O-Diphenols

PPO from tea reacts effectively with both 3 -4 and 3 -4 -5-hydroxylated catechins, with specificity for the o-diphenol (43,44). Studies defining the kinetics of PPO from tea in relation to substrate type are lacking as of this writing (ca 1997). Tea PPO has good functionaUty in the pH range 4.6—5.6 (43,45-48). 

Lillie RD, Pizzolato P, Dessauer HC, et al. Histochemical reactions at tissue arginine sites with alkaline solutions of /J-naphthoquinone-4-sodium sulfonate and other o-quinones and oxidized o-diphenols. J. Histochem. Cytochem. 1971 19 487 197. 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

As mentioned previously, PPO shows two catalytic activities the conversion of monophenols into o-diphenols (monophenolase activity) and the oxidation to the corresponding o-quinones (diphenolase activity) (Fig. 4.2) (Sanchez-Ferrer and others, 1995). 

The diphenolase activity involves the oxidation of two o-diphenols to two o-quinones... 

The oxidation of o-diphenols occurs via a 2e process because no semiquinone intermediate appears during the enzyme turnover. 

A) In the presence of a reducing agent, such as ascorbic acid or NADH, o-quinones are reduced to o-diphenols that are stable compounds, and the reducing agent is oxidized to dehydroascorbic acid or NAD+. 

Mason (30) and Pierpoint (31) have described the involvement of o-diphenols in plants and how they contribute to abnormal plant pigmentation. o-Diphenols are oxidized to o-quinones by enzymes of the phenolase complex (o-diphenol O2 oxidoreductase, E.C. 1.10.3.1) and by peroxidase (E.C. 1.11.1.7). o-Quinones react with amino acids, proteins, amines and thiol groups of proteins to polymerize and from reddish-brown pigments. Concentrations of caffeic acid are doubled in both bean (8) and peanut... 

Plant susceptibility to ozone as determined by visible injury may be very closely related to quantities of o-diphenols associated with the chloroplasts and specific requirements for activation of polyphenol oxidase enzymes. There is a significant correlation between ozone injury and concentrations of total phenols expressed as percent caffeic acid equivalents in peanut cultivars. This concept is not intended to underestimate the importance of membranes that separate phenols and enzymes. Perhaps future research will demonstrate that membranes of resistant alfalfa, green bean and other species differ both qualitatively and quantitatively from those of susceptible plants of these species. 

Tin glycolates or o-diphenolates undergo photochemical or thermal radical cyclization (Equation (31)) to 1,3,2-dioxastannolanes or benzo-l,3,2-dioxastannolenes in high yields <86JOM(303)87>. 

The basic forms of phenols (phenolate anions) are easily oxidized to semiquinone radicals through electron transfer. These radicals can then react with another radical to form an adduct through radical coupling or, in the case of o-diphenols, undergo a second oxidation step yielding o-quinones that are electrophiles as well as oxidants. Oxidation reactions are very slow in wine, due to the low proportion of phenolate ions at wine pH values, but take place extremely rapidly when oxidative enzymes are involved (see Section 5.5.2.2). 

Monophenol +V2O2 o-diphenol o-Diphenol +V2O2 o-quinone + H2O... 

Peroxidase (POD) catalyzes oxidation of a wide range of o-diphenolic substrates to o-quinones, using hydrogen peroxide as a co-substrate (3) ... 

So-called blue multinuclear copper oxidase enzymes, such as laccase and ascorbate oxidase, catalyze the stepwise oxidation of organic substrates (most likely in successive one-electron steps) in tandem with the four-electron reduction of O2 to water, i.e. no oxygen atom(s) from O2 are incorporated into the substrate (Eq. 4) [15]. Catechol oxidase, containing a type 3 center, mediates a two-electron substrate oxidation (o-diphenols to o-chinones), and turnover of two substrate molecules is coupled to the reduction of O2 to water [34,35]. The non-blue copper oxidases, e.g. galactose oxidase and amine oxidases [27,56-59], perform similar oxidation catalysis at a mononuclear type 2 Cu site, but H2O2 is produced from O2 instead of H2O, in a two-electron reduction. 

Dopa is converted by at least some insects into N-P-alanyldopamine, which is a preferred substrate for the o-diphenol oxidase of the insect pupal cuticle. Oxidation of this substrate plays a crucial role in the... 

Type m proteins 1. Deoxyhemocyanin (7-8 x 10° — 20i) 2. Met-hemocyanine or tyrosinase I/I II/II Proposed structure Cu1 Cu1 N L N u XCu" iL XN Colourless 670 14 925 350 to 600 Cu VCu1 Diamagnetic ESR silent Hydroxy monophenols Oxidizing o-diphenols 02— 2H20 (catalase) Oxygen bonding... 

PPO (known also as catecol oxidase, phenolase or diphenol-oxygen oxidereductase) and POD catalyse the oxidation of o-diphenols to o-diquinones, as in the hydroxylation of monophenols [13]. PPO is located exclusively in the plastids of healthy tissues, while most phenolic compounds are localized in the vacuoles, the two compounds thus being physically separated [13]. 

Boric acid has the particular ability to form stable complexes with compounds that present cis-hydroxyl groups (cis-diol groups) [117]. Several compounds, such as sugars and their derivatives and some phenolics (o-diphenols) have these cis-diol groups and therefore can form stable complexes with B [117]. 

The phenol-oxidizing enzyme tyrosinase has two types of activity (/) phenol o-hydroxylase (cresolase) activity, whereby a monophenol is converted into an o-diphenol via the incorporation of oxygen, and (2) cathecholase activity, whereby the diphenol is oxidized. The two reactions are illustrated in Figure 2-6, in the conversion of tyrosine (2.40) to L-DOPA (3,4-dihydroxyphenylalanine (2.41), dopaquinone (2.42), and indole-5,6-quinone carboxylate (2.43), which is further converted to the brown pigment... 

Fig. 17.3. Reactions catalyzed by PPO (17.5) Hydroxylation of monophenol to o-diphenol and (17.6) Dehydrogenation of o-diphenol to o-quinone. Reaction (17.7) is the electrochemical reduction of o-quinone to o-diphenol.

Fig. 17.4. Operation principles of a biosensor based on enzymatic oxidation of monophenol and/or o-diphenol by PPO and electrochemical detection by determining molecular oxygen or the oxidation product derived from monophenol and/or o-diphenol.

Dichlorophenoxyacetic acid 522 DABCYL 820 Dairy products 671 Decoupler 831, 839-840, 856 Dehydrogenation of o-diphenol 371, 372... 

The API epinephrine is an o-diphenol containing a hydroxyl group in the a-position that is easily oxidized by molecular oxygen (Fig. 87). Oxidation is proposed to occur through the transient formation of epinephrine quinone with subsequent formation of adrenochrome (126). This class of compounds (the adrenergics, including adrenaline and isoprenaline) also undergoes this reaction to the adrenochrome upon irradiation in aqueous solution (127). 

A similar oxidation and intramolecular cyclization is observed for the o-diphenol levarterenol (128). 

Synonyms 2-hydroxyphenol, o-diphenol, 1,2-benzenediol, o-diydroxyben-zene Formula C6H502 Structure ... 

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