The proposed research involves the development of methods for the synthesis of oligodeoxyribonucleotide analogs containing sulfuric acid diester bridges instead of the naturally occuring phosphoric acid diester linkages. Diesters of sulfuric acid and of phosphoric acid are geometrically very similar; they differ in that the latter have a negative charge in neutral aqueous medium, while the former are nonionic. The proposed sulfate-linked analogs are the most precise model possible for uncharged oligodeoxyribonucleotides. Comparison of these analogs with normal oligonucleotides will allow to accurately assess the importance of the electrical charge on oligonucleotides for their solution structure, for the free energy of DNA duplex formation, and for the interaction of oligonucleotides with proteins. It is expected that the oligonucleotide sulfate analogs will be sufficiently water soluble and also sufficiently stable at low temperatures in aqueous solution to allow the proposed experiments to performed. The research plan starts with the simplest unit, a dinucleoside monosulfate, specifically the sulfate analog (dAsdA) of deoxyadenylyl(3'-5')deoxyadenosine (dAPdA). The synthesis dAsdA will be undertaken by one of the following methods: condensation between a nucleoside sulfate and a nucleoside, using DCC, or mesitylenesulfonyl tetrazolide, or using the Mitsunobu method adapted to sulfate esters, or, alternatively, construction of the internucleoside sulfate by linking two nucleosides via the bifunctional agents thionyl chloride (SOC12) (followed by oxidation of the sulfite bridge) or sulfuryl chloride (SO2C12). The sulfate analog will be compared to the corresponding natural dinucleotide (dApdA) with respect to the UV, CD, and NMR spectra of their aqueous solutions, the stoichiometry and the stability of their complexes with polyuridylate and polydeoxythymidylate, and the susceptibility to several enzymes. The interaction of dAsdA with the sulfate-analog dimer complementary to it, dTsdT, in organic solvents will also be examined. This work may lead to the preparation of longer sulfate analogs of oligonucleotides. Of particular interest would be the synthesis of the self complementary tetramer, d-CsGsCsG, which would be used to probe the importance of electrostatic interactions for the equilibrium between the B and Z forms of poly(dC-dG).
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