BOVINE SERUM-ALBUMIN ADSORPTION ONTO IMMOBILIZED ORGANOTRICHLOROSILANE SURFACE - INFLUENCE OF THE PHASE-SEPARATION ON PROTEIN ADSORPTION PATTERNS

Octadecyltrichlorosilane silane (OTS) and [2-(perfluorooctyl)ethyl] tr ichlorosilane (FOETS) monolayers and their mixed monolayer were polyme rized on a water subphase and subsequently immobilized onto a silicon wafer surface by covalent bonding. Atomic force microscopic (AFM) obse rvation of the mixed (OTS/FOETS) monolayer revealed the formation of a phase-separated structure. Protein-adsorption behavior onto the monol ayers was investigated in situ on the basis of an attenuated total ref lection Fourier transform infrared (ATR-FT-IR) spectroscopic flow cell method and the morphology of the monolayer surface-adsorbed bovine se rum albumin (BSA) was observed by AFM. Protein adsorption behavior obs erved by ATR-FT-IR flow cell method revealed that the amount of BSA ad sorption onto the OTS and FOETS monolayers increased remarkably at an initial experimental stage and attained a steady state within a few mi nutes at pH 7.5. The amount of steady state adsorption was c. 0.18-0.2 mu g cm(-2). AFM observation of the monolayer after exposure to BSA s olution suggested that BSA adsorbed in the end-on adsorption state on OTS monolayer and side-on one in the FOETS monolayer, respectively. Ho wever, in the case of the mixed (OTS/FOETS) monolayer, ATR-FT-IR flow cell experiment revealed that the amount of steady state adsorption of BSA was suppressed. Also, AFM observation revealed that at pH 7.5, BS A preferentially adsorbed onto the FOETS phase of the mixed (OTS/FOETS ) monolayer, which had a higher interfacial free energy against water. On the other hand, BSA adsorbed homogeneously onto the OTS and FOETS phases at the isoelectric point of BSA (pH 4.7). These results indicat e that the preferential adsorption of BSA onto the FOETS phase in the mixed (OTS/FOETS) monolayer system is due to: (1) the minimization of the interfacial free energy between a monolayer surface and an aqueous solution; and (2) the electrostatic repulsion between BSA molecules b earing negative charges.