The elutability of fibrinogen by sodium dodecyl sulphate and alkyltrimethylammonium bromides

Abstract

The elutability of adsorbed fibrinogen by cationic surfactants of different chain lengths (dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, cetyltrimethylammonium bromide), and an anionic surfactant (sodium dodecyl sulphate (SDS)) was studied using in situ ellipsometry. The concentrations of the surfactants were twice the CMC in water and for fibrinogen, 0.4 mg ml −1. The investigation was carried out for two model surfaces: methylated silica (hydrophobic) and silica (hydrophilic and negatively charged, at pH 7). As a complement, a surface with a gradient in surface hydrophobicity was used. The end points of the gradient are similar to the methylated silica and silica surfaces with respect to hydrophobicity. All the surfactants adsorbed on the methylated silica surfaces, whereas only the cationic surfactants adsorbed on the silica surface. The adsorption of fibrinogen was 0.64 ± 0.03 μg cm −1 and 0.35 ± 0.03 μg cm −2 on the methylated silica and silica surfaces, respectively. Addition of surfactant led to a decrease in the amount of fibrinogen adsorbed on the methylated silica surface for all the surfactants, but only SDS affected the amounts adsorbed on the silica surfaces to any great extent. Despite the fact that the cationic surfactants adsorbed onto the silica surface, they did not affect the amount of fibrinogen adsorbed. The removal of protein decreased for the alkyltrimethylammonium bromides with increasing hydrophilicity of the gradient surfaces, and the amount of fibrinogen remaining after surfactant treatment decreased slightly for SDS. The effect of the chain length of the surfactant on elutability was small. The rate of removal of fibrinogen by the surfactants was found to be slower for SDS compared with the alkyltrimethylammonium bromides at the methylated silica surface, and at the hydrophobic end and in the intermediate part of the gradient. Adsorption from mixtures of surfactant and fibrinogen was also studied and the effects of cationic and anionic surfactants were quite different. The adsorption of fibrinogen was increased in the presence of the cationic surfactants, especially on the silica surface, but decreased in the presence of SDS. As surfactant adsorption onto clean surfaces is reversible with respect to dilution it might be assumed that the adsorbate mainly consists of fibrinogen. A trend was observed for the amounts of fibrinogen remaining after rinsing with buffer; the amounts increased with decreasing length of the surfactant hydrocarbon chain.