Thermodynamics of the conversion of penicillin G to phenylacetic acid and 6-aminopenicillanic acid.

The thermodynamics of the enzymatic conversion (penicillin acylase) of aqueous penicillin G to phenylacetic acid and 6-aminopenicillanic acid have been studied using both high-pressure liquid-chromatography and microcalorimetry. The reaction was carried out in aqueous phosphate buffer over the pH range 6.0-7.6, at ionic strengths from 0.10 to 0.40 mol kg-1, and at temperatures from 292 to 322 K. The data have been analyzed using a chemical equilibrium model with an extended Debye-Hückel expression for the activity coefficients. For the reference reaction, penicillin G- (aq) + H2O(l) = phenylacetic acid-(aq) + 6-aminopenicillanic acid-(aq) + H+ (aq), the following parameters have been obtained: K = (7.35 +/- 1.5) X 10(-8) mol kg-1, delta G0 = 40.7 +/- 0.5 kJ mol-1, delta H0 = 29.7 +/- 0.6 kJ mol-1, and delta C0p = -240 +/- 50 J mol-1 K-1 at 298.15 K and at the thermochemical standard state. The extent of reaction for the overall conversion is highly dependent upon the pH.