Solid-state 13C-NMR and diphenol analyses of sclerotized cuticles from stored product Coleoptera

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Sclerotized cuticles from eight species of stored product Coleoptera including the maize weevil, Sitophilus zeamais; rice weevil, S. oryzae; granary weevil, S. granarius; red flour beetle, Tribolium castaneum; confused flour beetle, T. confusum; sawtoothed grain beetle, Oryzaephilus surinamensis; lesser grain borer, Rhyzopertha dominica and flat grain beetle, Cryptolestes pusillus, were subjected to solid-state 13C-nuclear magnetic resonance and/or chemical analyses. Solid-state NMR was used to measure relative amounts of protein (30–38%), chitin (32–36%), diphenolic compounds (24–33%) and lipid (2–5%) in cuticles from the three weevil species and the confused flour beetle, as well as wild-type and black mutant strains of the red flour beetle. NMR difference spectroscopic data were consistent with melanin and β-alanine being more abundant in the black and wild-type strains, respectively. Cuticles were extracted in cold acid and the o-diphenolic compounds were quantitated by reversed phase HPLC with electrochemical detection. In all species, the major diphenols were 3,4-dihydroxyphenylacetic acid (6–58 μmol g −1) and 3,4-dihydroxyphenylethanol (5–48 μmol g −1). Dopamine, N- β-alanyldopamine, N- β-alanylnorepinephrine, N-acetylnorepinephrine, 3,4-dihydroxybenzaldehyde and 3,4-dihydroxyphenylalanine were detected at much lower concentrations. The diphenols extracted with cold acid accounted for less than 10% of the total estimated by solid-state NMR.