bumblebees (part 4)

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.

Origin of bumblebees, contents and enantiomeric purities of chiral terpenes in their marking pheromones
Species Origin of specimens* Relative abundance according to GC integration (non-chiral; chiral**)
Citronellol 2,3-Dihydrofarnesol 2,3-Dihydrofarnesal
B. terrestris Brno (CZ; lab. colony, 1999) - 28-32 %; >97.9 % (S) 1-3 %; >99.3 % (S)
B. pratorum Horní Albeřice (CZ, 1995)
Horní Albeřice (CZ, 1997)
0.2 %; >98.1 % (S)
0.3 %; >98.1 % (S)
0.02 %; ND
0.2 %; ND
-
-
B. lucorum Brno (CZ; lab. colony, 1999)
Pavlov (CZ; 1998)
-
-
0.2 %; >97.9 % (S)
0.05 %; >97.9 % (S)
-
-
B. jonellus Zonhoven (B; 2001)
Brdy-Jordán (CZ; 1995)
-
-
84 %; >97.9 % (S)
16 %; >97.9 % (S)
13 %; >99.3 % (S)
41 %; >99.3 % (S)
B. bohemicus East Pyrenees (F; 2001) 0.2 %; >98.1 % (S) - -
B. impatiens Montreal (CND; 2002) - 61 %; >97.9 % (S) 0.02 %; >99.3 % (S)
B. pyrenaeus East Pyrenees (F; 2001) - 0.3 %; >97.9 % (S) -
*  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).



Parts of chromatograms of citronellol enantiomers on the chiral column

citronellol enantiomers on the chiral column

A: citronellol from B. pratorum
B: coinjection of B. pratorum sample with racemic citronellol
C: racemic citronellol



Parts of chromatograms of 2,3-dihydrofarnesal and 2,3-dihydrofarnesol enantiomers on the chiral column

2,3-dihydrofarnesal and 2,3-dihydrofarnesol enantiomers on the chiral column

A: 2,3-dihydrofarnesal and 2,3-dihydrofarnesol from B. jonellus
B: coinjection of B. jonellus sample with racemic 2,3-dihydrofarnesal
    and 2,3-dihydrofarnesol

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

  1. Dietrich A., Maas B., Volker K., Kreis P., Lehmann D., Weber B., Mosandl A.:
    Stereoisomeric flavor compounds. Part LV: Stereodifferentiation of some chiral volatiles on heptakis(2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin.
    Journal of High Resolution Chromatography 15: 176-179, 1992.
     
  2. Borg-Karlson A.-K., Lindström M., Norin T., Persson M., Valterová I. .
    Enantiomeric composition of monoterpene hydrocarbons in different tissues of Norway spruce, Picea abies (L.).
    A Multidimensional gas chromatography study.
    Acta Chemica Scandinavica 47: 138-144, 1993.
     
  3. Valterová I., Svatoš A., Hovorka O.:
    Analysis of the labial gland secretion of the cuckoo-bumblebee (Psithyrus vestalis) males and synthesis of abundant geranylcitronellol.
    Collection of Czechoslovak Chemical Communications 61: 1501-1508 (1996).
     
  4. Ställberg-Stenhagen S. .
    The absolute configuration of terrestrol.
    Acta Chemica Scandinavica 24: 358-360,1970.
     

Research team

Anna Luxová, Klára Urbanová, Irena Valterová (Head of the Team), Michael Terzo*, Anna-Karin Borg-Karlson**

*  University of Mons-Hainaut, Laboratory of Zoology, Avenue du Champs de Mars 6, B-7000 Mons, Belgium
**Department of Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden


For detailed information see our paper

Luxová A., Urbanová K., Valterová I., Terzo M., Borg-Karlson A.-K.:
Absolute configuration of chiral terpenes in marking pheromones of bumblebees and cuckoo bumblebees.
Chirality 16: 228-233, 2004.


Related themes on our web pages

Bumblebees (part I)
Bumblebees (part II)
Bumblebees (part III)

Michal Hoskovec © 8.II.2007