II. EXPERIMENTAL

The contents (powder) of a terazosin 5 mg capsule (Sandoz Inc., Holzkirchen, Germany) were removed from the gelatin case then front packed into a standard sample holder. The X-ray powder diffraction pattern was measured on a Bruker D2 Phaser diffractometer using CuK α radiation (5–70°2θ, 0.0202144° steps, 0.5 s step⁻¹, 0.6° divergence slit, 2.5° Soller slits, 3 mm scatter screen height). The major phase was α-lactose monohydrate (Figure 2). The peaks from the minor phase(s) did not match either of the two PDF entries for terazosin hydrochloride dihydrate, or that of anhydrous material from US Patent 5 856 482 (Cannata et al., Reference Cannata, Ferrario and Galbiati1999). The peak intensities of the terazosin hydrochloride dihydrate entries in the Powder Diffraction File (PDF^®) (ICDD, Reference Kabekkodu2017) patterns differ, indicating that one or both of them might exhibit preferred orientation (Figure 3). PDF entry 00-047-2073 is a low-precision “blank” pattern, but 00-060-1210 is a high-quality “star” pattern. Unit-cell and connectivity searches in the Cambridge Structural Database (Groom et al., Reference Groom, Bruno, Lightfoot and Ward2016) did not yield any crystal structures of terazosin derivatives.

Figure 3. (Color online) X-ray powder diffraction pattern of powder removed from a terazosin 5 mg capsule, compared with the two PDF entries for terazosin hydrochloride dihydrate and α-lactose monohydrate. The relative intensities in the two terazosin hydrochloride dihydrate database patterns differ, indicating that one or both of the patterns may suffer from preferred orientation.

The pattern of PDF entry 00-060-1210 was measured using CuK α ₁ radiation and was indexed on a primitive triclinic unit cell with a = 10.893(1), b = 11.845(1), c = 10.005(1) Å, α = 108.232(3), β = 113.451(2), γ = 89.442(3)°, V = 1115.04 Å³, and Z = 2. The reported cell was converted to the reduced cell using a tool in the PDF database. The terazosin cation was built in Spartan ‘16 (Wavefunction, 2017) and the minimum energy conformation was determined. The molecule was saved as a .mol2 file and converted into a Fenske-Hall Z-Matrix using OpenBabel (O'Boyle et al., Reference O'Boyle, Banck, James, Morley, Vandermeersch and Hutchison2011).

The structure solution using the experimental PD3 pattern from entry 00-060-1210 was difficult, which we believe to be the result of preferred orientation (the refined texture index was 1.383). What ultimately seems to have been successful was to use terazosin, Cl, and 2O as fragments in FOX (Favre-Nicolin and Černý, Reference Favre-Nicolin and Černý2002). Included in the solution process were variation of the March–Dollase ratio and the direction of the preferred orientation. A reasonable hydrogen bonding pattern appeared, but on refinement one of the water molecules moved too close to the ketone group (O···O = 1.89 Å). There was a void in the structure at 00½, which was filled with a water molecule. The water–ketone distance became much more reasonable. The compound appeared to be a hemipentahydrate. Although the refinement yielded plausible residuals (R _wp = 0.1287, R _p = 0.0966, χ ² = 54.17) (Figure 4), some features of the structure seemed to be chemically unusual. The C₄N₂ ring was in a boat conformation, and a 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) showed that the tetrahydrofuran ring was in an unusual conformation. We felt that more accurate characterization required a better data set, so a synchrotron powder pattern was obtained.

Figure 4. (Color online) The Rietveld plot for the refinement of terazosin hydrochloride dihydrate using the laboratory data of PDF entry 00-060-1210. 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 scale as the other patterns.

Terazosin hydrochloride dihydrate was a commercial reagent, purchased from USP (Lot #G0F290), 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 Argonne National Laboratory using a wavelength of 0.457667 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 initial refinement used the known unit cell, the previous structural model, and the 2–26° portion of the diffraction pattern (d _min = 1.017 Å), but was not completely satisfactory (reduced χ ² = 4.59). The structure was re-solved using the synchrotron data with FOX (sinθ/λ _max = 0.3 Å⁻¹). A terazosin cation, a chlorine atom, and two oxygen atoms (water molecules) were used as fragments. Two of the 20 parallel tempering cycles had cost factors much lower than the others (10% success rate). This model has a chair conformation of the C₄N₂ ring and a different orientation of the tetrahydrofuran ring. One of the water molecules was unreasonable (overlapped other atoms), and was placed manually in a small void in the structure.

