Redetermination of guaninium chloride dihydrate

The title compound, (I), is a dihydrate salt of guanine, which is one of the two common purine bases found in ribose and deoxyribonucleic acids. The unit cell and space group of (I) were originally reported in 1951 (Broomhead, 1951), with a room-temperature X-ray determination performed 12 years later (Iball & Wilson, 1963, 1965). In this original determination, the intensities were recorded using Weissenberg photographs. All the atoms, including H atoms, were located by means of a difference Fourier synthesis and the structure refined to a final R value of 0.073. We have redetermined this crystal structure at 150 K, with a final R value of 0.032, to gain more precise data for our theoretical modelling studies.

The low-temperature redetermination of guaninium chloride dihydrate, C 5 H 6 N 5 O + ÁCl À Á2H 2 O, obtained as part of an experimental polymorph screen on guanine, is reported here.

Comment
The title compound, (I), is a dihydrate salt of guanine, which is one of the two common purine bases found in ribose and deoxyribonucleic acids. The unit cell and space group of (I) were originally reported in 1951 (Broomhead, 1951), with a room-temperature X-ray determination performed 12 years later (Iball & Wilson, 1963, 1965. In this original determination, the intensities were recorded using Weissenberg photographs. All the atoms, including H atoms, were located by means of a difference Fourier synthesis and the structure refined to a final R value of 0.073. We have redetermined this crystal structure at 150 K, with a final R value of 0.032, to gain more precise data for our theoretical modelling studies. In this low-temperature determination, the precision of the unit-cell dimensions was improved by an order of magnitude, and the unit-cell volume decreased by ca 14 Å 3 , consistent with the determination at low temperature. In general, the metric parameters are not significantly different, the exception being the C1-N2 bond length which is longer in the lowtemperature structure, while C1-N5 is shorter in the lowtemperature structure, both by ca 0.03 Å . The guanine molecule is protonated at N1 and N4, with the C-N bond lengths in the rings ranging from 1.3154 (18)  The packing consists of centrosymmetric dimers, the two components of which are linked by pairs of N-HÁ Á ÁN hydrogen bonds. These dimers are linked to four water molecules and two Cl atoms to form a planar unit (Fig. 2). These planar units are linked to one another through O-HÁ Á ÁCl hydrogen bonds within the plane, forming a ribbon structure, and through N-HÁ Á ÁCl and N-HÁ Á ÁO hydrogen bonds at an angle of approximately 80 from this plane, forming a complex three-dimensional hydrogen-bonded network (Fig. 3). The two H atoms on the NH 2 group form two very dissimilar hydrogen bonds. A strong bond [N5-H7Á Á ÁN2 iv = 3.0162 (17) Å ; see Table 1] is formed by one, while the second [N5-H6Á Á ÁCl1 i = 3.4368 (15) Å ; see Table 1] is weak.The N-HÁ Á ÁN distance within the centrosymmetric dimer is 3.0162 (17) Å , with the N-HÁ Á ÁO distances ranging from 2.6463 (17) to 3.0348 (17) Å . The N-HÁ Á ÁCl distances are 3.1281 (13) and 3.4368 (15) Å , and the O-HÁ Á ÁCl distances range from 3.1173 (14) to 3.1576 (13) Å . The O-HÁ Á ÁO hydrogen bond involving the carbonyl group is 2.7404 (15) Å .

Experimental
As part of an experimental polymorph screen on guanine, (I) was obtained from a solution of guanine in dilute hydrochloric acid which was allowed to evaporate at room temperature (10 ml solution, in 75 Â 25 mm vessels), forming block-shaped crystals. If the same guanine solution in dilute hydrochloric acid was allowed to evaporate at a slower rate by virtue of a smaller surface area, small block-like crystals of guaninium chloride monohydrate were obtained.
H atoms were refined independently using an isotropic model.  The hydrogen-bonded (dashed lines) planar unit in (I), showing the centrosymmetric dimer linked to four water molecules and two Cl À ions. The other hydrogen bonds have been omitted for clarity.

Figure 3
The crystal packing of (I), showing the complex three-dimensional hydrogen-bonding network (dashed lines).
Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2000) and MERCURY (Bruno et al., 2002); software used to prepare material for publication: SHELXL97. 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.