Interfacial energetics of protein adsorption from aqueous buffer to surfaces with varying hydrophilicity

Adsorption isotherms constructed from time-and-concentration-dependent advancing contact angles thetaa show that the profound biochemical diversity among ten different blood proteins with molecular weight spanning 10-1000 kDa has little discernible effect on the amount adsorbed from aqueous phosphate-buffered saline (PBS) solution after 1 h contact with a particular test surface selected from the full range of observable water wettability (as quantified by PBS adhesion tension tauoa=gammaolv cos thetaoa; where gammaolv is the liquid-vapor interfacial tension and thetaoa is the advancing PBS contact angle). The maximum advancing spreading pressure, Pimaxa, determined from adsorption isotherms decreases systematically with tauoa for methyl-terminated self-assembled monolayers (CH3 SAM, tauo=-15 mN/m), polystyrene spun-coated onto electronic-grade SiOx wafers (PS, tauo=7.2 mN/m), aminopropyltriethoxysilane-treated SiOx surfaces (APTES, tauo = 42 mN/m), and fully water wettable SiOx (tauo=72 mN/m). Likewise, the apparent Gibbs' surface excess [Gammasl-Gammasv], which measures the difference in the amount of protein adsorbed Gamma (mol/cm2) at solid-vapor (SV) and solid-liquid (SL) interfaces, decreases with tauo from maximal values measured on the CH3 SAM surface through zero (no protein adsorption in excess of bulk solution concentration) near tauo=30 mN/m (thetaa=65 degrees). These latter results corroborate the conclusion drawn from independent studies that water is too strongly bound to surfaces with tauo>or=30 mN/m to be displaced by adsorbing protein and that, as a consequence, protein does not accumulate within the interfacial region of such surfaces at concentrations exceeding that of bulk solution ([Gammasl-Gammasv]=0 at tauo=30 mN/m). Results are collectively interpreted to mean that water controls protein adsorption to surfaces and that the mechanism of protein adsorption can be understood from this perspective for a diverse set of proteins with very different amino acid compositions.