Quantitative Affinity of Genetically Engineered Repeating Polypeptides to Inorganic Surfaces

Binding kinetics of platinum-, silica-, and gold-binding peptides were investigated using a modified surface plasmon resonance spectroscopy (SPR). Platinum binding septa-peptides, quartz-binding dodecapeptides, and gold-binding 14-aa peptides were originally selected using phage or cell surface display libraries using the mineral or pure forms of these materials. All of the peptides were synthesized singly to investigate their binding kinetics and to assess quantitatively the specific affinity of each to its material of selection. The peptides were also postselection engineered to contain multiple copies of the same original sequences to quantify the effects of repeating units. SPR spectroscopy, normally using gold surfaces, was modified to contain a thin film (a few nm thick) of the material of interest (silica or platinum) on gold to allow the quantitative study of the adsorption kinetics of specific solid-binding peptides. The SPR experiments, carried out at different concentrations, on all three materials substrates, resulted in Langmuir behavior that allowed the determination of the kinetic parameters, including adsorption, desorption, and equilibrium binding constants for each of the solids as well as free energy of adsorption. Furthermore, we also tested multiple repeats of the peptide sequences, specifically three repeats, to see if there is a general trend of increased binding with increased number of binding domains. There was no general trend in the binding strength of the peptides with the increase of the repeat units from one to three, possibly because of the conformational changes between the single and multiple repeat polypeptides. In all cases, however, the binding was strong enough to suggest that these inorganic binding peptides could potentially be used as specific molecular linkers to bind molecular entities to specific solid substrates due to their surface recognition characteristics.