Revealing fibrinogen monolayer conformations at different pHs: electrokinetic and colloid deposition studies

Self-assembled fibrinogen–fibronectin hybrid protein nanofibers with medium-sensitive stability

Hybrid protein nanofibers (hPNFs) have been identified as promising nano building blocks for numerous applications in nanomedicine and tissue engineering. We have recently reported a nature-inspired, self-assembly route to create hPNFs from human plasma proteins, i.e., albumin and hemoglobin. However, it is still unclear whether the same route can be applied to other plasma proteins and whether it is possible to control the composition of the resulting fibers. In this context, to further understand the hPNFs self-assembly mechanism and to optimize their properties, we report herein on ethanol-induced self-assembly of two different plasma proteins, i.e., fibrinogen (FG) and fibronectin (FN). We show that by varying initial protein ratios, the composition and thus the properties of the resulting hPNFs can be fine-tuned. Specifically, atomic force microscopy, hydrodynamic diameter, and zeta potential data together revealed a strong correlation of the hPNFs dimensions and surface charge to their initial protein mixing ratio. The composition-independent prompt dissolution of hPNFs in ultrapure water, in contrast to their stability in PBS, indicates that the molecular arrangement of FN and FG in hPNFs is mainly based on electrostatic interactions. Supported by experimental data we introduce a feasible mechanism that explains the interactions between FN and FG and their self-assembly to hPNFs. These findings contribute to the understanding of dual protein interactions, which can be beneficial in designing innovative biomaterials with multifaceted biological and physical characteristics.

Hybrid protein nanofibers (hPNFs) have been identified as promising nano building blocks for numerous applications in nanomedicine and tissue engineering.

Microparticle Deposition on Human Serum Albumin Layers: Unraveling Anomalous Adsorption Mechanism

Human serum albumin (HSA) layers are adsorbed on mica under controlled diffusion transport at pH 3.5 and various ionic strengths. The surface concentration of HSA is directly determined by AFM imaging of single molecules. It is shown that the adsorption kinetics derived in this way is quantitatively described using the random sequential (RSA) adsorption model. The electrokinetic characteristics of the HSA layers at various pHs comprising their zeta potential are acquired in situ while using the streaming potential method. It is shown that at pH 3.5 the zeta potential of mica becomes positive for HSA concentrations above 3000 μm−2. At larger pHs, HSA layers exhibit negative zeta potential for the entire range of coverage. Thorough characteristics of these monolayers at various pHs were performed applying the colloid deposition method involving negatively charged polystyrene microparticles. The kinetics of their deposition and their maximum coverage are determined as a function of the HSA layer surface concentration, pH, and ionic strength. An anomalous deposition of microparticles on substrates also exhibiting a negative zeta potential is observed, which contradicts the Derjaguin, Landau, Vervey, Overbeek (DLVO) theory. This effect is interpreted in terms of heterogeneous charge distribution that results from molecule concentration fluctuations. It is also shown that the maximum concentration of microparticles abruptly decreases with the electric double-layer thickness that is regulated by changing ionic strength, which indicates that their deposition is governed by electrostatic interactions. One can argue that the results obtained in this work can be exploited as useful reference data for the analysis of deposition phenomena of bioparticles on protein layers.

Silver nanoparticle/fibrinogen bilayers - Mechanism of formation and stability determined by in situ electrokinetic measurements

The kinetics of negatively charged silver nanoparticle (AgNP) deposition on the supporting fibrinogen monolayers of well-characterized coverage was determined by the atomic force microscopy (AFM). The kinetics was quantitatively interpreted in terms of the hybrid random sequential adsorption model. The electrokinetic properties of the fibrinogen monolayers and fibrinogen/AgNP bilayers were thoroughly characterized in situ by the streaming potential measurements. These results were interpreted in terms of the general electrokinetic model expressing the particle coverage in terms of the zeta potential of the bilayers. This allowed one to determine the adsorption constants and the binding energy of AgNPs, which was equal to -20.8 and -21.3 kT for pH 3.5 and 7.4, respectively. These results confirmed the end-on mechanism of fibrinogen adsorption and the presence of positively charged spots at its molecule at pH 7.4 where it exhibits an average negative charge. Besides significance to basic science, the obtained results can be exploited for developing a procedure for producing AgNP monolayers of well-defined coverage and controlled particle release profile.

Revealing deposition mechanism of colloid particles on human serum albumin monolayers

Colloid particle deposition was applied in order to characterize human serum albumin (HSA) monolayers on mica adsorbed under diffusion transport at pH 3.5. The surface concentration of HSA was determined by a direct AFM imaging of single molecules. The electrokinetic characteristics of the monolayers for various ionic strength were done by in situ streaming potential measurements. In this way the mean-field zeta potential of monolayers was determined. It was shown that the initially negative potential changed its sign for HSA surface concentrations above 2800μm(-2) that was interpreted as overcharging effect. The monolayers were also characterized by the colloid deposition method where negatively charged polystyrene particles, 810nm in diameter were used. The kinetics of particle deposition and their maximum coverage were determined as a function of the HSA monolayer surface concentration. An anomalous deposition of particles on substrates exhibiting a negative zeta potential was observed, which contradicts the mean-field theoretical predictions. This effect was quantitatively interpreted in terms of the random site sequential adsorption model. It was shown that efficient immobilization of particles only occurs at adsorption sites formed by three and more closely adsorbed HSA molecules. These results can be exploited as useful reference data for the analysis of deposition phenomena of bioparticles at protein monolayers that has practical significance for the regulation of the bioadhesive properties of surfaces.