TEACHING LIGHT-SCATTERING SPECTROSCOPY - THE DIMENSION AND SHAPE OF TOBACCO MOSAIC-VIRUS

The tobacco mosaic virus is used as a model molecular assembly to illu strate the basic potentialities of light scattering techniques (both s tatic and dynamic) to undergraduates, The work has two objectives: a p edagogic one (introducing light scattering to undergraduate students) and a scientific one (stabilization of the virus molecular assembly st ructure by the nucleic acid). Students are first challenged to confirm the stabilization of the cylindrical shape of the virus by the nuclei c acid, at pH and ionic strength conditions where the coat proteins al one do not self-assemble, The experimental intramolecular scattering f actor is compared with the theoretical ones for several model geometri es. The data clearly suggest that the geometry is, in fact, a rod. Com paring the experimental values of gyration radius and hydrodynamic rad ius with the theoretical expectations further confirms this conclusion . Moreover, the rod structure is maintained over a wider range of pH a nd ionic strength than that valid for the coat proteins alone, The exp erimental values of the diffusion coefficient and radius of gyration a re compared with the theoretical expectations assuming the dimensions detected by electron microscopy techniques. In fact, both values are i n agreement (length approximate to 300 nm, radius approximate to 20 nm ).