Biomod/2013/NanoUANL/1

Abstract

Title: A model for silver nanoparticle synthesis by enzymatic reduction of silver ions in a protein cage

Although Ag-NP synthesis has been widely studied, the variables involved, such as substrate intake, enzymatic activity and atomic nucleation of silver, vary on a nanometric scale and within a specific geometry, which is of great interest. Our proposed model for Ag-NP synthesis involves constrained growth inside a modified viral capsid, aided by the enzyme HRP. A simplification of the system as a series of simple, more accessible scenarios is needed for computational analysis. Through the use of a variety of computer programs and algorithms for many-body problems (such as Monte Carlo), we expect to obtain a model to describe this phenomenon with more detail as a basis for future novel applications in the field of material synthesis.

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Introduction

A nanoparticle is an object between 1 and 100 nanometers in size, with unique size-dependant properties, as they are small enough to confine their electrons and produce quantum effects. A major limitation to existing synthesis protocols is the presence of a wide size distribution, restraining its usefulness in certain applications. The use of a viral capsid as a container for synthesis allows us to study the phenomenon in a controlled environment, ensuring a homogeneity in particle formation.

CCMV

3D structure of CCMV

Cowpea Chlorotic Mottle Virus, also know as CCMV, is a plant virus that infects Vigna ungulata, known as the cowpea plant. The CCMV capsid consists of 180 identical copies of a 20 kDa protein that self-assembles into a T=3 icosahedral protein cage with a ~280 Å external diameter and a ~180 Å diameter internal cavity.

The assembly of CCMV has been studied both in vivo and in vitro, and is therefore an appropriate system for constrained reactions. It was the first spherical virus to be assembled in vitro into an infectious form from its purified components.

Properties

A number of its physical properties can be taken advantage of for nanoscale construction. Some of these include a tolerance for high temperatures, pH's and stability in organic solvents. The viral capsid can undergo reversible pH-dependant structural transition that results in the formation of 60 ~20 Å pores that allow access between the interior and exterior. Under low pH (<6.0) and low ionic strength (i= 0.2) conditions, capsomers devoid of RNA self-assemble; while in higher pH (>7) and ionic strength (i> 1) the structure undergoes disassembly.

Mutagenesis of viral capsids is a well-established technique that allows the alteration of viral structure. These modifications can include changing the total charge or the attachment of ligands to the surface via interactions with the amino acids. Viral capsids have been show to allow the addition of small peptides, up to 30 amino acids. Systems for heterologous expression in a variety of organisms such as V. ungulata, E. coli and P. pastoris allow the production of both wild-type and modified virus-like particles (VLP's).

Silver

Silver nanoparticles have unique properties that make them suitable for a wide variety of applications: from photovoltaics to biological/chemical sensors. The high electrical conductivity presented by these particles make them specially suited for use as conductive links; their optical properties allow them to be used in molecular diagnostics. A popular application for this material is their integration into surfaces to generate an antimicrobial effect. Many products such as medical devices are starting to implement this protective coating.

Metal nanoparticles are commonly synthesized by two routes. The first is a physical approach (involving laser ablation and evaporation/condensation), the second is a chemical approach, promoting nanoparticle aggregation from metal ions using a reducing agent. Chemical methods can be divided into classical, using well-known agents (hydrazine, sodium borohydride) and radiation-chemical, where ionizing radiation generates solvated electrons.

The most popular chemical approaches, including chemical reduction using a variety of organic and inorganic reducing agents, electrochemical techniques, physicochemical reduction, and radiolysis are widely used for the synthesis of silver nanoparticles. Recently, there is growing attention to produce nanoparticles using environmentally friendly methods (green chemistry). The advantages to these methods is that they do not use any toxic and expensive chemical substances. . The bioreduction of metal ions by combinations of biomolecules found in the extracts of certain organisms (e.g., enzymes/proteins, amino acids, polysaccharides, and vitamins) is environmentally friendly, yet chemically complex, making this system fascinating to study. Silver nanoparticle synthesis has been reported in bacteria, fungi and plants.

Although frequently referred to as "silver" nanoparticles, many are in fact composed of silver oxide due to the surface-to-volume ratio.

HRP

Horseradish peroxidase is a ~44 kDa glycoprotein from the peroxidase family, with a known three-dimensional structure (Gajhede et al, 1997). Peroxidases typically catalyze a reaction in which a wide variety of both organic and inorganic compounds are oxidized.

There has been a great deal of scientific interest in the enzyme because of its commercial uses, primarily as a component of clinical diagnostic kits and for immunostaining. Its main role in these applications is that of a reporter system. The enzyme is usually conjugated to specific antibodies or streptavidin, which binds to the compound of interest, and activity is detected with substrates like TMB or ABTS, producing color.

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