Interaction between sodium n-dodecyl sulphate and chemically modified bovine catalases

Abstract

A series of catalases have been prepared in which a proportion of the carboxyl groups of glutamate and aspartate residues have been amidated with glycinamide. The physical properties of the amidated catalases have been investigated with specific reference to their interaction with sodium n-dodecyl sulphate (SDS). Amidation leads to an increase in SDS binding at pH 6.4. Microcalorimetric measurements show that the exothermic enthalpy of interaction with SDS increases with the extent of amidation in acid solution (pH 3.2–6.4). The increase in exothermicity is compensated by a decrease in entropy since the average Gibbs energy of SDS binding is independent of the extent of amidation. At pH 3.2 where the catalase carboxyl groups are largely un-ionized amidation still increase the exothermicity of the interaction with SDS. It is suggested that at low pH the SDS anion interacts favourably with the resonance stabilized O-protonated form of amidated side chains.