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. Neutral or positively charged tetramer structures, based on 9-methylxanthine (Xa) and 9-methyluric acid (Ua), were constructed and investigated by high-level DFT calculations.