Surface Chemistry Influences Interfacial Fibrinogen Self-Association

Surface chemistry modifications have been exploited in many applications in order to tune protein adsorption, layer formation, and aggregation. However, the kinetic processes by which surface chemistry influences protein adsorption and aggregation remain elusive. By combining intermolecular resonance energy transfer (RET) with high-throughput single-molecule tracking, we compared the dynamics of fibrinogen (Fg) interfacial self-associations on surfaces modified with hydrophobic trimethyl silane (TMS) or hydrophilic oligoethylene glycol (OEG). We directly observed interfacial, dynamic, and reversible Fg-Fg associations from low-RET (unassociated) to high-RET (associated) states. While isolated Fg molecule-TMS surface interactions were weaker than isolated Fg-OEG interactions, increasing protein concentration resulted in a more dramatic decrease in desorption from TMS than from OEG, such that at higher concentrations, Fg desorbed from TMS more slowly than from OEG. In addition to this observation, unassociated molecules were more likely to associate on TMS than on OEG, suggesting that the TMS surface promoted protein-protein associations. Importantly, increasing protein concentration also resulted in a greater increase in the length of time proteins remained associated (i.e., contact times) on TMS than on OEG, such that contact times were longer on TMS than on OEG at higher concentrations but shorter at low concentration, mirroring the behavior of the overall surface residence times. These findings strongly suggest that surface chemistry not only influences protein-surface interactions but can also promote interfacial aggregation on one surface (hydrophobic TMS) relative to another (hydrophilic OEG), and that the latter may well be the more important factor at higher surface coverage.