Conjugation of an α-Helical Peptide to the Surface of Gold Nanoparticles

Abstract

We are interested in functionalizing gold nanoparticles (AuNPs) with proteins using a biomimetic approach in which an intermediate peptide "glue" directs the orientation of a protein relative to the AuNP surface. The first step toward that goal is described in this presentation. Attachment of electrochemically active proteins to electrode surfaces is necessary for the interrogation of electron transfer kinetics in biological systems, as well as for the development of enzyme biosensors. Passive adsorption and covalent linking are the two main strategies used for protein-electrode attachment. Passive adsorption is experimentally simple but rarely controls protein orientation or stability, resulting in random structural orientation, denaturation, and/or uncontrolled desorption of proteins. Covalent linking provides greater binding strength to the protein-surface pair but exposes the protein to multiple abiological reagents, can disrupt structure due to the single rigid attachment point, and does not necessarily lead to optimal orientation. One strategy designed to overcome these limitations has been developed by Webb and coworkers, which is the formation of a surface layer of α-helical peptides on planar gold surfaces. This approach aims to design a tunable surface which can mimic protein-protein interactions and specifically orient proteins via the protein-peptide interaction. In order to make this approach more general and applicable to electrodes of various materials, we have endeavored to functionalize gold nanoparticles (AuNPs) with structure-controlled α-helical peptides. Using noble metal NPs as intermediates in electrode-protein attachment schemes is attractive due to the high surface to volume ratio and tunable surface curvature of NPs. Also, once attached to the NP, many surfaces can be functionalized with a protein via attachment of the NP-protein conjugate. The first step toward the goal of AuNP-protein attachment via structure-controlled peptide is described in this presentation. Specifically, we show that ~5 nm AuNPs can be functionalized with a mixed self-assembled monolayer consisting of oligo (ethylene glycol) alkanethiols terminated with either hydroxyl or azide groups, and that the resulting materials are stable and soluble in water. The azide groups on the surface of the AuNPs can subsequently be linked to alkyne-functionalized peptides via a copper-catalyzed azide-alkyne cycloaddition (click) reaction. Analysis of the resulting material by FTIR and circular dichroism spectroscopy demonstrates that the peptide is covalently linked to the SAM and that it exists in an α-helical conformation. These structure-controlled peptide-attached AuNPs are potentially useful in protein-electrode attachment schemes as intermediate structures between proteins and the electrode.