Enzymatic catalysis in cosolvent modified pressurized organic solvents

An important advantage of carrying out enzymatic catalysis in organic media is the increased solubility of hydrophobic substrates. This study compares a model lipase catalyzed esterification of cholesterol using vinyl acetate (VA) in two such nontraditional media: high-pressure hexane and supercriti cal (SCF) ethane. The effect of using one of the reactants (VA) as a cosolv ent to increase the solubility of the other reactant (cholesterol) in SCF e thane has been investigated. The thermodynamic activity of water (a(w)) in the reaction media was controlled by the direct addition of the salt hydrat e pair Na4P2O7/Na4P2O7. 10H(2)O. The a(w) of the salt hydrate system is sho wn to be a function of pressure and its variation over the pressure range 1 04-173 bar has been estimated. The initial reaction rate in pressurized hex ane was found to vary linearly with the cholesterol concentration. The reac tion rate was also a function of pressure-the effect being more pronounced in ethane than in hexane. This is consistent with the large negative partia l molar volumes observed in SCFs, although the sign of the resulting activa tion volume differs from previous investigations of lipase-catalyzed reacti ons in SCFs. When corrected for substrate concentration, the initial rate o f catalysis in SCF ethane was determined to be greater than in pressurized hexane over the conditions investigated. This study shows that proper solve nt choice can be used to regulate reaction rates in pressurized solvents. ( C) 1999 John Wiley & Sons, Inc.