Evidence for the formation of a quinone methide during the oxidation of the insect cuticular sclerotizing precursor 1,2-dehydro-N-acetyldopamine.

Abstract

1,2-Dehydro-N-acetyldopamine (dehydro-NADA) is an important catecholamine derivative involved in the cross-linking of insect cuticular components during sclerotization. Since sclerotization is a vital process for the survival of insects, and is closely related to melanogenesis, it is of interest to unravel the chemical mechanisms participating in this process. The present paper reports on the mechanism by which dehydro-NADA is oxidatively activated to form reactive intermediate(s) as revealed by pulse radiolysis, electron spin resonance spectroscopy, high performance liquid chromatography, and ultraviolet-visible spectroscopic analysis. Pulse radiolytic one-electron oxidation of dehydro-NADA by N3. (k = 5.3 x 10(9) M-1 s-1) or Br2.- (k = 7.5 x 10(8) M-1 s-1) at pH6 resulted in the rapid generation of the corresponding semiquinone radical, lambda max 400 nm, epsilon = 20,700 M-1 cm-1. This semiquinone decayed to form a second transient intermediate, lambda max 485 nm, epsilon = 8000 M-1 cm-1, via a second order disproportionation process, k = 6.2 x 10(8) M-1 s-1. At pH 6 in the presence of azide, the first order decay of this second intermediate occurred over milliseconds; the rate decreases at higher pH. At pH 6 in the presence of bromide, the intermediate decayed much more slowly over seconds, k = 0.15 s-1. Under such conditions, the dependence of the first order decay constant upon parent dehydro-NADA concentration led to a second order rate constant of 8.5 x 10(2) M-1 s-1 for reaction of the intermediate with the parent, probably to form benzodioxan "dimers." (The term dimer is used for convenience; the products are strictly bisdehydrodimers of dehydro-NADA (see "Discussion" and Fig. 11)) Rate constants of 5.9 x 10(5), 4.5 x 10(5), 2.8 x 10(4) and 3.5 x 10(4) M-1 s-1 were also obtained for decay of the second intermediate in the presence of cysteine, cysteamine, o-phenylenediamine, and p-aminophenol, respectively. By comparison with the UV-visible spectroscopic properties of the two-electron oxidized species derived from dehydro-NADA and from 1,2-dehydro-N-acetyldopa methyl ester, it is concluded that the transient intermediate exhibiting absorbance at 485 nm is the quinone methide tautomer of the o-quinone of dehydro-NADA. Sclerotization of insect cuticle is discussed in the light of these findings.