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

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.