II. EXPERIMENTAL

Atropine sulfate monohydrate was a commercial reagent, purchased from USP (Lomotil Lot # NOH159), and it was used as-received. The white powder was packed into a 1.5 mm diameter Kapton capillary and rotated during the measurement at ~50 Hz. The powder pattern was measured at 295 K at beamline 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 Argonne National Laboratory using a wavelength of 0.414157 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 with a = 19.2822, b = 6.9747, c = 26.9019 Å, β = 94.223°, V = 3608.18 Å³, and Z = 4 using DICVOL14 (Louër and Boultif, Reference Louër and Boultif2014). Analysis of the systematic absences using EXPO2014 (Altomare et al., Reference Altomare, Cuocci, Giacovazzo, Moliterni, Rizzi, Corriero and Falcicchio2013) suggested the space groups P2₁ and P2₁/n. Since this compound is known to be a racemate and the cell volume corresponded to Z = 4, P2₁/n was selected, and confirmed by successful solution and refinement of the structure. An unindexed peak at 3.491° 2θ shows that the sample contains an impurity. A reduced cell search in the Cambridge Structural Database (Groom et al., Reference Groom, Bruno, Lightfoot and Ward2016) combined with the chemistry C, H, N, O, and S only yielded no hits. Two copies of the hyoscyamine molecule from Kaduk et al. (Reference Kaduk, Gindhart and Blanton2019), a sulfate anion, and an oxygen atom (water molecule) were used as fragments. The structure was solved by Monte Carlo simulated annealing using DASH (David et al., Reference David, Shankland, van de Streek, Pidcock, Motherwell and Cole2006), including Mogul Distribution Bias and {010} preferred orientation (guessed based on the anisotropy of the lattice parameters). Only one of the 100 cycles yielded a chemically reasonable solution, with both N–H cation sites hydrogen-bonded to the sulfate anion; the success rate was 1%. The water molecule oxygen atom was too close to a cation, so it was removed from the model. There was a void (calculated by Mercury, with a probe radius of 1.2 Å) at 0.572, 0.653, 0.224 and reasonably placed for hydrogen bonding, so the water molecule oxygen atom O96 was placed there to begin refinement. Rietveld refinement was carried out using GSAS-II (Toby and Von Dreele, Reference Toby and Von Dreele2013). Only the 1.2–20.0° portion of the pattern was included in the refinement (d _min = 1.192 Å). 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 results were exported to a csv file. The Mogul average and standard deviation for each quantity were used as the restraint parameters and were incorporated using the new feature Restraints/Edit Restraints/Add MOGUL Restraints, which reads the bond distance and angle restraints from the csv file. The two aromatic rings were restrained to be planar. The restraints contributed 3.8% to the final χ ². The hydrogen atoms were included in calculated positions, which were recalculated during the refinement using Materials Studio (Dassault, 2017). Positions of the active hydrogens were derived by analysis of potential hydrogen bonding patterns. A common U _iso was refined for the carbon atoms of the two phenyl rings, separate U _iso for the atoms of the two bicyclic systems, another for the rest of the cation atoms, and a common U _iso for the O atoms of the sulfate anion. The U _iso for each hydrogen atom was constrained to be 1.3× that of the heavy atom to which it is attached. The background was modeled using a three-term shifted Chebyshev polynomial, and a five-term diffuse scattering function to model the Kapton capillary and any amorphous component.

The final refinement of 188 variables using 18 802 observations and 122 restraints yielded the residuals R _wp = 0.1264 and Goodness of Fit (GOF) = 2.59. The largest peak (0.24 Å from C2) and a hole in the difference Fourier map were 0.40 and −0.37(9) eÅ⁻³, respectively. The Rietveld plot is included in Figure 2. The largest errors in the fit are at the impurity peak at 3.491° and potentially other impurity peaks.

Figure 2.

(Colour online) Rietveld plot for the refinement of atropine sulfate monohydrate. The blue crosses represent the observed data points, and the green line is the calculated pattern. The cyan curve is the normalized error plot.

