Neutral and positively charged new purine tetramer structures: a computational study of xanthine and uric acid derivatives

Abstract

New tetramer structures, based on 9-methylxanthine (Xa), 9-methylxanthine protonated at N7 (XaH+) and 9-methyluric acid (Ua), were investigated by high-level density functional calculations. We have found that homo- and heterotetrads (XaH+)4, (XaH+–Xa)2, (XaH+–Ua)2 carrying positive charges can be formed by low barrier hydrogen bonds. Systems with zero charge [(Xa)4, (Xa–Ua)2, (Ua)4] were also constructed, investigated and compared to the guanine tetrad [(G)4]. It was shown that the new tetramers can bind cations and anions without the necessity of stacking interactions. Application of the calculated systems in higher-ordered structures (e.g. quadruplexes) is promising with or without intercalating ions.