Abstract
The association of chloroformโsoluble derivatives of uracil and adenine has been examined in chloroform solution in the presence of dissolved water. Analysis by infrared spectroscopy shows that complex formation still occurs in these conditions, and that the extent of association is substantially unchanged by the presence of water. Evidence is presented for the coexistence of two kinds of base pair (involving, respectively, the C~2~ and C~4~ carbonyl groups of the pyrimidine) in the solutions, and for some displacement in their relative balance by the added water. The binding of water to the C~2~ and C~4~ carbonyl groups can be separately observed in both the free uracil derivative and its 1:1 complex with 9โethyladenine. Little or no competition has been found to occur between the formation of base pairs and binding of water to the bases, as judged by measurements of water solubility in chloroform solutions of the bases individually and in 1:1 mixtures. The evidence suggests that this phenomenon can be largely explained by the formation of double hydrogen bonds by the uracil carbonyl groups. Taken together with recent published observations, the results indicate that hydrogen bonding may make a much greater energetic contribution to conformational stability of biopolymers in aqueous solution that has been supposed.