A density functional geometry optimization (fixed experimental unit cell) was carried out using VASP (Kresse and Furthmüller, Reference Kresse and Furthmüller1996) through the MedeA graphical interface (Materials Design, 2016). The calculation was carried out on 16 2.4 GHz processors (each with 4 GB RAM) of a 64-processor HP Proliant DL580 Generation 7 Linux cluster at North Central College. The calculation used the GGA-PBE functional, a plane wave cutoff energy of 400.0 eV, and a k-point spacing of 0.5 Å⁻¹ leading to a 1 × 2 × 1 mesh, and took ~240 h. A density functional geometry optimization was carried out from the (not-quite-converged) VASP output using CRYSTAL14 (Dovesi et al., Reference Dovesi, Orlando, Erba, Zicovich-Wilson, Civalleri, Casassa, Maschio, Ferrabone, De La Pierre, D-Arco, Noël, Causà and Kirtman2014). 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 sulfur was that of Peintinger et al. (Reference Peintinger, Vilela Oliveira and Bredow2013). The calculation was run on eight 2.1 GHz Xeon cores (each with 6 GB RAM) of a 304-core Dell Linux cluster at IIT, using eight k-points and the B3LYP functional, and took ~188 h.

III. RESULTS AND DISCUSSION

The powder pattern of atropine sulfate monohydrate obtained in this study matches that of the hydrated atropine sulfate in WO2014102829 A1 Figure 1 (Gore et al., Reference Gore, Joshi, Tripathi, Jadhav and Bhandari2013; Figure 3) well enough to conclude that the two compounds are the same. The refined atom coordinates of atropine sulfate monohydrate and the coordinates from the density functional theory (DFT) optimization are reported in the CIFs attached as Supplementary Material.

Figure 3.

(Colour online) Comparison of the synchrotron pattern of atropine sulfate monohydrate with the pattern from Gore et al. (Reference Gore, Joshi, Tripathi, Jadhav and Bhandari2013; Mylan). The literature pattern (measured using CuKα radiation) was digitized using UN-SCAN-IT (Silk Scientific, 2013) and scaled to the synchrotron wavelength of 0.414157 Å using Jade 9.8 (MDI, 2017).

Comparing the Rietveld refined and the optimized structures, the root-mean-square Cartesian displacements of the non-H atoms in the atropine cations are 0.481 and 0.278 Å [Figures 4(a) and 4(b)]. The agreement between the refined and optimized structures is at the upper end of the range for correct structures (van de Streek and Neumann, Reference van de Streek and Neumann2014). Most of the displacement coefficients (especially those of the sulfate anion) are large, indicating the possibility of disorder. This discussion concentrates on the CRYSTAL-optimized structure. The asymmetric unit (with the atom numbering) is illustrated in Figure 5, and the crystal structure is presented in Figure 6.

Figure 4.

(Colour online) (a) Comparison of the Rietveld refined (red) and CRYSTAL14-optimized (blue) structures of atropine cation 1. (b) Comparison of the Rietveld refined (red) and CRYSTAL14-optimized (blue) structures of atropine cation 2.

Figure 5.

(Colour online) Asymmetric unit of atropine sulfate monohydrate, with the atom numbering. The atoms are represented by 50% probability spheroids.

Figure 6.

(Colour online) Crystal structure of atropine sulfate monohydrate, viewed down the b-axis.

The two independent atropine cations have different conformations (Figure 7). All of the bond distances and angles and almost all of the torsion angles fall within the normal ranges indicated by a Mercury/Mogul Geometry Check (Macrae et al., Reference Macrae, Bruno, Chisholm, Edington, McCabe, Pidcock, Rodriguez-Monge, Taylor, van de Streek and Wood2008). In cation 1 (the lower atom numbers), the O22–C4–C3–C5 torsion angle is flagged as unusual; it lies in a minor gauche portion of the distribution of mainly trans conformations. In cation 2, the torsion angles involving rotation about the C48–C50 and C48–C63 bonds lies in the tails of broad distributions of similar torsion angles. The O67–C49–C48–C63 torsion angle is truly unusual. Cation 2 is more unusual than cation 1.

Figure 7.

(Colour online) Comparison of the conformations of atropine cations 1 (green) and 2 (purple).

Quantum chemical geometry optimizations (DFT/B3LYP/6-31G*/water) using Spartan ‘18 (Wavefunction, 2018) indicated that the observed conformation of cation 1 is 2.0 kcal mol^-1 lower in energy than that of cation 2. The difference is close to the expected accuracy of such calculations, so the two conformations can be considered similar in energy. Molecular mechanics conformational analysis indicated that cations 1 and 2 are 2.7 and 4.3 kcal mol^-1 higher in energy than the global minimum energy conformation. The root-mean-square Cartesian displacements from the global minimum energy conformation are 1.148 and 0.927 Å, respectively. The conformational differences are relatively small and are spread throughout the molecules. Intermolecular interactions play a role in determining the observed solid-state conformations.