Rietveld refinement was carried out using GSAS (Toby, Reference Toby2001; Larson and Von Dreele, Reference Larson and Von Dreele2004). Only the 2.0–26.0° portion of the pattern was included in the refinement (d _min = 1.017 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 Mogul average and standard deviation for each quantity were used as the restraint parameters. The restraints contributed 7.3% to the final χ ². The hydrogen atoms were included in calculated positions, which were recalculated during the refinement using Materials Studio (Dassault, 2016). Positions of the active hydrogens were derived by the analysis of potential hydrogen bonding patterns. A common U _iso was refined for the non-H atoms of the fused-ring system, another U _iso for the non-H substituent atoms, another for the non-H atoms of the C₄N₂ ring, another for the non-H atoms of the tetrahydrofuran ring, and a common U _iso for the water molecules. The U _iso for 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 (Thompson et al., Reference Thompson, Cox and Hastings1987; Finger et al., Reference Finger, Cox and Jephcoat1994), which includes the Stephens (Reference Stephens1999) anisotropic strain broadening model. The background was modeled using a three-term shifted Chebyshev polynomial, with a seven-term diffuse scattering function to model the Kapton capillary and any amorphous component. The final refinement of 138 variables using 24118 observations (24037 data points and 81 restraints) yielded the residuals R _wp = 0.0807, R _p = 0.0594, and χ ² = 4.006. The largest peak (1.10 Å from C42) and hole (1.48 Å from C6) in the difference Fourier map were 0.40 and −0.41 eÅ⁻³, respectively. The Rietveld plot is included as Figure 5. The largest errors in the fit are in the shapes of some of the strong peaks.

Figure 5. (Color online) The Rietveld plot for the refinement of terazosin hydrochloride dihydrate using the synchrotron data. The black crosses represent the observed data points, and the red line is the calculated pattern. The blue curve is the difference pattern, plotted at the same vertical scale as the other patterns. The vertical scale has been multiplied by a factor of 5 for 2θ > 7.4°, and by a factor of 40 for 2θ > 17.0°.

A density functional geometry optimization (fixed experimental unit cell) was carried out 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, and O atoms were those of Gatti et al. (Reference Gatti, Saunders and Roetti1994), and the basis set for chlorine 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 8 k-points and the B3LYP functional, and took ~48 h.

III. RESULTS AND DISCUSSION

The synchrotron and laboratory powder patterns match that of Figure 2 of US Patent 5 294 615 well enough (Figure 6) to conclude that the sample studied here is the crystalline terazosin hydrochloride dihydrate marketed as Hytrin^®.

Figure 6. (Color online) Comparison of the synchrotron pattern with that of terazosin hydrochloride dihydrate from Figure 2 of US Patent 5 294 615. The patent pattern (measured using CuK α radiation) was digitized using UN-SCAN-IT, corrected for a 1° shear in the patent figure using Adobe Illustrator, and re-scaled to the synchrotron wavelength of 0.457667 Å using Jade 9.7.

The refined atom coordinates of terazosin hydrochloride dihydrate and the coordinates from the density functional theory (DFT) optimization are reported in the Crystallographic Information Framework (CIF) files attached as Supplementary Material. The root-mean-square Cartesian displacement of the non-hydrogen atoms in the terazosin cations is 0.177 Å (Figure 7). The largest deviation is 0.351 Å at C51 in the tetrahydrofuran ring. The good agreement between the refined and optimized structures is evidence that the experimental structure is correct (van de Streek and Neumann, Reference van de Streek and Neumann2014). This discussion uses the DFT-optimized structure. The asymmetric unit (with atom numbering) is illustrated in Figure 8, and the crystal structure is presented in Figure 9. The packing diagram hints that some π–π interactions may be present, but there is no evidence for them in the DFT calculation.

Figure 7. (Color online) Comparison of the refined and optimized structures of the cation in terazosin hydrochloride dihydrate. The Rietveld refined structure is in red, and the DFT-optimized structure is in blue.

Figure 8. (Color online) The asymmetric unit of terazosin hydrochloride dihydrate, with the atom numbering. The atoms are represented by 50% probability spheroids.

Figure 9. (Color online) The crystal structure of terazosin hydrochloride dihydrate, viewed down the b-axis.

