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Introduction Chiral acyclic terpene alcohols play an important role in chemical communication of bumblebees. Citronellol (1), 2,3-dihydrofarnesol (2), geranylcitronellol (4), and the corresponding aldehydes are often found in the male labial gland secretions of bumblebees and cuckoo bumblebees. Since the secretions are species-specific and the specificity is mostly reached by different proportions of components, the knowledge of the enantiomeric purity of the chiral pheromonal components is needed for full characterization of individual compounds and for further studies of their ecological importance.
Results Absolute configurations of citronellol, 2,3-dihydrofarnesol, and 2,3-dihydrofarnesal in male marking pheromones of seven species of bumblebees and cuckoo bumblebees were determined using enantioselective gas chromatographic separations on a capillary column coated with 60 % heptakis(2,3-di-O-acetyl-6-O-TBDMS)-β-cyclodextrin in polysiloxane PS 268 [1]. A two-dimensional gas chromatography (2D-GC) technique [2] was used for separations. Enantiomers of citronellol were purchased from Aldrich, the other standards were synthesised in the laboratory [3]. Pure (-)-S-enantiomers of all three terpenes were found in the labial glands of all investigated specimens of the following species: Bombus (Bombus) terrestris, B. (Bombus) lucorum, B. (Pyrobombus) pratorum, B. (Pyrobombus) pyrenaeus, B. (Pyrobombus) jonellus, B. (Pyrobombus) impatiens, and the cuckoo bumblebee B. (Ashtonipsithyrus) bohemicus. Within species, specimens were collected in different years at different localities in the Czech Republic, Belgium, France, and in Canada.
* One sample per species and locality was analysed if not stated otherwise.
**Max. possible contamination by the (R)-enantiomer was calculated from GC parameters (difference in r.t., peak height, and peak tailing).
Discussion Some of the chiral terpenes are major components of the labial gland secretions (2,3-dihydrofarnesol - 84 % in B. jonellus, 61 % in B. impatiens, 30 % in B. terrestris), some species on the other hand produce these compounds in minor amounts (2,3-dihydrofarnesol - 0.02 % in B. lucorum, 0.3 % in B. pyrenaeus; citronellol - 0.3 % in B. pratorum, 0.2 % in B. bohemicus) (Table 1). Considering that one gland extract contains approximately 50 mg of the secretion (mixture of up to 100 components), it is impossible to use classical ways (separation and measuring optical rotation) or chemical derivatisation for determination of the enantiomeric purity especially of the minor compounds. The use of enantioselective gas chromatography in a two-dimensional system2 simplified the analyses and enabled the determination of the enantiomeric ratio of terpenes in the amount as small as 10 ng in complex mixtures with high accuracy. All three terpenes, citronellol, 2,3-dihydrofarnesol, and 2,3-dihydrofarnesal were found to be pure (S)-enantiomers (within detection limits) in all species studied. No traces of (R)-enantiomers were detected in case of all 3 terpenes. In B. terrestris, our result was in agreement with a previous report on enantiomeric purity of 2,3-dihydrofarnesol (terrestrol, [4]). Despite of the fact that the species studied belong to 3 different subgenera and thus they are not closely related, their enzymatic system producing terpenic pheromonal components must be very similar and highly selective in all bumblebee species.
References
Research team
Anna Luxov�, Kl�ra Urbanov�, Irena Valterov� (Head of the Team), Michael Terzo*, Anna-Karin Borg-Karlson**
Luxov� A., Urbanov� K., Valterov� I., Terzo M., Borg-Karlson A.-K.:
Bumblebees (part I)
Michal Hoskovec © 8.II.2007
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