Autooxidation of phenolic compounds

Auto-oxidation refers to the formation of cross-linked structures as a result of exposure to light and oxygen. Under the influence of light, oxygen can abstract a proton, thereby generating a radical. This is particularly likely to occur if the proton is adjacent to a double bond, because the radical electron can be delocalized, thus lowering the energy.

Given their aromatic nature, phenolic compounds are easily auto-oxidized. The radical that is generated can subsequently react with other radicals to form a dimer. Since the radical electron is delocalized, several structures can be formed depending on the precise location of the radical electrons at the time of the reaction.

Figure 2-4 shows how radicals of catechol (2.11) can react to form mixtures of tetrahydroxy-biphenyls (2.36) and quinines (2.37). Another example, shown in Figure 2-5, shows the formation of dimers of p-cresol (2.38).

A variety of complex compounds can arise through these mechanisms, including biflavonyls and bianthraquinones. An example of the latter is the compound iridoskyrim (2.39) formed by the fungus Penicillium islandicum.

Figure 2-4. Auto-oxidation of catechol can result in the formation of different dimers.

(2.38)

Figure 2-4. Auto-oxidation of catechol can result in the formation of different dimers.

Figure 2-5. Auto-oxidation ofp-cresol can result in the formation of different dimers.

Figure 2-5. Auto-oxidation ofp-cresol can result in the formation of different dimers.

1.8.2 Enzymatic oxidation of the phenolic hydroxyl group

An alternative mechanism for the oxidation of phenolic compounds is enzyme-catalyzed oxidation. Several classes of enzymes can catalyze this reaction. According to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB), these enzymes are part of the E.C. 1 class of oxidoreductases (see the Internet web site: http://www.chem.qmul.ac.uk/iubmb/enzyme/EC1). The three main classes of enzymes that catalyze the oxidation of phenolic compounds are the oxidoreductases that use oxygen as electron acceptor (E.C. 1.10.3), the peroxidases (E.C. 1.11.1), and monophenol monooxygenase (E.C. 1.14.18.1).

This class includes enzymes that use diphenols or related compounds as electron donors and oxygen as the acceptor, thereby forming the oxidized donor and water. Members include catechol oxidase (E.C. 1.10.3.1), laccase (E.C. 1.10.3.2), and o-aminophenol oxidase (E.C. 1.10.3.4). Laccase is also known as p-diphenoloxidase, whereas catechol oxidase is also known as diphenoloxidase, phenoloxidase, polyphenoloxidase, o-diphenolase, phenolase and tyrosinase. Many of these names are also used in reference to a different enzyme, monophenol monooxygenase (E.C. 1.14.18.1). This enzyme will be discussed further in Section 1.8.2.2.

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 melanin via enzyme-mediated oxidation (reviewed by Sänchez-Ferrer et al. (1995)). Melanin is the major determinant of skin color in humans (Sturm, 1998), and is formed when the cut surfaces of fruits, such as apples, bananas and avocados, are exposed to air.

Figure 2-6. Tyrosinase-catalyzed oxidation of tyrosine results in precursors of melanin.

Figure 2-6. Tyrosinase-catalyzed oxidation of tyrosine results in precursors of melanin.

Laccase catalyzes the oxidation of /-diphenols to /-quinones. Shown in Figure 2-7 is the oxidation of 1,4-dihydroxybenzene (2.44) to /-quinone (2.45).

laccase

laccase

1/2 o2 h2o

Figure 2-7. The oxidation of 1,4-dihydroxybenzene top-quinone.

This enzyme exhibits no hydroxylase activity and is involved in the final synthesis of many naturally occurring /-quinones, e.g. the naphthaquinone juglone in walnut (1.58) and the benzoquinone arbutin (hydroquinone-P-D-glucopyranoside; 2.46). Arbutin is a plant cryo-protectant that stabilizes membranes (Hincha et al., 1999). This compound has medicinal properties and has, for example, been used to treat urinary tract infections in humans. It is also used to lighten skin color, because it inhibits tyrosinase and hence the formation of melanin. The derivative deoxyarbutin (2.47; note the difference in the sugar molecule) was recently reported to be considerably more effective as a skin-lightening compound (Boissy et al., 2005).

1/2 o2 h2o

While initially controversial, there is evidence that laccases play a role in the polymerization of the cell wall polymer lignin (see Chapter 1, section 3.12), which occurs via the oxidative coupling of monolignol radicals with reactive (oxidized) sites on the lignin polymer. The evidence for their involvement comes from a number of studies in which laccases were localized to lignifying tissues in woody species through the use of histochemical stains. Furthermore, the expression of the laccase genes was shown to be specific for lignifying tissues. In addition, when laccases purified from these tissues were mixed with monolignols under aerobic conditions, a dehydrogenation polymer (DHP) with lignin-like characteristics was formed (Sterjiades et al., 1992; Driouich et al., 1992; Boa et al., 1993; Ranocha et al., 1999). Down-regulation of laccase genes in poplar through the introduction of antisense constructs did not, however, impact lignin content nor subunit composition, but did have an effect on cell wall structure (Ranocha et al., 2002).

Figure 2-8 shows laccase-mediated generation of radicals of coniferyl alcohol (2.48), with the stoichiometry of the reaction adjusted for coniferyl alcohol. The radical electron is delocalized, enabling the formation of various interunit linkages, as discussed in Chapter 1.

Figure 2-8 shows laccase-mediated generation of radicals of coniferyl alcohol (2.48), with the stoichiometry of the reaction adjusted for coniferyl alcohol. The radical electron is delocalized, enabling the formation of various interunit linkages, as discussed in Chapter 1.

(2.48)

Figure 2-8. Laccase-catalyzed formation of coniferyl alcohol radicals.

Was this article helpful?

0 0

Post a comment