The determination of solvation descriptors for terpenes, and the prediction of nasal pungency thresholds

Abstract

We have previously put forward two solvation equations for the correlation of chemical and biochemical phenomena, including nasal pungency thresholds (NPT) in man. In order to make use of these equations, a number of descriptors need to be assigned to any given solute, viz.π₂^H the dipolarity/polarizability, Σα₂^H and Σβ₂^H the overall hydrogen-bond acidity and basicity, and log L¹⁶ where L¹⁶ is the solute gas–hexadecane partition coefficient. We show that these descriptors can be obtained from data from a number of processes, including gas-liquid chromatography, high performance liquid chromatography, gas–water partitions, gas–solvent partitions and water–solvent partitions. In this way, the above descriptors have been obtained for eleven terpenes, including hydrocarbons, aldehydes, ketones, alcohols and an ether. The π₂^H, Σα₂^H and log L¹⁶ descriptors have also been obtained for a further 21 terpenes, but Σβ₂^H now has to be estimated. Through our previously reported equation, we have predicted NPT values for the total 32 terpenes. Small hydrocarbons such as α-pinene are predicted to have large NPT values, but aldehydes (e.g. geranial) and ketones (e.g. camphor) are predicted to have moderate values, and alcohols (e.g. menthol and borneol) and their acetates are predicted to have very small NPT values.