J. George Pomonis

Physical properties of insect cuticular hydrocarbons: The effects of chain length, methyl-branching and unsaturation

Abstract The waterproofing abilities of insect cuticular lipids, consisting mainly of hydrocarbons, are thought to depend upon their biophysical properties. However, little is known regarding the effects of specific structural changes upon cuticular lipid properties. We examined the phase behavior of pure hydrocarbons differing in chain length, methyl-branching pattern, and unsaturation, using Fourier transform infrared spectroscopy. Melting temperatures ( T m ) of 21–40 carbon n -alkanes increased by 1–3°C for an increase in backbone chain length of one carbon atom. The effects of methyl-branching on hydrocarbon properties depended upon the location of the methyl group along the molecule. Melting temperatures of 25-carbon long methylpentacosanes decreased by over 30°C as the location of the methyl moiety was shifted from the terminal portion of the molecule to more internal positions. Addition of a second methyl branch had additional effects on T m . Unsaturation decreased T m by 50°C or more.

Insect hydrocarbons. 2. Mass spectra of dimethylalkanes and the effect of the number of methylene units between methyl groups on fragmentation.

Dimethylalkanes with one, three, four, five, and seven methylene units separating the methyl branches were synthesized and analyzed by mass spectrometry for comparison with the spectra of dimethylalkanes isolated from insect cuticular wax. Fragmentation reactions of these synthetic dimethylalkanes under electron impact verified assumptions based on the mass spectra of the natural products. Dimethylalkanes with 4 methylene units separating the two methyl branches had spectra that were unique to their structure.

A 13C-NMR study of the biosynthesis of 3-methylpentacosane in the American cockroach

13C-NMR spectrometry was used to examine the in vivo incorporation of 13C-labeled precursors into 3-methylpentacosane in the cockroach Periplaneta americana. Natural abundance 13C-NMR of 3-methylpentacosane showed that carbons 1 through 6, 23 through 25 and the branching methyl carbon (C26) each gave distinct signals, with carbons 7 through 22 indistinguishable from each other. The label from dipotassium 2-[methyl-13C]methylmalonate was incorporated primarily into the methyl branch of 3-methylpentacosane, demonstrating the 2-methylmalonate is the precursor to the methyl branch unit. The carboxyl carbon from sodium [1-13C]propionate was incorporated exclusively into the 4-position. This indicates that propionate, as a 2-methylmalonyl derivative, is incorporated as the second unit during chain synthesis rather than toward the end of the elongation process. The labeled carbon from sodium [1-13C]acetate was incorporated into carbons 2, 6 and 24 and the labeled carbon from [2-13C]acetate was incorporated into carbons 1, 5, 23 and 25 of 3-methylpentacosane, respectively. These data are consistent with an elongation-decarboxylation pathway for 3-methylpentacosane biosynthesis.

Incorporation of [2,3-13C]succinate into methyl-branched alkanes in a termite

Abstract Carbon-13 nuclear magnetic resonance was used to examine the incorporation of [2,3-13C]succinate into methylalkanes in the termite Zootermopsis angusticollis. Carbons 2 and 3 of succinate were preferentially incorporated into the branching methyl group(s) and the tertiary carbon(s) of monomethyl- and dimethylalkanes. These data support a pathway in which succinate is metabolized to methylmalonyl-coenzyme A, and is then incorporated in place of malonyl-coenzyme A at specific points during chain elongation.

Identification of compounds in an HPLC fraction from female extracts that elicit mating responses in male screwworm flies,Cochliomyia hominivorax

When hexane extracts of mature screwworm females were chromatographed on a silica gel column, mating stimulant activity was concentrated in a fraction that eluted with hexane-ether (94∶6, v/v). Separation of this fraction with HPLC (acetonitrile-acetone; 60∶40, isocratic) resulted in a chromatogram of some 20 peaks. Only peaks 4–11 elicited mating responses. Peaks 5–10 had most of the activity, with peak 8 producing the highest response. Sixteen compounds were characterized from peak 8 by gas chromatography-mass spectrometry: six unbranched secondary acetates (C31H62O2); seven previously unreported methyl-branched secondary acetates (C32H64O2); one unbranched ketone (C31H62O); and one methyl-branched ketone (C32H64O). The isomeric acetates were not completely resolved from each other by capillary gas chromatography (CGC) on methyl silicone columns. The sixteenth compound was an aldehyde (C30H60O) that was present only in occasional peak 8 preparations. These compounds and several derivatives were characterized by capillary gas chromatography-mass spectrometry (CGC-MS). The position of the acetate group was ascertained by conversion to a keto group or by replacement of the acetate with a methyl group. Pheromone activity was not observed in peaks trapped either from CGC or by recombination of the trapped CGC peaks from HPLC peak 8. This apparent loss of activity from CGC peaks or from TLC cannot currently be explained.

ALKANES FROM SURFACE LIPIDS OF SUNFLOWER STEM WEEVIL, Cylindrocopturus adspersus (LECONTE)

The stem weevil,Cylindrocopturus adspersus (LeConte) (Coleoptera: Curculionidae) yields 3% of its body weight as extractable lipids (40 μg/ weevil). The alkane fraction was composed ofn-alkanes (38%) and branched alkanes (62%). The compounds were characterized by gas chromatography-mass spectrometry (GC-MS). The chromatogram contained several single-component peaks (9 of 25). Only seven dimethylalkanes were isolated (17.8%): 9,19- and 9,21-dimethylheptacosane; 9,19- and 9,21-dimethylnonacosane; 9,21- and 11,21-dimethylhentriacontane; and 11,21-dimethyltritriacontane. Important methylalkanes were: 2-methyltetra- and hexacosanes and 10-methylhexa- and octacosanes. Late-eluting gas chromatography peaks were composed of simple alkane mixtures or a single component.

Biosynthesis of a pheromone, 1,7-dioxaspiro[5,5]undecane, from 14C-substrates in vivo and by explanted female rectal glands of the olive fruit fly, Dacus oleae (Gmel.): a preliminary study

Summary The olive fruit fly, Dacus oleae, female incorporated dietary 14 C-labeled malonate, succinate, glutamate, and propionate into the primary pheromone component, 1,7-dioxaspiro[5,5]undecane (1); but did not significantly incorporate acetate. Explanted pheromone glands when incubated in pH 7 phosphate buffer with [14C]malonate incorporated the label into 1, but did not incorporate [14C]glutamate. Incorporation of labeled malonate by incubating explanted glands was temperature and age dependent.