Adsorption, retention and biologic activity of proteins adsorbed on gas discharge treated surfaces

Surface-MALDI mass spectrometry in biomaterials research

Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) has been used for over a decade for the determination of purity and accurate molecular masses of macromolecular analytes, such as proteins, in solution. In the last few years the technique has been adapted to become a new surface analysis method with unique capabilities that complement established biomaterial surface analysis methods such as XPS and ToF-SSIMS. These new MALDI variant methods, which we shall collectively summarize as Surface-MALDI-MS, are capable of desorbing adsorbed macromolecules from biomaterial surfaces and detecting their molecular ions with high mass resolution and at levels much below monolayer coverage. Thus, Surface-MALDI-MS offers unique means of addressing biomaterial surface analysis needs, such as identification of the proteins and lipids that adsorb from multicomponent biological solutions in vitro and in vivo, the study of interactions between biomaterial surfaces and biomolecules, and identification of surface-enriched additives and contaminants. Surface-MALDI-MS is rapid, experimentally convenient, overcomes limitations in mass resolution and sensitivity of established biochemical techniques such as SDS-PAGE, and can in some circumstances be used for the quantitative analysis of adsorbed protein amounts. At this early stage of development, however, limitations exist: in some cases proteins are not detectable, which appears to be related to tight surface binding. This review summarizes ways in which Surface-MALDI-MS methods have been applied to the study of a range of issues in biomaterials surfaces research.

Covalent attachment and non-specific binding of reactive probe molecules onto surfaces

Optimization of strategies for the covalent attachment of proteins onto polymer surfaces requires the development of analytical methods which can differentiate between proteins that are covalently attached and those that are non-covalently bound (physisorbed). We probed for the surface density of reactive amine, carbonyl, and hydrazide groups using solution phase derivatization reactions to mimic and explore protein immobilization reaction strategies. Labeling compounds investigated were fluorescein derivatives, which were quantified by adsorption spectroscopy, and fluorinated phenyl compounds which were quantified by XPS. Control experiments consisted of performing the same labeling reactions using surfaces without reactive groups, or immersing the polymer surface into the labeling solution after blocking the reactive group of the labeling compound by a covalent reaction in solution. We always found non-negligible contributions arising from physisorption of the derivatization labels. Multiple control surfaces and a novel 'crossover derivatization-XPS' method were studied with the aim of improving compensation for physisorption. Our documentation of surprisingly large physisorption components even for small molecule labels, together with the known propensity of proteins to adsorb onto polymers, suggests caution in quantitative analysis of surface groups by derivatization, and in interpreting covalent protein immobilizations onto polymeric surfaces.