Making sense of adsorption : Attempting to explain the adsorption of histatin 5 with models, metaphors, and machines

Abstract: This thesis summarises two studies in which the main purpose was to find out how and why the amount of the protein histatin 5 that adsorbs to negatively charged surfaces changes with pH and ionic strength. Histatin 5 is an intrinsically disordered saliva protein, and in the oral environment it adsorbs to tooth enamel. The approach here is to simplify the biological system by using an aqueous buffer solution instead of saliva, and a silica surface to represent the tooth enamel. Ellipsometry measurements were performed in order to answer the question of how the adsorbed amount changes with pH and ionic strength. Coarse-grained Monte Carlo simulations were used to investigate the molecular mechanisms behind the adsorption of histatin 5 and try to elucidate why the experiments give certain trends. The obtained results could have implications for the understanding of the role of histatin 5 in the oral environment, as well as for fundamental understanding of the adsorption of flexible proteins or polyelectrolytes.The main take-home messages are the following: (i) the adsorbed amount of histatin 5 changes with ionic strength – but the trends are different depending on the pH of the solution, (ii) the change in surface charge with pH and ionic strength can strongly affect the adsorbed amount, (iii) the electrostatic interactions between charged groups are not enough to account for the experimentally observed adsorption of histatin 5 to silica surfaces, and (iv) the coarse-grained model used in these studies cannot explain the experimentally observed pH-dependence of the adsorbed amount as a function of ionic strength. However, the simulations cast light on the balance between the electrostatic attraction between the protein and the surface and the electrostatic repulsion between adsorbed proteins, the deficiencies of the Langmuir isotherm, and the implications of protein charge regulation for adsorption.