Higher Affinity Quadruply Hydrogen-Bonded Complexation with 7-Deazaguanine Urea

[structure: see text] UG forms a highly stable quadruply hydrogen-bonded heterocomplex with DAN, but the fidelity of the complex is lowered somewhat by the Hoogsteen-side oligomerization of UG (K(assoc) approximately 230 M(-)(1), CDCl(3)). DeUG was prepared as a more robust analogue of UG lacking the Hoogsteen nitrogen atom. Remarkably, the deaza analogue, DeUG, forms a much more stable complex with DAN (>10-fold higher K(assoc) for DeUG.DAN vs UG.DAN) but also dimerizes more strongly (K(dim) = 880 +/- 40 M(-)(1), CDCl(3)) by adopting a conformation preorganized for both binding and dimerization.