Quinones

Quinones are distributed widely from opilionids and millipedes to grasshoppers, cockroaches and caddis flies, but are most frequently found in beetles (Coleoptera). Phenols have been shown to be oxidized to quinones in both millipedes and beetles. It was shown some time ago in the beetle Eloides longicollis, that quinones can arise by two independent pathways. Labelled tyrosine, acetic, propionic and malonic acids were all used. Benzoquinone itself was preferentially made from labelled tyrosine. Simple alkylquinones were produced by the acetate pathway (Meinwald, Happ, Labows and Eisner, Science, 1966, 151, 79). Propionic acid was

-nhg

^COOH

homogentisic acid ch2-cooh

-co2

methylhydroquinone tyrosine ho-<\ h—ch2

p-hydroxyphenylpyruvic acid homogentisic acid methylhydroquinone

Figure 8.11 An unconfirmed way in which quinones can be produced from phenyl-C3

only incorporated well into ethylbenzoquinone. Similar results were obtained with another tenebroid beetle Zophobas rugipes. It has been said that opilionids and millipedes make quinones from pre-existing aromatic substances while insects can make them from acetates. The work of Meinwald et al. indicates that this is not completely so. The benzoquinone and methylbenzoquinone in the secretion of the millipede Narceus gardanus were shown to be made from 6-methylsalicylic acid (Figure 4.2), indicating a polyketide pathway. Another research group found that three other millipedes required tyrosine, while acetic and malonic acid were not incorporated into quinones. Tyrosine can also be degraded via homogentisic acid (Figure 8.11), which requires a 1,2-shift of the side chain and oxidation. If this is followed by another decarboxylation, methylhydroquinone is produced.

The explosive mixture produced by bombardier (brachynid) beetles has been studied in detail by Schildknecht. A pygidial gland produces a mixture of hydroquinones in an aqueous solution of hydrogen peroxide, the concentration of the latter can be up to 28%. This mixture is stored in an inner sac. When the beetle is disturbed the mixture is discharged into an outer chamber, which is supplied with many small glands secreting a mixture of catalase and peroxidase. The resulting reactions are given in Figure 8.12. Quinones, gas and heat are evolved in a very rapid reaction, with an audible 'plop', taking the temperature of the exploded mixture to 100 °C It is noteworthy that the temperature optimum of the catalase is between 70 and 80 °C. The beetle can turn its abdominal tip through 360° to direct the explosion at any predator.

Quinone, hydroquinone, methyl-, methoxy- dimethyl-, and ethylqui-nones have all been identified in beetles, and some of them in cockroaches and grasshoppers. The tenebrionid beetle Argoporus alutacea compounds

2 h202

catalase

2 h202

catalase

Figure 8.12 The defensive reaction of bombardier beetles

Figure 8.12 The defensive reaction of bombardier beetles

6-alkylnaphthoquiriones

Figure 8.13 Compounds a, b and c are examples of millipede quinones, d and e are from opilionids. The naphthoquinones are from the beetle Argoporus alutacea

6-alkylnaphthoquiriones

Figure 8.13 Compounds a, b and c are examples of millipede quinones, d and e are from opilionids. The naphthoquinones are from the beetle Argoporus alutacea produces a viscous, orange defensive secretion consisting of four naphthoquinones (Figure 8.13) as well as benzoquinone and its methyl and ethyl derivatives. The aphin pigments (see later) are also accompanied by simpler naphthoquinones.

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