Icosahedral virus particles as addressable nanoscale building blocks.

Nanochemistry is the synthesis and study of well-defined structures with dimensions of 1 ± 100 nanometers (nm), and thus spans the size range between molecules and materials.[1] While supramolecular chemistry (making small molecules bigger) and microfabrication techniques (making big structures smaller) attack from the flanks, biology employs many constructs of this size. Examples include the photosynthetic reaction center, the ribosome, and membrane-bound receptor-signaling complexes, all notable because of their sophisticated yet modular function. The burgeoning field of nanotechnology[2] seeks to mimic the information-handling, materials-building, and responsive sensing capabilities of biological systems at the nanometer scale. The special requirements of this enterprise would be well served by building blocks of the proper size with predictable and programmable chemistry. Cowpea mosaic virus (CPMV) particles are 30 nm-diameter icosahedra, formed by 60 copies of two different types of protein subunits (Figure 1a).[3] The physical, biological, and genetic properties of CPMV have been well characterized.[4] Approximately one gram of virus is easily and routinely obtained from a kilogram of infected leaves of the black-eye pea plant. The structure of CPMV has been characterized at 2.8 ä resolution by X-ray crystallography and an atomic model of the particle has been constructed.[5] The virion displays icosahedral symmetry to the resolution of the crystal structure and an infectious clone of the virus allows sitedirected and insertional mutagenesis to be performed in a straightforward and rapid manner.[6] The particles are remarkably stable; they maintain their integrity at 60 C (pH 7) for at least one hour and at pH values from 3.5 to 9 indefinitely at room temperature. Different crystal forms of the virus can be readily produced under well-defined conditions (Figure 1d).[7, 8] Here we report on the selective Experimental Section