Salting-in and salting-out effects of ionic and neutral osmotica on limonene and linalool Henry’s law constants and octanol/water partition coefficients

Foliar emission rates of plant-generated volatile monoterpenes depend on monoterpene partitioning between air, aqueous and lipid-phases in the leaves. While Henry’s law constants (Hpc, equilibrium gas/water partition coefficient) and octanol/water partition coefficients (KOW) for pure water have been previously used to simulate monoterpene emissions from the leaves, aqueous phase in plants is a complex solution of electrolytes and neutral osmotica. We studied the effects of dissociated compounds KCl and glycine and sugars glucose, sorbitol and sucrose with concentrations between 0 and 1 M on Hpc and KOW values for limonene and linalool. Linalool with ca. 1500-fold lower Hpc (2.62 Pa m3 mol−1 for pure water at 30 °C) and ca. 30-fold lower KOW (955 mol mol−1 for pure water at 25 °C) is the more hydrophilic compound of the two monoterpenes. Hpc of both monoterpenes increased with increasing concentration of both ionic compounds and sorbitol, but decreased with increasing glucose and sucrose concentrations. The salting-out coefficients for Hpc (kH) were ca. an order of magnitude larger for more hydrophilic compound linalool than for more hydrophobic limonene. For linalool, co-solutes modified Hpc by 30–50% at the highest concentration (1 M) tested. The effect of temperature on the salting-out coefficient of KCl was minor. As with Hpc, KOW increased with increasing the concentration of KCl, glycine and sorbitol, and decreased with increasing glucose and sucrose concentrations. For limonene, co-solutes modified KOW by 20–50% at the highest concentration used. For linalool, the corresponding range was 10–35%. Salting-out coefficients for Hpc and KOW were correlated, but the lipid–solubility was more strongly affected than aqueous solubility in the case of limonene. Overall, these data demonstrate physiologically important effects of co-solutes on Hpc and KOW for hydrophilic monoterpenes and on KOW for hydrophobic monoterpenes that should be included in current emission models.