Proteins adsorb strongly on most surfaces and the process involved have been extensively studied. Broadly speaking, they can adsorb onto hydrophobic surfaces via their hydrophobic amino acid residues. Since, in soluble proteins, the hydrophobic residues will be organized into the interior of the protein, adsorption on to hydrophobic surfaces generally requires denaturation of the protein and a considerable rearrangement of its structure. This process will generally be irreversible. The adsorption of water soluble proteins on to hydrophilic surfaces will be driven by the interaction of the hydrophilic groups on the outside of the protein and may require little or no reorganization of the protein, i.e. the protein does not necessarily denature. However, it may denature and the process may take some time. once denatured it may become difficult or impossible for it to desorb, in which case the adsorption is effectivly irreversible.
Any surface coming into contact with a biological system will rapidly be coated with a layer of protein. However, the composition of the surface will depend on the rate of adsorption of different proteins from the mixture in the system and any cooperativity in the adsorption of pairs of proteins. At short times of exposure there will also be the possibility of displacement of one protein by another and this will obviously be sensitive to any changes in the biological environment. To add to the complexity, one can expect further protein to be adsorbed on to the outside layers of the system. From the point of view of assessing the possible biological impact of nanoparticles, it is important to be able to follow and understand how the process of protein coating evolves and particularly to determine the final state of the surface. This task constitutes amsall part of an EU funded project called Nanointeract. We are using neutron reflectometry from model flat surfaces to probe the elements of the processes outlined above, e.g. protein self-exchange, and cooperative and competitive adsorption from protein mixtures.