Elution process of adsorbed fibrinogen by SDS: Competition between removal and anchoring

The elution of proteins from various surfaces by surfactants was widely studied in the literature in connection with practical applications but also as a tool to understand protein adsorption behavior. In this paper we present the results obtained for the following system: fibrinogen adsorbed at a silica surface eluted by sodium dodecyl sulfate (SDS) solutions of various concentrations and at various ionic strengths. The process was followed by measuring the radioactivity of labeled protein. The elution kinetics was determined and described quantitatively by an apparent rate constant k and an elutability parameter. An unexpected nonmonotonic dependence of k as a function of SDS concentration c(SDS) was obtained for the first time. This nonmonotonic behavior of k was explained assuming double action of SDS molecules on the fibrinogen adsorption layer: SDS with its detergent ability removes adsorbed protein but, being at the same time a strong denaturing agent, it is able to transform adsorbed molecules into an irreversibly bound nonelutable form. The observed dependence of k on c(SDS) is then the result of competition between these two processes. Two-step experiments were performed in which the existence of the SDS-induced fastening of fibrinogen molecules to the surface was confirmed. A simple kinetic model was proposed in which two parameters related to the removal and anchoring subprocesses were introduced. The comparison between the experimental data and the model allowed us to get access to these parameters.