Osmium Tetroxide Modification and the Study of DNA-Protein Interactions

Osmium tetroxide is an evil-smelling chemical used as a fixative in electron microscopy that can also be used to modify thymidine residues within DNA (1 ,2 ). The ability of osmium tetroxide to modify DNA is very sensitive to DNA conformation, In particular, osmium tetroxide will attack thymidines that are unstacked, either because they are in a single-stranded region or, for example, because the DNA is bent or because it is overwound (3 ).