Adeninium cytosinium sulfate

In the title compound, C5H6N5 +·C4H6N3O+·SO4 2−, the adeninium (AdH+) and cytosinium (CytH+) cations and sulfate dianion are involved in a three-dimensional hydrogen-bonding network with four different modes, viz. AdH+⋯AdH+, AdH+⋯CytH+, AdH+⋯SO4 2− and CytH+⋯SO4 2−. The adeninium cations form N—H⋯N dimers through the Hoogsteen faces, generating a characteristic R 2 2(10) motif. This AdH+⋯AdH+ hydrogen bond in combination with AdH+⋯CytH+ H-bonds leads to two-dimensional cationic ribbons parallel to the a axis. The sulfate anions interlink the ribbons into a three-dimensional hydrogen-bonding network and thus reinforce the crystal structure.

The protonated nucleobases are present in many biochemical processes, such as enzymatic reactions and the stabilization of triplex structures, and they play a key role in a newly emerging feature of nucleic acid chemistry, namely acid-base catalysis (Lippert, 2005). There ability to form hydrogen-bonded networks is obviously the most important and interesting characteristic, because the self-assembly of hydrogen-bonded networks of these compounds or there derivatives has been used to design or construct highly ordered supramolecular nanostructures which are of interest for their potential applications as molecular devices (Lehn, 1995& Gottarelli et al., 2000.
The main purpose of the present study is to examine the hydrogen bonding engineered in crystal formed by two monoprotonated nucleobases and one dianion: adeninium cytosinium sulfate [AdH + , CytH + , SO 4
Adeninium cations can be either mono-or diprotonated and the bond lengths and angles are dependent on the degree of protonation (Hingerty et al., 1981;Langer & Huml, 1978). This form contains three basic N atoms, the most basic site is N1, which accepts the first proton, and the next protonation occurs at N7 and then at N3.
The adeninium cation in this structure is monoprotonated at N1 atom. The protonation on this site is evident from the C-N-C bond angle, indeed we note an increase in the C2A-N1A-C6A bond angle [123.35 (6)°] compared with the corresponding value found in the neutral adenine [119.8°; Voet & Rich, 1970]. The location of the H-atom bonded to N1 in a difference Fourrier map and the successful refinement of its position confirms the protonation on this site.
In the sulfate anion, S atom is linked to four equivalents short bonds of 1.4706 (10) Å to O1 and O2, 1.4895 (10) Å to O3 and 1.4905 (10) Å to O4, which confirm the absence of proton in this anion.
The asymetric unit, of the title compound, is thus formed by one adeninium cation, one cytosinium cation and one sulfate dianion (Fig. 1).
The three-dimensional crystal structure is stabilized by thirteen hydrogen bonds with four different modes viz.

Experimental
Colourless needle crystals of the title compound [AdH + , CytH + , SO 4 2-], were obtained by slow evaporation at room temperature of an equimolar solution of adenine, cytosine and sulfuric acid.

Refinement
All the H atoms were located in the difference electron density maps. All the H atoms attached to C were treated as riding with C-H = 0.93 Å (aromatic) with U iso (H) = 1.2U eq (C). The coordinate parameters of the H atoms attached to N were freely refined with U iso (H) = 1.2U eq (N). Fig. 1. The title molecule with the atom-numbering scheme. The displacement ellipsoids are drawn at the 50% probability level Fig. 2. The cation-cation (AdH + ···AdH + and AdH + ···CytH + ) hydrogen bonds.  Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F^2^ against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F^2^, conventional R-factors R are based on F, with F set to zero for negative F^2^. The threshold expression of F^2^ > σ(F^2^) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F^2^ are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.