Physicochemical and conformational studies of papain/sodium dodecyl sulfate system in aqueous medium

Interaction between a globular protein, papain and the anionic surfactant, sodium dodecyl sulfate (SDS) has been studied in aqueous medium in detail using conductometric, tensiometric, calorimetric, fluorimetric, viscometric, circular dichroism techniques. The physicochemical properties, e.g. critical micellar concentration (CMC), counterion binding, free energies, enthalpies and entropy of micellization, interfacial adsorption, micellar aggregation number and micellar polarity of SDS have been determined in presence of papain. The results show that the CMC values of SDS increase with the increasing concentration of papain. The energetics of micellization of papain–SDS system is endothermic and the interaction of SDS with papain is an entropy controlled process. Such physicochemical studies in presence of protein are rare. Also, the conformational behavior of papain in aqueous solution has been investigated in the presence of SDS. The results show the high α-helical content and unfolded structure of papain in the presence of SDS due to strong electrostatic repulsion.