Structure of bovine serum albumin adsorbed on silica investigated by quartz crystal microbalance

• QCM-D indicate that pH strongly affect the effectiveness of BSA adsorption on silica surface.
• Contact angle values are very sensitive to the structure of BSA monolayers formed on the silica surface.
• AFM method revealed various BSA structures depending on the pH at which they were adsorbed.
• Conformational changes of BSA are reflected in the contact angle and the thickness of the protein layers.

Adsorption of bovine serum albumin (BSA) on the surface of SiO2 was investigated mainly using quartz crystal microbalance (QCM) with energy dissipation mode. BSA adsorption kinetics and coverage were determined as a function of pH of the solution with the ionic strength of 1 × 10−2 M. The maximum coverage was obtained in the pH range 4.5–5.4, near the protein isoelectric point. For pH larger than 5.4 and smaller than 4.5 a monotonic decrease of the adsorbed mass was observed. The images obtained using atomic force microscopy (AFM) method revealed various BSA structures depending on the pH at which they were adsorbed. The identified conformations were assigned to the compact triangular form with an effective length Lef = 8.3 nm and the elongated one with length Lef = 26.7 nm. The dependence between the structure of the protein layers formed on the surface and BSA conformations in solution was additionally followed by contact angle (CA) measurements. It turned out that CA values are very sensitive to the structure of BSA monolayers formed on the silica surface. Comparison of QCM and AFM data with those obtained by other indirect methods (CA) additionally enabled interpretation of the dependence of BSA adsorption on the pH of the solution. The obtained results confirmed the significant role of the highly anisotropic surface charge distribution of BSA molecules on the process of their adsorption.

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