Since atropine is the racemic version of hyoscyamine, it is worth comparing the cations in this structure to the two independent cations in the hyoscyamine crystal structure (Kaduk et al., Reference Kaduk, Gindhart and Blanton2019). The root-mean-square Cartesian displacements of the non-H atoms are reported in Table I. The differences are perhaps larger than expected and indicate that the molecule is flexible enough to accommodate differences in hydrogen bonding and van der Waals interactions.

TABLE I.

Root-mean-square Cartesian displacements (Å, Mercury molecule overlay) between the two independent cations in the atropine structure and the two independent cations in the hyoscyamine structure.

Analysis of the contributions to the total crystal energy using the Forcite module of Materials Studio (Dassault, 2017) suggests that angle distortion terms are significant in the intramolecular deformation energy, as might be expected for a bicyclic system. The intermolecular energy is dominated by electrostatic attractions, which in this force-field-based analysis include hydrogen bonds. The hydrogen bonds are better analyzed using the results of the DFT calculation.

Hydrogen bonds are prominent in the crystal structure (Table II). Each protonated nitrogen atom (N19–H1 and N64–H46) participates in a strong hydrogen bond to the sulfate anion. The energies of these N–H⋯O hydrogen bonds were calculated according to the correlation of Wheatley and Kaduk (Reference Wheatley and Kaduk2019). Only one of the water molecule hydrogen atoms acts as a hydrogen bond donor to the sulfate anion. The energy of this O–H⋯O hydrogen bond was calculated by the correlation of Rammohan and Kaduk (Reference Rammohan and Kaduk2018). Each of the two hydroxyl groups O67–H90 and O22–H45 acts as a donor in a hydrogen bond to the sulfate anion. Many C–H⋯O hydrogen bonds apparently also contribute to the crystal energy.

TABLE II.

Hydrogen bonds (CRYSTAL14) in atropine sulfate monohydrate.

* indicates an intramolecular distance.

The hydrogen bonds are all discrete but link the cations, anion, and water molecule along [101] (Figure 6). Prominent in the crystal structure are clusters of four phenyl rings. Visually, phenyl–phenyl interactions seem to play a role in the packing, but this is not obvious from an examination of the Mulliken overlap populations. The atropine and hyoscyamine crystal structures share some features, such as hydrogen bonding and phenyl–phenyl packing, but as the powder patterns (Figure 8) show that the structures are very different. The CRYSTAL14 energies suggest that the hyoscyamine structure is more stable.

Figure 8.

(Colour online) Comparison of the powder patterns of atropine (red) and hyoscyamine (cyan) sulfate monohydrate.

The volume enclosed by the Hirshfeld surface (Figure 9; Hirshfeld, Reference Hirshfeld1977; Turner et al., Reference Turner, McKinnon, Wolff, Grimwood, Spackman, Jayatilaka and Spackman2017) is 891.35 Å³, 98.74% of one-fourth of the unit cell volume. The molecules are thus not tightly packed. All of the significant-close contacts (red in Figure 9) involve the hydrogen bonds. The volume/non-H atom is 18.8 Å³.

Figure 9.

(Colour online) Hirshfeld surface of atropine sulfate monohydrate. Intermolecular contacts longer than the sums of the van der Waals radii are colored blue, and contacts shorter than the sums of the radii are colored red. Contacts equal to the sums of radii are white.

The Bravais–Friedel–Donnay–Harker (Bravais, Reference Bravais1866; Friedel, Reference Friedel1907; Donnay and Harker, Reference Donnay and Harker1937) morphology suggests that we might expect needle-like morphology for atropine sulfate monohydrate, with {010} as the long axis, consistent with the anisotropy of the lattice parameters. A sixth-order spherical harmonic model was included in the refinement. The texture index was 1.093, indicating that preferred orientation was significant in this rotated capillary specimen. The powder pattern of atropine sulfate monohydrate from this synchrotron data set has been submitted to ICDD for inclusion in the Powder Diffraction File™.