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

Abstract

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 ( H pc, equilibrium gas/water partition coefficient) and octanol/water partition coefficients ( K OW) 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 H pc and K OW values for limonene and linalool. Linalool with ca. 1500-fold lower H pc (2.62 Pa m 3 mol −1 for pure water at 30 °C) and ca. 30-fold lower K OW (955 mol mol −1 for pure water at 25 °C) is the more hydrophilic compound of the two monoterpenes. H pc 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 H pc ( k H) were ca. an order of magnitude larger for more hydrophilic compound linalool than for more hydrophobic limonene. For linalool, co-solutes modified H pc 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 H pc, K OW increased with increasing the concentration of KCl, glycine and sorbitol, and decreased with increasing glucose and sucrose concentrations. For limonene, co-solutes modified K OW by 20–50% at the highest concentration used. For linalool, the corresponding range was 10–35%. Salting-out coefficients for H pc and K OW 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 H pc and K OW for hydrophilic monoterpenes and on K OW for hydrophobic monoterpenes that should be included in current emission models.