US researchers have used spectroscopy to identify the absolute configuration of the mealybug sex pheromone, with potential uses in pest control.

An interdisciplinary team worked together to determine the 3D arrangements of the atoms in this molecule.  Entomologist Jocelyn Millar and organic chemist Bruno Figadère at the University of California, Riverside, teamed up with an expert in vibrational circular dichroism (VCD) spectroscopy, Philip Stephens from the University of Southern California, Los Angeles.

Millar explained the importance of the finding 'This insect is a worldwide pest of grape vines and numerous other plants. To determine whether the pheromone can be incorporated into pest management, full identification was required.' 

The chiral pheromone of the mealybug Pseudococcus viburni had been made diastereoselectively by Millar before, and its relative configuration established using NMR.  The absolute configuration could not be confirmed as only a few micrograms of the natural compound are available, preventing measurement of its optical rotation.  In the absence of one enantiomer of known configuration, Millar had no way of determining which enantiomer was the natural pheromone.

'To break this impasse we reasoned that if at least one enantiomer could be resolved from the synthetic racemate, it might be possible to determine its absolute configuration by comparing its experimentally-determined VCD spectrum to the theoretical VCD spectra of the two enantiomers, calculated using density functional theory,' explained Figadère. This proved to be the case and enabled unambiguous assignment of the absolute configuration of the pheromone.

'Comparison of experimental and theoretical VCD spectra has again been shown to be a valuable method for determining the absolute configuration of chiral compounds without enantioselective synthesis of one or both enantiomers,' remarked Stephens. 'This method should be particularly useful for newly-identified natural products where the very small amounts of sample may eliminate the use of other methods of determining absolute configuration.'  

'There are a number of chemical and enzymatic methods of resolving racemates, and the ability to determine the absolute configurations of the resulting enantiomers without having to do a chiral synthesis will save a lot of time and effort,' Millar added. 

Hans-Erik Högberg, a Professor in Organic Chemistry at the Mid Sweden University, Sundsvall, agreed 'Millar presents an elegant solution to this problem and provides researchers in chemical ecology with a powerful new tool for determining absolute stereochemistry. I expect that this will have a great impact on future research in chemical ecology.'

Michael Spencelayh