Analysis of 2,4,6-nonatrienal geometrical isomers from male flea beetles, Epitrix hirtipennis and E. fuscula

Authors: Zilkowski, Bruce; Bartelt, Robert; Vermillion, Karl

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2008
Publication Date: 6/14/2008
Citation: Zilkowski, B.W., Bartelt, R.J., Vermillion, K. 2008. Analysis of 2,4,6-nonatrienal geometrical isomers from male flea beetles, Epitrix hirtipennis and E. fuscula. Journal of Agricultural and Food Chemistry. 56(13):4982-4986.

Interpretive Summary: Chemicals known as 2,4,6-nonatrienals occur in foods and have importance as aroma compounds, and they also occur in some insects where they have been shown to function as pheromones. In this study, compounds in this class were detected from males of the tobacco flea beetle, Epitrix hirtipennis. As a prerequisite to further pheromone research, it was necessary to identify which forms (isomers) were emitted from the beetles (there are eight possible isomers). Ideally, synthetic standards of these isomers would be on hand, for comparison, to aid in the identification process; but these are not available commercially, and synthesis of individual isomers would be a daunting task. Furthermore, the compounds are very labile when exposed to bright light. However, in this research we took advantage of this light sensitivity to help solve the analytical problem. When the one readily accessible isomer (called (2E,4E,6E)-2,4,6-nonatrienal) was exposed to sunlight, a mixture of six isomers was produced; and these were isolated and characterized to determine exactly which isomers they were. Using these as the needed standards for comparison, we learned that the main isomers from the tobacco flea beetle were (2E,4Z,6Z)- and (2E,4E,6Z)-2,4,6-nonatrienal, and several minor components were similarly identified. This novel and relatively simple approach to identification of 2,4,6-nonatrienals is expected to be of use in the future to food scientists, entomologists, and analytical chemists.

Technical Abstract: Geometrical isomers of 2,4,6-nonatrienal have been reported from a variety of food- and insect-related sources. It was discovered recently that the eggplant flea beetle, Epitrix fuscula, uses the (2E,4E,6Z) and (2E,4E,6E) isomers as components of its male-produced aggregation pheromone. Here, we learned that the related species, E. hirtipennis, also emits a blend of 2,4,6-nonatrienals, including isomers not previously characterized. Patterns in emission and response suggest a pheromonal function. In an effort to acquire standards to aid in identification, we found that exposing (2E,4E,6E)-2,4,6-nonatrienal (or any other available isomer) to light readily generated an equilibrium mixture of six geometrical isomers. Configurations of these were determined by NMR, and chromatographic behavior (GC and HPLC) was documented. Based on chromatographic comparison to the standards, the most abundant new compound from E. hirtipennis was concluded to be (2E,4Z,6Z)-2,4,6-nonatrienal. Minor components from both E. hirtipennis and E. fuscula were also characterized. The analytical approach given here would also be of use in the food industry, where 2,4,6-nonatrienals are important as aroma compounds.