Polarimetry equations and software for characterizing semiochemical enantiomers of insects

Many compounds in nature are chiral such as various plant chemicals affecting herbivory, sex and aggregation pheromones, and allomones of insects. Each chiral semiochemical usually consists of 2 enantiomer structures that are non-superimposable mirror images. These contain at least one asymmetric carbon yielding absolute R- or S-stereo configurations that rotate plane-polarized light in a polarimeter in opposite directions to specific amounts. Most chiral insect semiochemicals consist of only one bioactive enantiomer functioning in communication with conspecifics (pheromones) or in interspecific interactions (allomones). Once the absolute configuration of each enantiomer is established, usually by chemical syntheses, and associated with the (±) optical rotations via polarimetry, then chiral gas chromatography-mass spectrometry (GC-MS) and polarimeters are used to identify the bioactive component in behavioral research. We present equations that use the optical rotations of compounds in a polarimeter to characterize the optical purity, specific rotation, enantiomeric excess, and percentage of each enantiomer in solution. We extend the polarimetry equations iteratively to solve for the specific rotation of a compound of interest in solutions of multiple chiral impurities with known proportions and specific rotations (from literature and analyzed by chiral GC-MS and polarimetry). The software allows for improved accuracy in measuring specific rotations of chiral semiochemicals in mixtures, as exemplified in previous literature and test samples. The equations implemented in Java software or webpages for personal computers and smartphones will aid research on the identification of volatile chiral molecules functioning in insect-insect and insect-plant interactions as well as in pest management.