Protein adsorption from seawater onto solid substrata: II. Behavior of bound protein and its influence on interfacial properties

Modification of solid surfaces (e.g. minerals, biogenic debris, engineered materials, etc.) and exchanges across their interfaces in seawater can be influenced by the composition and behavior of adsorbed solutes, such as proteins. The present study illustrates that the degrees to which extracellular proteins adsorb from seawater solutions, denature in the bound state, after the wettability of the interface, and impede dissolution and oxidation of the substratum strongly depend on protein concentration and substratum type. For all variables measured, a transition in state of the interface is evident within the surface concentration range required to establish a monomolecular layer. The plant enzyme, ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO), was used to examine the behavior of protein adsorbed to well-defined titanium and copper surfaces in seawater. Secondary structure of proteins in thin films on Ti were most altered while those in thicker films and in Cu-bound films more closely resembled native protein as indicated by relative intensities of amide II and I, vibrational frequency shifts, and amide III features in spectra obtained by Fourier Transform Infrared Reflectance-Absorbance Spectrometry (FT-IRAS). Critical surface tension (CST) of Ti oxides was significantly reduced in the presence of very thin films, but was enhanced on coated Cu oxide surfaces. Differences in interfacial CST reflect a higher degree of denaturation upon adsorption onto Ti oxides than onto Cu oxides, i.e. greater exposure of hydrophobic protein residues on less wettable substrata. Potential substratum oxidation rates and surface oxide accumulation derived from Electrochemical Impedance Spectroscopy (EIS) and ellipsometric measurements, respectively, were inversely related to protein surface concentration. Results consistently illustrate that substratum composition and protein concentration control film organization, protein denaturation and interfacial attributes which are manifested as interfaces with varying wetting properties and as solid phases with varying susceptibility to oxidation and dissolution.