Modeling the Cooperative Adsorption of Solid-Binding Proteins on Silica: Molecular Insights from Surface Plasmon Resonance Measurements

Abstract

Combinatorially selected solid-binding peptides (SBPs) provide a versatile route for synthesizing advanced materials and devices, especially when they are installed within structurally or functionally useful protein scaffolds. However, their promise has not been fully realized because we lack a predictive understanding of SBP-material interactions. Thermodynamic and kinetic binding parameters obtained by fitting quartz crystal microbalance and surface plasmon resonance (SPR) data with the Langmuir model whose assumptions are rarely satisfied provide limited information on underpinning molecular interactions. Using SPR, we show here that a technologically useful SBP called Car9 confers proteins to which is fused a sigmoidal adsorption behavior modulated by partner identity, quaternary structure, and ionic strength. We develop a two-step cooperative model that accurately captures the kinetics of silica binding and provides insights into how SBP−SBP interactions, fused scaffold, and solution conditions modulate adsorption. Because cooperative binding can be converted to Langmuir adhesion by mutagenesis, our approach offers a path to identify and to better understand and design practically useful SBPs.

The role of human serum and solution chemistry in fibrinogen peptide–nanoparticle interactions

“…The weakened binding as sodium chloride concentrations increase is indicative of an electrostatic peptide-NP interaction. 40 Zeta potential results clearly indicated the silica NP is negatively charged. The GF peptide did not signicantly shi the zeta potential; yet the one arginine and two lysines in the peptide sequence, as opposed to only one negative glutamate, indicate the peptide is slightly positively charged (calculated charge at pH 7.4 ¼ 1.8).…”

“…56 The assumption that the GF peptide binds in a monolayer and reversibly to silica NPs is consistent with previous peptide-silica NP studies. 22,24,40 Experiments were designed to ensure samples were fully equilibrated by collecting data aer a 2 h incubation. Fluorescence anisotropy measurements at shorter time points proved noisier (Fig.…”

“…Although anisotropy techniques do not provide detailed insight into the binding mechanism, these results, combined, are consistent with other studies of similar peptides binding to silica NPs via positively charged amino acids and hydrogen bonds. 24,[39][40][41] In these uorescence anisotropy binding studies, measurable dissociation constants were obtained at HS concentrations 500Â lower and sodium chloride concentrations 5Â lower than that in the bloodstream of a healthy adult. Although we studied binding at more typical physiological sodium chloride concentrations, 145 mM, the binding prole no longer conforms to a Langmuir adsorption model.…”

“…39 In studies of silica NPspeptide interactions, NP size and functionalization mediate binding, as do the peptide biophysical properties. 22,24,[39][40][41] A comparison of positively, negatively, and neutrally charged peptides identies electrostatic, hydrophobic, and hydrogen bonding interactions at the aqueous silica interface. 39,41,42 These interactions are dominated by electrostatics as the silica NP size increases.…”

“…22,24,39 Thus, changes in pH that disrupt or charge H-bonds are expected to alter binding, 22,39 while increases in sodium chloride may shield charged interactions and weaken peptide-silica NP complexes. 24,40 On the other hand, the glucose concentrations we chose to study are higher than physiological relevance to…”