I. INTRODUCTION
Levalbuterol, as a β 2-adrenergic receptor on airway smooth muscle, is used for the treatment of asthma and chronic obstructive pulmonary disease (Jat and Khairwa, Reference Jat and Khairwa2013). The systematic name (CAS registry number 50293-90-8) is (R)-α 1-{[(1,1-dimethylethyl)amino]methyl}-4-hydroxy-1,3-benzenedimethanol hydrochloride. A two-dimensional (2D) molecular structure diagram is shown in Figure 1.
The molecular structure of levalbuterol hydrochloride.
The presence of high-quality reference powder patterns in the Powder Diffraction File (PDF; ICDD, Reference Kabekkodu2014) is important for phase identification, particularly by pharmaceutical, forensic, and law enforcement scientists. The crystal structures of a significant fraction of the largest dollar volume pharmaceuticals have not been published, and thus calculated powder patterns are not present in the PDF-4 databases. Sometimes experimental patterns are reported, but they are generally of low quality. Accordingly, a collaboration among ICDD, Illinois Institute of Technology (IIT), Poly Crystallography Inc., and Argonne National Laboratory has been established to measure high-quality synchrotron powder patterns of commercial pharmaceutical ingredients, include these reference patterns in the PDF, and determine the crystal structures of these active pharmaceutical ingredients (APIs).
Even when the crystal structure of an API is reported, the single-crystal structure was often determined at low temperature. Most powder measurements are performed at ambient conditions. Thermal expansion (often anisotropic) means that the peak positions calculated from a low-temperature single-crystal structure often differ significantly from those measured at ambient conditions. These peak shifts can result in failure of default search/match algorithms to identify a phase, even when it is present in the sample. High-quality reference patterns measured at ambient conditions are thus critical for easy identification of APIs using standard powder diffraction practices.
II. EXPERIMENTAL
Levalbuterol hydrochloride was commercial reagent, purchased from United States Pharmacopeia (USP) (Lot FOG162), and was used as-received. The white powder was packed into a 1.5 mm diameter Kapton capillary, and rotated during the measurement at ~50 cycles s−1. The powder pattern was measured at 295 K at beam line 11-BM (Lee et al., Reference Lee, Shu, Ramanathan, Preissner, Wang, Beno, Von Dreele, Ribaud, Kurtz, Antao, Jiao and Toby2008; Wang et al., Reference Wang, Toby, Lee, Ribaud, Antao, Kurtz, Ramanathan, Von Dreele and Beno2008) of the Advanced Photon Source at the Argonne National Laboratory using a wavelength of 0.413 914 Å from 0.5° to 50° 2θ with a step size of 0.001° and a counting time of 0.1 s step−1. The pattern was indexed on a primitive monoclinic unit cell having a = 8.4982, b = 6.5092, c = 13.1788 Å, β = 102.107°, V = 712.80 Å3, and Z = 2 using DICVOL06 (Louër and Boultif, Reference Louër and Boultif2007). A search of this cell in the Cambridge Structural Database (Allen, Reference Allen2002) yielded 42 hits, but no crystal structure for levalbuterol hydrochloride. Analysis of the systematic absences using EXPO2009 (Altomare et al., Reference Altomare, Camalli, Cuocci, Giacovazzo, Moliterni and Rizzi2009) indicated the space group to be P21 (a common space group for chiral organic compounds), which was confirmed by successful solution and refinement of the structure.
A levalbuterol cation was built and its conformation optimized using Spartan′14 (Wavefunction, 2013), and saved as a mol2 file. This file was converted into a Fenske–Hall Z-matrix file using OpenBabel (O'Boyle et al., Reference O'Boyle, Banck, James, Morley, Vandermeersch and Hutchison2011). This molecule and a chlorine atom were used to solve the structure with FOX (Favre-Nicolin and Černý, Reference Favre-Nicolin and Černý2002). The maximum sinθ/λ used in the solution was 0.26 Å−1.
Rietveld refinement was carried out using GSAS (Larson and Von Dreele, Reference Larson and Von Dreele2004). Only the 1.7–30° 2θ portion of the pattern was included in the refinement. All non-H bond distances and angles were subjected to restraints, based on a Mercury/Mogul Geometry Check (Bruno et al., Reference Bruno, Cole, Kessler, Luo, Motherwell, Purkis, Smith, Taylor, Cooper, Harris and Orpen2004; Sykes et al., Reference Sykes, McCabe, Allen, Battle, Bruno and Wood2011) of the molecule. The Mogul average and standard deviation for each quantity were used as the restraint parameters. The C1–C6 aromatic ring was subjected to a planar restraint with a standard deviation of 0.01 Å. The restraints contributed 0.5% to the final χ 2. Isotropic displacement coefficients were refined, grouped by chemical similarity. The hydrogen atoms were included in calculated positions, which were recalculated during the refinement. The U iso of each hydrogen atom was constrained to be 1.3× that of the heavy atom to which it is attached. The peak profiles were described using profile function #4, which includes the Stephens (Reference Stephens1999) anisotropic strain broadening model. The background was modeled using a 12-term shifted Chebyshev polynomial. The final refinement of 84 variables using 28 343 observations yielded the residuals wRp = 0.0806, Rp = 0.0665, and χ 2 = 1.969. The largest peak (0.20 Å from N9) and hole (0.187 Å from O17) in the difference Fourier map were 0.52 and −0.57 e(Å−3), respectively. The Rietveld plot is included as Figure 2. The largest errors are in the shapes and positions of the lowest-angle peaks, and may reflect subtle changes in the specimen during the measurement.
