Selective Staining of Nucleic Acids with Indium
Heavy metal compounds are used to increase contrast in electron microscope specimens by virtue of strong electron scattering properties. Chemical properties of heavy metal atoms cause them to discriminate amongst available ligands, a property useful for biological specimens. Decreasing available specimen ligand types produces a narrow reaction spectrum. Taking advantage of both these controlling factors, one can produce a very narrow reaction spectrum and thus high selectivity. This approach was utilized in the development of methods for the use of indium as an “electron stain” for nucleic acids. Here selectivity is greatly enhanced by reduction and acylation reactions which precede exposure to indium trichloride dissolved in anhydrous acetone. Nucleic acid phosphate is the predominant binding site but some non-nucleic acid ligands still bind indium. To further increase selectivity and provide greater contrast (thus lower detection limits), more efficient blocking reactions of broader chemical reactivity plus techniques to increase the amount of heavy metal bound to nucleic acid were sought.