Binding of 9-N-butyladenine with carboxylic acids : Studies on the watson-crick vs. Hoogsteen selectivity

Abstract

It’s been shown that if the carboxylic group of the acid is in resonance with the adjacent R group, a net preference for the HG site is observed. If the resonance is disrupted in case of aromatic acids by tilting the plane of the phenyl ring the association constant decreases. If the carboxylic group is insulated by an sp³ centre from the aromatic fragment the HG binding diminishes, and the association constant at the WC site is governed by the pK? of the acid. In case of an aliphatic alkyl chain the binding takes place exclusively at the WC site. And if the binding constant is fairly large the specificity is lost, though not to a significant extent. The dependence of the association constant on pK? of the acid was verified. We have shown that the (H–C?) of the aliphatic carboxylic acids undergoes a well?behaved chemical shift as a function of acid concentration (fig. 1.14, 1.15, 1.18) on complexation with 9?N?butyladenine and can possibly be used for determination of association constants. The calculated geometry of HG and WC complexes with acetic and benzoic acid at 3? 21G, 6?31G, 6?31G* levels revealed that WC site is geometrically more favorable than HG, as the H?bonds at WC site were slightly shorter, less angular and directed closer to the lone pair direction. The interaction energies were overestimated at all these levels of theory; one possible explanation could be that the solvation effects have not been taken care of. The findings, possibly with improved basis set and higher levels of theory, the experimental findings can be reproduced by the calculations.