Protein arrays: concepts and subjects

Two-dimensional nanoparticle arrays derived from ferritin monolayers

A scalable technique for making silica coatings with embedded two-dimensional arrays of iron oxide nanoparticles is presented. The iron oxide nanoparticle arrays were formed by depositing quasi-crystalline ferritin layers, an iron storage protein with an iron oxide mineral core, on solid substrates by a spread-coating technique based on evaporation-induced convective assembly. The layer of protein molecular arrays was then encapsulated in a silica matrix film deposited from a sol precursor. The organic protein shell of the ferritin molecules was then removed by controlled pyrolysis, leaving ordered iron oxide cores bound in the silica matrix. This article is the first report on combining convective self-assembly of proteins with sol-gel techniques of oxide film formation. The technique is technologically feasible and scalable to make coatings of encapsulated ordered magnetic clusters tens of cm(2) or larger in size.

Voltammetry and in situ scanning tunnelling microscopy of de novo designed heme protein monolayers on Au(111)-electrode surfaces

In the present work, we report the electrochemical characterization and in situ scanning tunnelling microscopy (STM) studies of monolayers of an artificial de novo designed heme protein MOP-C, covalently immobilized on modified Au(111) surfaces. The protein forms closely packed monolayers, which remain electroactive upon immobilization. In situ STM images show circular structures indicating that MOP-C stands upright on the surface in accordance with the molecular design. Despite the large spatial extension of MOP-C, about 5 nm in height, conditions could be found where tip/sample interaction is minimal and proteins could be imaged without detectable tip interference. The results indicate further that the structural sensitivity of (in situ) STM depends to a significant extent on associated electron transfer kinetics. In the present case, the heme group does not contribute significantly to the tunnelling current, apparently due to slow electron transfer kinetics. As a consequence, STM images of heme-containing and heme-free MOP-C did not reveal any notable differences in apparent height or physical extension. The apparent height of heme-containing MOP-C did not show any dependence on the substrate potential being varied around the redox potential of the protein. The mere presence of an accessible molecular energy level is not sufficient to result in detectable tunnelling current modulation.

Influence of electrostatic interactions on the surface adsorption of a viral protein cage

The Cowpea chlorotic mottle virus (CCMV) provides a useful protein-cage architecture that can be used for the size- and shape-constrained chemistry of nanomaterials. The control of surface assembly is necessary for the realization of many applications of these nanoscale reaction vessels. Electrostatic interactions provide a useful (and reversible) method for controlled surface assembly. CCMV absorption behavior was studied on Formvar, bare Si, Formvar-coated Si, and Si modified by aminopropyltriethoxysilane (APS). Transmission electron microscopy (TEM), atomic force microscopy (AFM), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) were used to characterize the CCMV surface adsorption. Combined AFM and ATR-FTIR data indicated that the viral coverage on the modified surfaces was approximately 84% of the jamming limit predicted by the random sequential adsorption (RSA) model. According to the ATR-FTIR results, surface coverage was not increased at higher ionic strengths nor at a pH near the isoelectric point (pI) of the virus. The Langmuir model was used to provide a description of the kinetic absorption behavior.

2-D array formation of genetically engineered viral cages on au surfaces and imaging by atomic force microscopy

The preparation and subsequent imaging of a two-dimensional array of a genetically and chemically modified cowpea chlorotic mottle virus (CCMV) is described. The genetic mutation provides symmetrically dispersed exposed thiol groups on the outer surface of the virus capsid. These functional groups can be used to covalently bind the capsid to smooth Au substrate. AFM imaging suggests that the genetic mutation by itself does not promote array formation but, rather, aggregation through disulfide linkages. However, breaking the symmetry of the capsid using a solid-phase approach and chemically passivating the exposed thiol groups with iodoacetic acid results in a capsid with exposed thiols only on one side of the particle. These symmetry-broken capsids were able to form self-assembled monolayers (SAM) on a Au surface.

Protein folding at the air-water interface studied with x-ray reflectivity

We report the results of x-ray reflectivity measurements of thin films formed by different water-soluble proteins at the air-aqueous solution interface. It is demonstrated that glucose oxidase, alcohol dehydrogenase, and urease molecules denaturate at the air-aqueous solution interface to form 8- to 14-A-thick peptide sheets. X-ray reflectivity data indicate that the spreading of a lipid monolayer at the aqueous solution surface before protein injection does not prevent proteins from unfolding. On the other hand, crosslinking of proteins results in intact enzyme layers at the subphase surface. A model that involves interaction of glucose oxidase molecules with a phospholipid monolayer is proposed. In this model, an observed decrease of the lipid electron density in the protein presence is explained in terms of "holes" in the monolayer film caused by protein molecule adsorption.