Single Crystal X-Ray Diffraction Structure of a Pseudo-polymorph of ( ± )-3-( ethoxycarbonyl )-2-( imidazol-1-yl ) Propionic Acid ( IEPA )

±)-3-(Ethoxycarbonyl)-2-(imidazol-1-yl) propionic acid (IEPA) crystallized in two different forms when recrystallized from absolute ethanol (Form I) or water (Form II), respectively. The single crystal X ray diffraction study of Form II unambiguously established that IEPA yielded a pseudopolymorph with a different crystalline packing when crystallized from water. This behaviour is additionally confirmed by Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG).


Introduction
(±)-3-(Ethoxycarbonyl)-2-(imidazol-1-yl) propionic acid (IEPA) (Figure 1) is currently the most appropriate extrinsic probe for non-invasive, preclinical measurements of extracellular pH by magnetic resonance 1 H spectroscopic imaging (MRSI) methods (Gil, Zaderenko, Cruz, Cerdán, & Ballesteros, 1994;Pacheco-Torres et al., 2011;Hashim, Zhang, Wojtkowiak, Martinez, & Gillies, 2011).Furthermore, it has been recently proposed as a convenient therapeutic buffer for the treatment of cancer and metastasis (Hashim et al., 2011).Detailed analyses of crystal structures and intra and intermolecular interactions are essential for the complete characterization and understanding of the functional properties of solids, as well as for the design of new materials (Hilfiker, 2006).We have described previously that IEPA, crystallized in two different forms when it was recrystallized either from absolute ethanol (Form I) or water (Form II) (Ubide-Barreda, Rojas, Martínez-de Paz, Righi, & Ballesteros, 2011).Comparison of powder X-ray diffraction pattern of Form I and that from Form II, showed clearly that the structures obtained in both cases were different.
In order to confirm this interesting feature we have performed here the single crystal X-ray diffraction analysis of Form II.This information will be useful to prepare and characterize improved IEPA formulations for oral administration (Hashim et al., 2011).

X-Ray Data Collection and Structure Refinement
Data collection was carried out at room temperature on a Bruker Smart CCD diffractometer using graphite-monochromated Mo-K radiation (λ=0.71073Å) operating at 50 kV and 35 mA.The data were collected over a hemisphere of the reciprocal space by combination of three exposure sets.Each exposure of 20s covered 0.3 in ω.The cell parameters were determined and refined by a least-squares fit of all reflections.The first 100 frames were recollected at the end of the data collection to monitor crystal decay, and no appreciable decay was observed.A summary of the fundamental crystal and refinement data is given in Table 1.
The structure was solved by direct methods and refined by full-matrix least-square procedures on F 2 (SHELXL-97) (Sheldrick, 1997).All non-hydrogen atoms were refined anisotropically.
The hydrogen atoms were included in their calculated positions and refined riding on the respective carbon atoms with the exception of hydrogens H5A and H5B bonded to O5 that were located in a Fourier synthesis and refined riding on the respective bonded atoms.
Further crystallographic details for the structure reported in this paper may be obtained from the Cambridge Crystallographic Data Center, on quoting the depository number CCDC-852977.

Thermal Analysis (TG and DSC)
Thermal analysis was carried out using a Seiko SSC 5200 TG-TA 320 System.A Sample of Form II (about 20 mg) was heated in nitrogen from 30 up to 125 ºC (flow rate = 50 mL•min −1 ) with a heating rate of 10 ºC•min −1 , soaked at this temperature for 3 min, then heated up to 160 ºC at a heating rate of 0.5 ºC•min -1 and finally from 160 up to 400 ºC at a heating rate of 10 ºC•min −1 .

Molecular and Crystal Structure of Form II
Bond distances and angles of Form II (Figure 1) are similar to those of Form I described previously (López et al., 1996).Tables 2 and 3 summarize bonds, angles, hydrogen bonds and torsion angles obtained from single crystal X-ray analysis of IEPA Form II.The chains are connected by hydrogen bonds involving the water molecules and the second oxygen atom of the carboxylate group with distances O5•••O2= 2.936(4) Ǻ and O5•••O2'=2.903(3)Ǻ.This interaction is extended along the "a" crystal axis.It becomes important to remark here the importance of the water molecule interactions in the stabilization of the crystal packing of Form II (Figure 2a).No intermolecular interactions are found involving the ester carboxylate oxygen.Torsion angles Degree (º) Torsion angles Degree (º)

Conclusions
The results obtained from single crystal X-ray diffraction study of Form II confirm unambiguously that (±)-3-(ethoxycarbonyl)-2-(imidazol-1-yl)propionic acid (IEPA) yields a pseudopolymorph when crystallized from water.The chains in the crystal are connected by hydrogen bonds involving the water molecules and the second oxygen atom of the carboxylate group with distances O5•••O2 = 2.936(4) Ǻ and O5•••O2'=2.903(3)Ǻ.This interaction is extended along the "a" crystal axis.The presence of water in the solvate is also confirmed by thermogravimetry.The results obtained herein will be useful to prepare and characterize new IEPA formulations for oral administration with improved bioavailability.

Figure 1 .
Figure 1.ORTEP view of IEPA (Form II) showing the atomic numbering; displacement ellipsoids are drawn at the 20% probability level Figure 2. Crystal packing of Form II.(a) Three-dimensional view of the zig-zag chains (b) View of one zig-zag chain along the "a" axis

Figure 3 .
Figure 3. TG (solid line) and DSC (dotted line ) thermograms of Form II

Table 1 .
Crystal data and structure refinement for C 9 H 14 N 2 O 5