A predictive thermodynamic model for the bioreduction of acetophenone to phenethyl alcohol using resting cells of Saccharomyces cerevisiae

Equilibrium conversions were observed in the range of 60.2-76.0% with different initial compositions of reaction media for the bioreduction of acetophenone using resting cells of Saccharomyces cerevisiae in aqueous solutions at 30°C. The reduction of acetophenone in the cells under anaerobic conditions is considered to be coupled with the oxidation of ethanol to acetate in the cytoplasm. A biphasic thermodynamic model is proposed which includes a nonuniform distribution of reagents across the cell membrane, a transmembrane pH gradient, ideal and nonideal solution models, and a basic reaction stoichiometry (ACP + 1/2 EtOH + 1/2 H₂O mutually implies PEA + 1/2 Ac^- + 1/2 H⁺). The intracellular activity coefficients were based on the Lewis-Randall rule for acetophenone, phenethyl alcohol, and H₂O and Henry's law for ethanol, acetate anion, and H⁺. The overall standard Gibbs free energy was estimated to be -0.11 kcal/mol at a pH 7, 25°C, and 1 atm. The intracellular thermodynamic activity coefficients of acetophenone and phenethyl alcohol were predicted to be 471.2 and 866.4, respectively, using the measured initial distribution coefficients and calculated extracellular activity coefficients. The model reflected a zero Gibbs free energy change at calculated conversions within 4% of the measured equilibrium conversions. The analysis verified the effect of the concentration ratio of the substrate acetophenone to the cosubstrate ethanol on the conversion efficiency and suggested that the intracellular pH and the pH gradient across the cell transmembrane significantly affect the predicted equilibrium conversion. The intracellular pH of resting, viable cells of Bakers' yeast at the bioconversion conditions was determined experimentally to be 5.77.