Adsorption of Fibrinogen on Silica Surfaces-The Effect of Attached Nanoparticles.

When a biomaterial is inserted into the body, proteins rapidly adsorb onto its surface, creating a conditioning protein film that functions as a link between the implant and adhering cells. Depending on the nano-roughness of the surface, proteins will adsorb in different amounts, with different conformations and orientations, possibly affecting the subsequent attachment of cells to the surface. Thus, modifications of the surface nanotopography of an implant may prevent biomaterial-associated infections. Fibrinogen is of particular importance since it contains adhesion epitopes that are recognized by both eukaryotic and prokaryotic cells, and can therefore influence the adhesion of bacteria. The aim of this study was to model adsorption of fibrinogen to smooth or nanostructured silica surfaces in an attempt to further understand how surface nanotopography may affect the orientation of the adsorbed fibrinogen molecule. We used a coarse-grained model, where the main body of fibrinogen (visible in the crystal structure) was modeled as rigid and the flexible α C-chains (not visible in the crystal structure) were modeled as completely disordered. We found that the elongated fibrinogen molecule preferably adsorbs in such a way that it protrudes further into solution on a nanostructured surface compared to a flat one. This implicates that the orientation on the flat surface increases its bio-availability.

Integrating All-Atom and Coarse-Grained Simulations-Toward Understanding of IDPs at Surfaces.

We present a scheme for transferring conformational degrees of freedom from all-atom (AA) simulations of an intrinsically disordered protein (IDP) to coarse-grained (CG) Monte Carlo (MC) simulations using conformational swap moves. AA simulations of a single histatin 5 peptide in water were used to obtain a structural ensemble, which is reweighted in a CGMC simulation in the presence of a negatively charged surface. For efficient sampling, the AA trajectory was condensed using two approaches: RMSD clustering (based on the root-mean-square difference in atom positions) and a "naïve" truncation, where only every 100th frame of the trajectory was included in the library. The results show that even libraries with few structures well reproduce the radius of gyration and interaction free energy as functions of the distance from the surface. We further observe that the surface slightly promotes the secondary structure of histatin 5 and more so if using explicit surface charges rather than smeared charges.

Adsorption of polyelectrolyte-like proteins to silica surfaces and the impact of pH on the response to ionic strength. A Monte Carlo simulation and ellipsometry study.

HYPOTHESIS: The adsorbed amount of the polyelectrolyte-like protein histatin 5 on a silica surface depends on the pH and the ionic strength of the solution. Interestingly, an increase in ionic strength affects the adsorbed amount differently depending on the pH of the solution, as shown by ellipsometry measurements (Hyltegren, 2016). We have tested the hypothesis that the same (qualitative) trends can be found also from a coarse-grained model that takes all charge-charge interactions into account within the frameworks of Gouy-Chapman and Debye-Hückel theories. EXPERIMENTS: Using the same coarse-grained model as in our previous Monte Carlo study of single protein adsorption (Hyltegren, 2016), simulations of systems with many histatin 5 molecules were performed and then compared with ellipsometry measurements. The strength of the short-ranged attractive interaction between the protein and the surface was varied. FINDINGS: The coarse-grained model does not qualitatively reproduce the pH-dependence of the experimentally observed trends in adsorbed amount as a function of ionic strength. However, the simulations cast light on the balance between electrostatic attraction between protein and surface and electrostatic repulsion between adsorbed proteins, the deficiencies of the Langmuir isotherm, and the implications of protein charge regulation in concentrated systems.

Adsorption of the intrinsically disordered saliva protein histatin 5 to silica surfaces. A Monte Carlo simulation and ellipsometry study.

HYPOTHESIS: The adsorption of histatin 5 to hydrophilic silica surfaces is governed by electrostatic attractive forces between the positive protein and the negative surface. Hence pH and ionic strength control the adsorbed amount, which can be described by coarse-grained Monte Carlo simulations accounting for electrostatic forces and charge regulation of the protein. EXPERIMENTS: The amount of histatin 5 adsorbed to hydrophilic silica surfaces at different pH and ionic strengths was measured using null ellipsometry. The results were compared with coarse-grained Monte Carlo simulations of a single histatin 5 molecule and a surface with a fixed, smeared charge set according to experimental values for silica. The Langmuir isotherm was used to calculate the surface coverage from the simulation results. The effect of charge regulation of the protein was investigated. FINDINGS: Even though electrostatic attractive forces are important for the investigated system, a non-electrostatic short-ranged attraction with a strength of about 2.9kBT per amino acid was needed in the simulations to get surface coverages close to experimental values. The importance of electrostatics increases with increasing pH. Charge regulation of the protein affected the results from the simulations only at high surface charge and low ionic strength.