Crystal structure of 2,4-diamino-7-(hydroxymethyl)pteridin-1-ium nitrate

In the crystal of the title molecular salt, C7H9N6O+·NO3 −, the cations and anions are linked via N—H⋯O and O—H⋯O hydrogen bonds, forming sheets parallel to (100). Within the sheets there are numerous hydrogen-bonding ring motifs.

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 Pteridine derivatives are found as the core structure of folic acid flavin adenine dinucleotide (FAD) and function as cofactors for enzymes involved in hydroxylation (Benkovic & Annu,1980) and methyl transfer (Blakeley, 1969;Van Beelen et al., 1984), as redox mediators (Dolphin, 1980) and as pigments for eyes and wings in certain insects (Pfleiderer, 1982). Variation of the substituents on the pteridine core of folates has provided synthetic anticancer drugs (Blakely & Cocco, 1985;Pfleiderer & Taylor, 1964). Pterdines are metabolites formed by a bicyclic pyrimidine-pyrazine moiety that occurs in a wide range of living systems and contributes in relevant biological functions (Muller et al., 1991). Pteridine derivatives have good diuretic activity and are also used as simple models for therapeutically valuable antifolate drugs (Weinstock et al., 1968). In order to study potential hydrogen bonding interactions, the title compound was synthesized and we report herein on its crystal structure.
During the synthesis of the molecular salt a proton was transferred from the hydroxyl group of nitric acid to atom N3 of the pteridine ring. The pteridine ring system (C1-C7/N1-N6/O1) is planar with a maximum deviation of 0.001 (1) Å for all the non H atoms. The bond lengths and angles are close to those found for similar compounds, viz. 2,4-diamino-6,7-dimethylpteridine hydrochloride monohydrate (Schwalbe & Williams, 1986) and triamterenium tetraphenylborate acetonitrile solvate (Robertson et al., 1998).
In the crystal, Fig. 2, the protonated N3 atom and the protonated 2-amino group (N6) are hydrogen-bonded to the nitrate O atoms (O2 and O4) via a pair of N3-H1N3···O2 and N6-H2N6···O4 hydrogen bonds, forming an R 2 2 (8) ring motif (Bernstein et al., 1995). This type of interaction is similar to the carboxylate-trimethoprim interaction observed in the trimethoprim cation-dihydrofolate reductase complex (Kuyper, 1990) and to the cyclic hydrogen bonded motif observed in many organic crystal structures (Allen et al., 1998). An R 1 2 (4) ring motif indicates a bifurcated hydrogen bond formed by N6-H1N6 to the two acceptors (O3 and O4). The 4-amino group and the hydroxyl group form hydrogen bonds with the O atoms of the nitrate ion leading to an R 3 3 (8) ring. The three center and bifurcated hydrogen bonds and fork like interaction form an R 4 4 (12) ring. Two R 3 3 (8) motifs and an R 2 2 (8) ring motif generate a new R 3 2 (18) ring motif. The above interactions lead to the formation a two dimensional network parallel to the bc plane (Table 1 and Fig. 2).

S2. Synthesis and crystallization
A few drops of nitric acid were added to a hot methanol solution (20 ml) of 2,4-diamino-6-(hydroxymethyl)pteridine (43 mg, Aldrich) which had been warmed over a heating magnetic stirrer hotplate for a few minutes. The resulting solution was allowed to cool slowly at room temperature and crystals of the title molecular salt appeared after a few days.

S3. Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 U eq (C).

Figure 1
The molecular structure of the title salt, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The N-H···O hydrogen bonds are shown as dashed lines (see Table 1 for details).

Figure 2
A view along the a axis of the crystal packing of the title molecular salt. The N-H···O hydrogen bonds are shown as dashed lines (see Table 1 for details). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.09 e Å −3 Δρ min = −0.15 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (