Lipase-surfactant interactions studied by neutron reflectivity and ellipsometry

The interaction between a microbial lipase and an anionic and a cationic su rfactant at the air-water interface has been studied by neutron reflectivit y. Sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TT AB) were used as anionic and cationic surfactant, respectively. The same en zyme-surfactant systems were also studied at the interface between a hydrop hobic solid surface and water by ellipsometry, and the results from the two techniques were compared. Surface tension measurements were also performed in order to monitor complex formation in bulk. The data obtained from neut ron reflectivity and from ellipsometry were in very good agreement with eac h other. Both techniques show that lipase adsorbs readily at the interfaces and that SDS at low concentration does not interact strongly with this pro tein layer. At higher SDS concentration, the protein is displaced from the surface. On the other hand, when TTAB is added at low concentration, a thic k lipase-surfactant layer is formed at the surfaces. This compact layer is solubilized by further addition of the cationic surfactant.