Protein Immobilization on Gold Nanoparticles: Quantitative Analysis.

Conjugation of gold nanoparticles (AuNPs) with biologically relevant molecules underpins many applications in medicine and biochemistry. Immobilization of functional proteins on AuNPs often affects protein structure and function. Such effects are protein dependent and require thorough investigation using suitable quantitative tests. Good experimental design and the use of a comprehensive set of control samples are essential when characterizing the consequences of protein immobilization and its effect on protein structure and function. However, traditional approaches to making control samples, that is, immobilized protein versus protein in solution in absence of any nanoparticles, do not provide sufficiently identical reaction conditions and complicate interpretation of the results. Accurate quantification of protein conjugation to AuNPs and ensuring complete removal of unconjugated protein remain the two key challenges in such functional assays. This report describes a simple and straightforward procedure allowing for quantitative analysis of protein conjugation to AuNPs. The principles are illustrated using fluorescence and circular dichroism measurements, and can be applied to other analytical techniques or be adapted with minor modifications for use with other proteins.