INTERACTION OF GLUCOSE-OXIDASE WITH IONIC SURFACTANTS - A MICROCALORIMETRIC STUDY

The enthalpies of interaction of glucose oxidase at 25-degrees-C with a homologous series of n-alkyltrimethylammonium bromides (TABs) at pH 10 and a homologous series of n-alkylsulfates at pH 3.2 have been meas ured by microcalorimetry. For the n-dodecyl member of each series, DTA B and sodium n-dodecylsulfate (SDS), the binding of the surfactants to glucose oxidase as measured by equilibrium dialysis has been used in combination with the enthalpy data to obtain the Gibbs energy (DELTAG( vBAR)), enthalpy (DELTAH(vBAR)) and entropy (DELTAS(vBAR)) of binding per surfactant molecule as a function of the number of surfactant mole cules bound (vBAR). The thermodynamic parameters for the glucose oxida se interaction with DTAB at pH 10 and SDS at pH 3.2 are very similar a nd show that the interactions are entropically driven. The observed en thalpies of interaction of glucose oxidase with the homologous n-alkyl sulfates have been analysed in terms of the interactions between the a nionic surfactant head group and cationic sites on the protein, hydrop hobic binding and the thermal contributions arising from protein unfol ding. At surfactant concentrations of 0.5 c.m.c., the enthalpy of unfo lding of glucose oxidase is estimated to be 3610 +/- 560 kJ mol-1.