All of the bond distances, bond angles, and 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). The N26–C10 and N39–C40 bonds, which link the C₄N₂ ring to the other parts of the molecule, are significantly shorter (1.357 Å) than the C–N bonds within this ring (~1.465 Å). This shorter distance might provide evidence of multiple bonding, but the Mulliken overlap populations in these bonds do not differ significantly from the other C–N single bonds. Quantum chemical geometry optimizations (DFT/631G*/water) using Spartan ‘16 (Wavefunction, 2017) indicated that the observed conformation of terazosin in terazosin hydrochloride dihydrate is 9.0 kcal mole⁻¹ higher in energy than the local minimum energy conformation. The rms Cartesian displacement is 0.180 Å, and the largest differences are in the tetrahydrofuran and C₄N₂ rings of the molecule. Molecular mechanics conformational analysis indicated that the global minimum energy conformation (−8.4 kcal mole⁻¹) has a more compact conformation with the tetrahydrofuran end of the molecule curled toward the rest of the molecule, an indication intermolecular interactions are important in determining the solid-state conformation.

Two independent structure solutions and density functional optimizations yielded structures with slightly different orientations of the tetrahydrofuran ring. The crystal energies were within 0.03 kcal mole⁻¹ of each other, suggesting the possibility of disorder on that end of the molecule. The U _iso of the atoms in the tetrahydrofuran and the C₄N₂ rings are larger than those in the other portion of the molecule, but the difference Fourier map shows no clear evidence for disorder.

Analysis of the contributions to the total crystal energy using the Forcite module of Materials Studio (Dassault, 2016) suggests that angle, bond, and torsion distortion terms are significant in the intramolecular deformation energy, as might be expected from a fused ring system. The intermolecular energy contains significant contributions from 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.

As expected from the chemistry, hydrogen bonds are important in the crystal structure (Table I). The chloride anion accepts hydrogen bonds from three of the four protons of the water molecules. The energies of these three hydrogen bonds were calculated using a correlation described in Kaduk (Reference Kaduk2002). The chloride also acts as the acceptor in a N9–H54···Cl55 hydrogen bond from the protonated ring nitrogen; the short N···Cl distance established that the protonation occurred on N9. The Cl also participates in two weaker C–H···Cl hydrogen bonds. The amino group N23 acts as a hydrogen bond donor to two water molecules. The energies of these hydrogen bonds were calculated using the correlation in Wheatley and Kaduk (Reference Wheatley and Kaduk2018). For the remaining water molecule, hydrogen acts as a donor to the ketone oxygen O41. The energy of this hydrogen bond was calculated by the correlation of Rammohan and Kaduk (Reference Rammohan and Kaduk2018). The aromatic ring carbon C8 makes a strong C–H···O hydrogen bond to the water molecule O57. C–H···O and C–H···N hydrogen bonds also contribute to the crystal energy. Two of these are intramolecular, and presumably help determine the conformation of the molecule. In addition to the ketone oxygen O41, both of the ether oxygen atoms O13 and O14 act as hydrogen bond acceptors. Most of these hydrogen bonds are discrete, but the water molecule H60–O57–H61 and the chlorine Cl55 form a ring with graph set (Etter, Reference Etter1990; Bernstein et al., Reference Bernstein, Davis, Shimoni and Chang1995; Shields et al., Reference Shields, Raithby, Allen and Motherwell2000) R2,4(8). The terazosin molecules lie roughly in the (328) plane. The R2,4(8) hydrogen bond pattern links the planes of the molecules. The other hydrogen bonds contribute to a three-dimensional hydrogen bond network.

TABLE I. Hydrogen bonds (CRYSTAL14) in terazosin hydrochloride dihydrate.

*Intramolecular.

The volume enclosed by the Hirshfeld surface (Figure 10; Hirshfeld, Reference Hirshfeld1977; McKinnon et al., Reference McKinnon, Spackman and Mitchell2004; Spackman and Jayatilaka, Reference Spackman and Jayatilaka2009; Wolff et al., Reference Wolff, Grimwood, McKinnon, Turner, Jayatilaka and Spackman2012) is 546.43 Å³, 97.74% of 1/2 the unit-cell volume. The molecules are thus not tightly packed. All of the significant close contacts (red in Figure 10) involve the hydrogen bonds.

Figure 10. (Color online) The Hirshfeld surface of terazosin hydrochloride dihydrate. 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 blocky morphology for terazosin hydrochloride dihydrate, with {001}, {010}, and {011} as the principal faces. A sixth-order spherical harmonic preferred orientation model was included in the refinement; the texture index was 1.0210, indicating that preferred orientation was not significant in this rotated capillary specimen. The powder pattern of terazosin hydrochloride dihydrate from this synchrotron data set has been submitted to ICDD for inclusion in the Powder Diffraction File.