Adsorption energetics of diverse proteins, spanning 3 orders-of-magnitude in molecular weight (MW), from aqueous-buffer solutions to variably hydrophilic solid-water (solid-liquid, SL) surfaces are studied via contact angle goniometry, a time-honored method critiqued in the first chapter. Whereas energetics observed for the most hydrophilic SL surface are remarkably similar to the hydrophobic water-air (liquid-vapor, LV) interface, decreasing adsorption is observed with increasing hydrophilicity of the adsorbent. In addition, increasing diversity amongst adsorbing proteins within the data set on each surface with incrementally increasing hydrophilicity is observed. However adsorption to each SL surface still follows the “Traube-rule-like” progression first observed for proteins at the LV interface and therefore bulk molar concentration required to reach a given spreading pressure decreases with increasing MW.
A relatively straightforward postulate of protein adsorption, predicated on the interfacial packing of hydrated spherical molecules with dimensions scaling as a function of MW, accounts for the essential physical chemistry of protein adsorption and rationalizes significant experimental observations. From this theory, it is evident that displacement of interfacial water by hydrated proteins adsorbing from solution places an energetic cap on protein adsorption to surfaces and this cap scales with hydrophilicity. This phenomenon is generic to all proteins. Whereas charged surfaces, for example, may display additional adsorptive modes unlike those for adsorbent-surfaces herein, the role of water in delimiting energetics is comparatively greater than protein-solute amphilicity for the proteins and surfaces observed. As a consequence, protein adsorption is not found to vary significantly among diverse protein types, a finding further supported by our observation of protein mixtures (plasma and serum of several mammalian species).
Collectively, observations including the “Traube-rule-like” ordering of interfacial tension (IFT) reduction and dependence of apparent Gibbs’ surface excess (quantitative measure of adsorption) on hydrophobicity of the adsorbents imply that water plays a dominant, controlling role in the adsorption of proteins to surfaces and that the mechanism of protein adsorption can be quite comprehensively understood from this perspective that solvent and not solute molecules are determiners of adsorption.
Cha, Paul A.
Pennsylvania State University