Benchmark experimental data set and assessment of adsorption free energy for peptide-surface interactions

Abstract

With the increasing interest in protein adsorption in fields ranging from bionanotechnology to biomedical engineering, there is a growing need to understand protein-surface interactions at a fundamental level, such as the interaction between individual amino acid residues of a protein and functional groups presented by a surface. However, relatively little data are available that experimentally provide a quantitative, comparative measure of these types of interactions. To address this deficiency, the objective of this study was to generate a database of experimentally measured standard state adsorption free energy (DeltaGoads) values for a wide variety of amino acid residue-surface interactions using a host-guest peptide and alkanethiol self-assembled monolayers (SAMs) with polymer-like functionality as the model system. The host-guest amino acid sequence was synthesized in the form of TGTG-X-GTGT, where G and T are glycine and threonine amino acid residues and X represents a variable residue. In this paper, we report DeltaGoads values for the adsorption of 12 different types of the host-guest peptides on a set of nine different SAM surfaces, for a total of 108 peptide-surface systems. The DeltaGoads values for these 108 peptide-surface combinations show clear trends in adsorption behavior that are dependent on both peptide composition and surface chemistry. These data provide a benchmark experimental data set from which fundamental interactions that govern peptide and protein adsorption behavior can be better understood and compared.

Figure 1

Response curves (SPR signal (RU) vs. time for TGTG-L-GTGT on (A) SAM-CH₃ and (B) SAM-OH surface. (Not all of the concentration curves are listed for clarity sake because some of the low concentration curves overlap one another and are thus not separately distinguishable).

Figure 2

Corresponding adsorption isotherm for TGTG-L-GTGT on both of SAM-OH solid line SAM-CH₃ surfaces (dotted line). Note that the adsorption response plotted on the y-axis includes bulk-shift effects, which are linearly related to solution concentration. (Error bar represents 95% C.I., N = 6.)

Figure 3

ΔG^o_ads (kcal/mol) vs. cosine (contact angle) for TGTG-X-GTGT on SAM surfaces with various functionalities. The ΔG^o_ads values represent the average value of all of the host-guest peptides that exhibited reversible adsorption behavior on each SAM surface (i.e., peptides with X = A, F, and V, which tended to adsorb irreversibly, were excluded from these average values). The blue line shows the linear regression for the non-charged SAM surfaces with r²=0.95. (The error bar represents the 95% C.I. with N = 9.)

Figure 4

Comparisons of ΔG^o_ads for each peptide on the hydrophobic surfaces (water contact angle > 65°): SAM–CH₃, SAM–OCH₂CF₃ and SAM–OC₆H₅ surfaces. The amino acid label across the top of each set of columns designates the X residue of the TGTG-X-GTGT peptide. Peptides are ordered by a standard hydrophobicity scale from the most-to-least degree of hydrophobicity. (An asterisks (*) indicates that adsorption was irreversible. The error bars represent the 95% C.I. with N = 6.)

Figure 5

Comparisons of ΔG^o_ads for each peptide on neutrally charged relatively hydrophilic surfaces (water contact angle < 65°): SAM–COOCH₃, SAM–NHCOCH₃, and SAM–EG₃OH surfaces. The amino acid label across the top of each set of columns designates the X residue of the TGTG-X-GTGT peptide. The SAM-OH surface is not included in this plot because ΔG^o_ads = 0.0 for each reversibly adsorbed peptide on this surface. (An asterisks (*) indicates that adsorption was irreversible. The error bar represent the 95% C.I. with N = 6.)

Figure 6

Comparisons of ΔG^o_ads for each peptide on the charged surfaces: SAM–COOH and SAM–NH₂ surfaces. The amino acid label across the top of each set of columns designates the X residue of the TGTG-X-GTGT peptide. An asterisks (*) indicates that adsorption was irreversible. The error bar represent the 95% CI. with N = 6.)