Hydrogen-Bonding Complexes of 5‑Azauracil and Uracil Derivatives in Organic Medium

Uracil derivatives form strong complexes with complementary 2,4-diaminotriazine and adenine compounds, whereas derivatives of 5-azauracil (2,4-dioxotriazine) are known to form weak complexes in aqueous medium. However, herein we report that in organic medium (CDCl<sub>3</sub>), the 5-azauracil moiety forms hydrogen-bond-mediated complexes with complementary 2,4-diaminotriazine and adenine compounds, with strengths comparable to those formed by uracil compounds. Such dichotomous base-pairing behavior of the 5-azauracil moiety, in organic versus aqueous media, is found to be consistent with the ionization of the 5-azauracil moiety in aqueous medium leading to competitive interference from water molecules (via solvation), which is absent (lack of such ionization and solvent interference) in organic medium. This discriminating role of solvent (e.g., water) could have been an important factor in the selection of molecules, based on their physicochemical properties, and subsequently in the emergence of potential primordial informational oligomers that would have played a role in the origins of life.