Abstract
In the past fifteen years, there has been considerable interest in metallic nanoparticles for various biomedical, environmental, and industrial applications. Metallic nanoparticles, such as gold nanoparticles (AuNPs) feature prominently in plasmonics, biosensing, photoacoustic and photothermal imaging. In combination with proteins and other biomolecules, optical and electromagnetic properties of gold nanoparticles can be modulated. Melanin is one such biomolecule of interest, which has also gained lot of interest due to its broad absorption spectrum and has potential for optical, photo-acoustic, and biosensing applications. Combining such optically active materials could lead to a composite material with tunable optical properties. As the complexity of composite material synthesis increases, the development of simple design rules become essential, and using biological building blocks is one method to achieve this. Bacterial synthesis offer immense potential for an efficient synthesis process, ease of functionalization, modification and repeatability. Genetically modified E. coli can synthesize extracellular curli nanofibers, which is an amyloid, fibrous, structural component of the biofilms. We have designed and assembled two DNA constructs to produce curli fibers with the ability to reduce gold nanoparticles and a melanin-binding fusion protein. Two E. coli strains were transformed with the constructs to produce the melanin-coated well-organized gold nanoparticles. Curli fibers were assembled with alternating structural curli CsgA (protein repeat unit) segments, where one of the CsgA units was conjugated to SpyTag protein, and the other was conjugated to the peptide fragment (FlgA3), which binds gold nanoparticle and has been shown to reduce AuNPs from auric chloride solution. A fusion protein comprised of the SpyCatcher protein conjugated to the RPT region of the Pmel17 amyloid fiber was created, and the fusion lead to the formation of an apparent gold nanoparticle-melanin composite. In this study we have designed and produced novel melanin-bound gold nanoparticles using synthetic biology approaches, as a preliminary model for future investigation of the feasibility and advantages of bacterially-produced tunable biocomposite materials, which could find applications in plasmonics and photocatalysis.