 

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