Protein adsorption Subject

It has become customary to refer to the action of surfaces on proteins as surface denaturation. This is an unfortunate term in some ways. First, it does not satisfy the requirements of a scientific term that needs to be simple and unambiguous. Second, it tends to discourage further interest in the subject whereas, in natural systems, proteins must be continually interacting with interfaces and undergoing changes in conformation and properties. It therefore seems preferable to examine protein adsorption, adhering to strict terminology in order to describe the processes which occur. 

The only authors to study the more relevant protein adsorption characteristics to date have been Ruan et al. [97], However, they observed neither qualitative nor quantitative differences in adsorption from 1 1 mixed parotid -submandibular saliva samples, from 6 caries-free and 6 caries-active subjects, onto powdered enamel and cementum. The small number of samples compared may be the reason for a lack of discrimination. The authors did find that much more proline-rich protein and cysteine-containing protein were adsorbed to enamel than to cementum though they could not explain why. 

Surveying the literature, it appears that the interfacial behavior of proteins is a controversial subject. The main reason is that many studies have been performed under insufficiently defined conditions and/or that conclusions have been drawn on the basis of too scanty experimental evidence. Furthermore, the theoretical description of adsorbed layers of simple, flexible polymers is still in its infancy (5,6). As the structure of proteins is much more complex than that of those simple polymers, theories of polymer adsorption need to be greatly extended to become applicable to proteins. Clearly, our current knowledge of protein adsorption mechanisms and of the structure of the adsorbed layer is far from complete. 

Furthermore, on polystyrene it was found that the adsorption of HPA in the low surface coverage region increased with increasing temperature, except at the isoelectric point (i.e.p.) of the protein where the adsorption appeared to be independent of the temperature. According to Clapeyron s law a positive value for (6r/6T) implies an endothermic adsorption process under isosteric conditions. Although with protein adsorption isosteric conditions are difficult to establish, the qualitative conclusion is that at pHf i.e.p. the adsorption enthalpy is positive. Hence, under those conditions, adsorption must be entropically driven. We will return to this subject in section 5. 

Although the phenomenon of protein adsorption at the liquid/air and solid/liquid interfaces has been the subject of a large number of investigations during the past several years, the answer to the key questions what is the behavior of proteins at interfaces and why do proteins behave as they do at interfaces, remains unclear and further research effort has to be directed toward understanding of the mechanism of protein adsorption. In particular there is still a lack of direct experimental evidence on the organisation of various adsorbed protein layers and on their composition when protein adsorption takes place from multicomponent mixtures. 

Samples, one half coated with SiOa and the other half with Ti02, were used for quantitative surface analysis after each of the siuface treatment steps (cleaning, self-assembly, and polymer and protein adsorption, section 2). These samples exhibit material contrast on a macroscopic scale and are discussed in section 3.1. Micropat-temed surfaces were subjected to identical siuface modification procedures and characterized qualitatively by imaging ToF-SIMS (section 3.2) and fluorescence microscopy (section 3.3) and were used in the cell experiments (section 3.4). In both types of samples, material contrast (on a macroscopic or microscopic scale, Figure la) is converted into contrast with respect to protein adhesion (Figure Ic) via a series of surface modification steps (self-assembly of DDP, adsorption of PLL-g-PEG section 2). 

SAMs of alkanethiolates on gold provide excellent model systems for studies on interfacial phenomena (e.g., wetting, adhesion, lubrication, corrosion, nucleation, protein adsorption, cell attachment, and sensing). These subjects have been reviewed previously [125,183-185]. Here we focus on applications that involve using chemical synthesis of functional SAMs after their assembly. 

At extremely small separations, after coming into physical contact with the interface, coUoid particles and proteins are subject to surface deformation and reconformation processes, ion exchange, or specific chemical-type interactions, like hydrogen-bound formation or sintering. All of these time-dependent phenomena lead to particle immobilization (localized adsorption), which is responsible for the partial or total irreversibifity of colloid and protein adsorption. 

Before I move to the core of the subject matter, I would like to point out the fact that there are numerous models, developed by other investigators, which deal with protein adsorption equilibria [31-35]. 

Human serum albumin (HSA) may be used as a protectant against adsorptive loss of proteins present at low concentrations. HSA is present at higher concentration than the active substance and is preferentially adsorbed, coating the surface of interest and preventing adsorption of the drug. For example, insulin is subject to adsorptive loss to hydrophobic materials. Addition of 0.1-1.0% HSA has been reported to prevent this adsorptive loss [9],... 

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