Nuclear magnetic resonance study of the interactions of guanosine and cytidine in dimethyl sulfoxide.

Guanosine forms hydrogen-bonded dimers in DMSO. The equilibrium constant for dimer formation is 0.18 l./mol at 32”, and there is no evidence of higher order aggregates. The structure of the dimer is probably similar to the hydrogen bonding thought to occur in guanylic acid gels but it is not possible from our results to assign a unique hydrogen-bonding scheme. Cytidine and 1-methylguanosine show little or no evidence of dimer formation in DMSO. When both guanosine and cytidine are present in DMSO a Watson-Crick hydrogenbonded dimer results. The equilibrium constant for dimer formation is 3.7 f 0.6 at 32”, and the enthalpy of formation is about 6 kcal/mol. There is no evidence of base stacking or the formation of larger hydrogen-bonded complexes in DMSO. Thus this solvent is an excellent system in which to study the effects of base pairing on the formation of specific complexes between monoor oligonucleotides. basic feature of the macromolecular structure of A nucleic acids is the occurrence of guanine-cytosine and adenine-thymine (or uracil) base pairs. It would be of considerable interest to know just how much the interaction between two hydrogen-bonded bases contributes to the stability of double strand DNA or RNA helicesa2 One would also like to understand whether the specificity involved in base-pair formation is a result of the structures of the individual monomers themselves, or is mostly due to the constraints of forming monomers into an ordered helix. This is of special importance when we consider the possible types of interactions which might arise in a transfer RNA or other nucleic acid that is less than 100% double helix. It is not surprising that there have been a large number of studies on the interactions between nucleic acid bases or nucleosides. :j-14 These represent the simplest systems which may yield information about the specificity of base-base interactions. Mononucleosides can be expected to interact by either base stacking or hydrogen bonding. There is a great deal of evidence that vertical stacking interactions occur between mononucleosides in aqueous solution. Ts’o and his collaborators have pioneered efforts to determine the extent of these interactions by studying (1) J. D. Watson and F. H. C. Crick, Nature, 171, 737, 964 (1953). (2) H. DeVoe and I. Tinoco, Jr., J . Mol. Bio!., 4, 500 (1962). (3) P. 0. P. Ts’o. I . S. Melvin. and A. C. Olson. J . Am. Chem. Soc., \ , ~. 85, 1289 (1963). ibid., 86, 4182 (1964). (4) S. I . Chan, M. P. Schweizer, P. 0. P. Ts’o, and G. I<. Helnikamp, ( 5 ) M. P. Schweizcr, S. I. Chan, and P. 0. P. Ts’o, ibid., 87, 5241