Rational selection of biphasic reaction systems for geranyl glucoside production by Escherichia coli whole-cell biocatalysts

Geranyl glucoside, the glucosylated, high-value derivative of the monoterpenoid geraniol, has various applications in the flavor and fragrance industry and can be produced through whole-cell biotransformation of geraniol with Escherichia coli whole-cell biocatalysts expressing the glucosyltransferase VvGT14a. However, the low water solubility and high cytotoxicity of geraniol require the design of a proper biphasic system where the second, non-aqueous phase functions as an in-situ substrate reservoir. In this work, a rational selection strategy was applied for choosing suitable sequestering phases for geranyl glucoside production by whole-cell biotransformation of geraniol. Hansen solubility parameters and octanol/water distribution coefficients were used as first principle methods in combination with extensive database research to preselect 12 liquid and 6 solid sequestering phases. Subsequently, experimental approaches were applied to determine physicochemical characteristics and the distribution of geraniol and geranyl glucoside between the phases. Moreover, the effects of the sequestering phases on the whole-cell biocatalysts and on the produced geranyl glucoside concentration were measured during parallel biotransformations in milliliter-scale stirred-tank bioreactors. The fatty acid ester isopropyl myristate emerged as the best choice due to its low viscosity, very poor water solubility, low price and compatibility with the whole-cell biocatalyst. The biphasic system containing 20% (v/v) of this solvent boosted geranyl glucoside production (4.2-fold increase of geranyl glucoside concentration in comparison to aqueous system) and exhibits advantageous partitioning of geraniol into the organic phase (logD of 2.42±0.03) and of geranyl glucoside into the water phase (logD of −2.08±0.05). The systematic selection of a suitable biphasic system will serve as basic groundwork for the development of new bioprocesses involving geraniol. Moreover, this study can serve as a guideline for selecting sequestering phases for other whole-cell biotransformation processes.