Dimers, trimers, and tetramers of cytosine with guanine
Cytosine and guanine have been shown previously to form Watson-Crick type base pairs in nonaqueous solvents, suggesting that the monomers can be used to understand and possibly to predict structures of the polymeric nucleic acids. Yet, the poor solubility properties of cytosine and guanine (and their corresponding nucleosides) have limited the utility of the monomeric model of polymeric nucleic acids. The 2'-deoxynucleosides, which are substituted at both ribose hydroxyls with triisopropylsilyl groups, have high solubilities (greater than 200 mM) in nonpolar solvents such as chloroform-d. These substituted nucleosides are appropriate for detailed 1H NMR study of hydrogen bonding between cytosine and guanine over a wide temperature range. In this DNA model system, cytosine and guanine form the stable Watson-Crick type dimer, as would be expected from previous studies of bases in higher dielectric solvents or at lower concentrations. We report here that the bases form such dimers and additional, more intricate hydrogen-bonded complexes. Cytosine and guanine monomers form both trimers (cytosine:guanine2) and tetramers [(cytosine:guanine)2] in low-dielectric solution. Thus, the interactions of monomers are consistent with formation of two-, three-, and four-stranded nucleic acid polymers. © 1989, American Chemical Society. All rights reserved.
Williams, NG; Williams, LD; Shaw, BR
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