Osmium-modified oligonucleotides

When a metal complex is formed with the nucleic acid base of single-stranded DNA (ssDNA), such ssDNA can be used as an alternative electrochemically-active DNA-binding ligand. For example, Palecek and coworkers reported the formation of a reversible redox-active metal complex by the reaction of osmium tetraoxide-pyridine with the thymine (T) base of ssDNA, and they developed an electrochemical gene detection method based on this modified oligonucleotide as a DNA probe [3]. 

Fig. 2 Emission spectra of double-stranded DNA molecules composed of an oligonucleotide modified at its 5 -end with ruthenium(II) nucleoside 5 and its complementary strand a without modification, and with a related osmium(II) label at a distance of b 61, c 52, d 43, e 31, f 21 and g 16 A. The Ru-Os distances are based on a helical DNA model [34]...

The distortions induced in the DNA double helix by the interstrand cross-links have been characterized by several techniques. As judged by chemical probes (diethyl pyrocarbonate, hydroxylamine, osmium tetroxide), antibodies to cisplatin-modified poly(dG-dC)-poly(dG-dC), natural (DNase I) and artificial (1,10-phenanthroline-copper complex) nucleases, the cytosine residues are accessible to the solvent, and the distortions are located at the level of the adduct [48-50]. From the electrophoretic mobility of the multimers of double-stranded oligonucleotides containing a single interstrand cross-link [50] it is deduced that the DNA double helix is unwound (79°) and its axis is bent (45°). 

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