pH-Triggered Growth of Gold Nanoparticles

Abstract

Methanotrophs are bacteria that live exclusively on methane. They produce a small biomolecule called methanobactin (mb), an 11 amino acid “peptide”, which is found inside and outside the cell. Methanobactins are of interest because they are associated with enzymes (produced by methanotrophic bacteria) that show great promise in bioremediation and environmental applications since they can oxidize halogenated hydrocarbons; they also catalyze the conversion of methane to methanol. Two types of methanobactin have been previously characterized: SB2 and OB3b, each isolated from a different strain of methanotrophic bacteria. This study focused only on OB3b mb. Methanobactin’s role is to bind copper (II) ions in the environment and chaperone the ions back inside the cell. We have previously found that methanobactin reduces copper (II) to copper (I) upon binding the metal ion. In addition to copper, methanobactins have been shown to reduce several other metals, including gold. Specifically, it can reduce Au3+ (in solution in the form AuCl4 -) to Au0 (atomic gold). The atomic gold then aggregates to form gold nanoparticles which can be characterized using TEM and UV-visible spectroscopy. Here we show how pH affects the reaction kinetics between AuCl4 – and mb. This work has implications for biochemical oxidation-reduction reactions that likely have analogs in other organisms and for other metals. In addition, our results could improve our understanding of the metabolic pathway used by methanotrophs to oxidize methane to methanol.