(Colour online) The Rietveld plot for the refinement of levalbuterol hydrochloride polymorph A. The red crosses represent the observed data points, and the green line is the calculated pattern. The magenta curve is the difference pattern, plotted at the same vertical scales as the other patterns. The vertical scale has been multiplied by a factor of 10 for 2θ > 10.5°, and by a factor of 50 for 2θ > 21.0°.
A density functional geometry optimization (fixed experimental unit cell) was carried out using CRYSTAL09 (Dovesi et al., Reference Dovesi, Orlando, Civalleri, Roetti, Saunders and Zicovich-Wilson2005). The basis sets for the H, C, N, and O atoms were those of Gatti et al. (Reference Gatti, Saunders and Roetti1994), and the basis set for Cl was that of Apra et al. (Reference Apra, Causa, Prencipe, Dovesi and Saunders1993). The calculation used 8 k-points and the B3LYP functional.
III. RESULTS AND DISCUSSION
The observed powder pattern corresponds to the pattern of levalbuterol hydrochloride Polymorph A reported in U.S. Patent 7,465,831 (Merli et al., Reference Merli, Mantovani, Bianchi, Daverio, Spreafico, Aronhime and Kovacsne-Mezei2008). The pattern of Polymorph B is reported in U.S. Patent 7,488,758 (Merli et al., Reference Merli, Mantovani, Bianchi, Daverio, Spreafico, Aronhime and Kovacsne-Mezei2009). The refined atom coordinates of levalbuterol hydrochloride polymorph A are reported in Table I, and the coordinates from the density functional theory (DFT) optimization in Table II. The root-mean-square deviation of the non-hydrogen atoms is 0.13 Å, and the maximum deviation is 0.30 Å, at O17 (Figure 3). The discussion of the geometry uses the DFT-optimized structure. The asymmetric unit (with atom numbering) is illustrated in Figure 4, and the crystal structure is presented in Figure 5.
(Colour online) Comparison of the refined and optimized structures of levalbuterol hydrochloride polymorph A. The Refined structure is colored red, and the DFT-optimized structure is in blue.
(Colour online) The molecular structure of levalbuterol hydrochloride polymorph A, with the atom numbering. The atoms are represented by 50% probability spheroids.
(Colour online) The crystal structure of levalbuterol hydrochloride polymorph A, viewed down the b-axis.
Rietveld refined fractional atomic coordinates and isotropic displacement parameters (Å2) of levalbuterol hydrochloride polymorph A. Space group P21, a = 8.499 352(15), b = 6.511 726(13), c = 13.182 256(25) Å, β = 102.1157(2)°, V = 713.327(2) Å3, and Z = 2.
DFT (CRTYSTAL09) optimized fractional atomic coordinates and isotropic displacement parameters (Å2) of levalbuterol hydrochloride polymorph A. Space group P21, a = 8.499 352(15), b = 6.511 726(13), c = 13.182 256(25) Å, β = 102.1157(2)°, V = 713.327(2) Å3, and Z = 2.
All bond distances and angles fall within the normal ranges indicated by a Mercury Geometry Check. The O15–C14–C3–C2 and O15–C14–C3–C4 torsion angles, which describe the relation of the hydroxymethyl and phenyl groups, are flagged as unusual. The position of the C15–C14–C3–C2 torsion in the distribution is illustrated in Figure 6. A conformational energy analysis in Spartan suggests that the cation conformation in the crystal structure is higher in energy than the minimum energy conformation. Presumably the cation has distorted to accommodate the hydrogen bonding.
(Colour online) The O15–C14–C3–C2 torsion angle in relation to the distribution from the Mogul geometry search. This angle is unusual, but not unprecedented.
An analysis of the contributions to the total crystal energy using the Forcite module of Materials Studio (Accelrys, 2013) suggests that the bond, angle, and torsion angle distortion contributions are small. The energy appears to be dominated by electrostatic contributions, which in this force-field-based analysis include hydrogen bonds. The hydrogen bonds are better analyzed using the results of the DFT calculation.
The most prominent feature of the structure is the two different types of hydrogen bonds (Table III). Both of the ammonium hydrogens and O17–H40 act as donors to Cl18. The graph sets (Etter, Reference Etter1990; Bernstein et al., Reference Bernstein, Davis, Shimoni and Chang1995; Shields et al., Reference Shields, Raithby, Allen and Motherwell2000) are C1,2(4) and C1,2(7). The result is a zig-zag chain parallel to the b-axis. The two hydroxy groups at the phenyl end of the molecule form another set of chains, both with graph set, C1,1(6), also resulting in a chain parallel to the b-axis. The bulk of the molecule connects these chains in the (101) plane to form a three-dimensional network.
Hydrogen bonds in levalbuterol hydrochloride polymorph A.
The Bravais–Friedel–Donnay–Harker (Bravais, Reference Bravais1866; Friedel, Reference Friedel1907; Donnay and Harker, Reference Donnay and Harker1937) morphology suggests that we might expect platy morphology for levalbuterol hydrochloride polymorph A, with {001} as the principal faces; no preferred orientation correction was necessary. The powder pattern of levalbuterol hydrochloride polymorph A is included in the PDF as entry 00-064-1494.
SUPPLEMENTARY MATERIALS AND METHODS
The supplementary material for this article can be found at http://www.journals.cambridge.org/PDJ
ACKNOWLEDGMENTS
Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work was partially supported by the International Centre for Diffraction Data. The authors thank Lynn Ribaud for his assistance in data collection.
