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Adsorption behavior of acidic and basic proteins onto citrate-coated Au surfaces correlated to their native fold, stability, and pI.

The adsorption of eight different proteins (alpha-lactalbumin (types I and III), bovine serum albumin, hemoglobin, myoglobin, cytochrome c, alpha-casein, and lysozyme) onto a model anionic surface was performed at equivalent bulk (solvent, ionic strength, pH) and surface conditions. Adsorption was monitored on a quartz crystal microbalance with dissipation monitoring (QCM-D) with citrate-coated gold surfaces as adsorbents and has been correlated to native fold stability determined from near- and far-UV circular dichroism (CD) measurements. The proteins studied here were chosen based on their pI and documented knowledge about their structural stability and flexibility. Protein adsorption was found to be independent of global protein charge. Rather, binding occurs through oppositely charged patches on protein and surface. Moreover, data indicate that there is a correlation between secondary and tertiary structure stability and the adsorption characteristics at interfaces. Also, protein surface coverage, layer thickness, and flexibility can be tuned as a function of deposition method. This is discussed in terms of adsorption/spreading kinetics and intermolecular (protein-surface and protein-protein) interactions. Adsorption to surfaces can induce formation of supramolecular structures such as micelles (in the case of alpha-Cas) and multilayers (as for Hb). In the case of alpha-casein, this phenomenon depends on the deposition method and protein concentration. When ranking the surface coverage for proteins added in excess, the order is Lyz < Cyt c < Mb < BSA < alpha-La I < alpha-Cas < alpha-La III < Hb, which can be correlated to the proteins ability to form supramolecular structures (alpha-Cas, Hb), overall conformational flexibilities, and ability to form stable intermediates.