Article pubs.acs.org/molecularpharmaceutics
Myelography Iodinated Contrast Media. 2. Conformational Versatility of Iopamidol in the Solid State Barbara Bellich,† Silvia Di Fonzo,‡ Letizia Tavagnacco,† Marco Paolantoni,§ Claudio Masciovecchio,‡ Federica Bertolotti,∥ Giovanna Giannini,† Rita De Zorzi,† Silvano Geremia,† Alessandro Maiocchi,⊥ Fulvio Uggeri,⊥ Norberto Masciocchi,*,∥ and Attilio Cesàro*,†,‡ †
Laboratory of Physical and Macromolecular Chemistry, Department of Chemical and Pharmaceutical Sciences, University of Trieste, via Giorgieri 1, I-34127 Trieste, Italy ‡ Elettra Sincrotrone Trieste, Area Science Park, I-34149 Trieste, Italy § Department of Chemistry, Biology and Biotechnologies, University of Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy ∥ Department of Science and High Technology, To.Sca.Lab and INSTM, University of Insubria, via Valleggio 11, I-22100 Como, Italy ⊥ Centro Ricerche Bracco, Bracco Imaging SpA, via Ribes 5, I-10010 Colleretto Giacosa, Turin, Italy S Supporting Information *
ABSTRACT: The phenomenon of polymorphism is of great relevance in pharmaceutics, since different polymorphs have different physicochemical properties, e.g., solubility, hence, bioavailability. Coupling diffractometric and spectroscopic experiments with thermodynamic analysis and computational work opens to a methodological approach which provides information on both structure and dynamics in the solid as well as in solution. The present work reports on the conformational changes in crystalline iopamidol, which is characterized by atropisomerism, a phenomenon that influences both the solution properties and the distinct crystal phases. The conformation of iopamidol is discussed for three different crystal phases. In the anhydrous and monohydrate crystal forms, iopamidol molecules display a syn conformation of the long branches stemming out from the triiodobenzene ring, while in the pentahydrate phase the anti conformation is found. IR and Raman spectroscopic studies carried out on the three crystal forms, jointly with quantum chemical computations, revealed that the markedly different spectral features can be specifically attributed to the different molecular conformations. Our results on the conformational versatility of iopamidol in different crystalline phases, linking structural and spectroscopic evidence for the solution state and the solid forms, provide a definite protocol for grasping the signals that can be taken as conformational markers. This is the first step for understanding the crystallization mechanism occurring in supersaturated solution of iopamidol molecules. KEYWORDS: iopamidol, conformational versatility, XRD, Raman, IR, thermodynamics
1. INTRODUCTION The existence of multiple conformers of molecular species in solution can give rise to anomalous crystallization processes, making the determination of the structure and energetics of the crystal phases, and the assessment of the conformational stability in solution, compulsory steps for the definition of the mechanism driving molecules in solution to assemble into distinct conformational polymorphs or solvate forms. Polymorphism and conformational polymorphism in molecular crystals have been widely illustrated in the literature and © 2017 American Chemical Society
originate from the interplay between intra- and intermolecular interactions, i.e., molecular flexibility and packing.1 Still, according to the literature, molecular flexibility in conformational polymorphs must refer to conformers that do not Received: Revised: Accepted: Published: 468
October 7, 2016 January 5, 2017 January 6, 2017 January 6, 2017 DOI: 10.1021/acs.molpharmaceut.6b00902 Mol. Pharmaceutics 2017, 14, 468−477
Article
Molecular Pharmaceutics populate the same potential well.2 Indeed, as nucleation occurs in solution and crystals grow from these elusive seeds, it is commonly assumed that a strict structural relation exists between the (not necessarily most probable) conformation of the molecules in solution and the emerging crystal structure. Before, or even during, the nucleation process, a flexible molecule may also undergo a conformational change, driven by the nature and strength of the molecular interactions in solution and, ultimately, in the crystal for the selection of the resultant crystal structure. Several studies have appeared on the correlation between multiple conformational equilibria in solution and the potential mechanism of selecting the “right conformer”, i.e., the conformational form occurring in the solid.3−5 Specifically, since different polymorphs have different physicochemical properties, e.g., solubility, bioavailability, and process rheology, the phenomenon of polymorphism (including the conformational one) has become the focus of many basic and applied science studies, with a steadily increasing relevance in pharma industry (and patent regulation issues). Atropisomerism is a kind of isomerism due to the restricted rotation about a single bond giving rise to conformers that can be isolated as distinct chemical species. A relevant example of atropisomeric molecule is represented by iopamidol, a nonionic, low-osmolar iodinated contrast agent.6−11 Aqueous solutions are used as injectable contrast media for radiological analysis, such as urography, angiography, and mielography. A high concentration is required to provide good quality images and short exposure time. Its molecular structure (Figure 1) is
characterized by an aromatic ring bearing three iodine atoms, covalently linked in positions 2, 4, and 6, and three hydrophilic side arms (in position 1 via amino group and in positions 3 and 5 via carbonyl group),12 that increase the solubility in water. The rotation of the three hydrophilic side arms is relatively hampered due to the presence of iodine atoms with a slow interconversion between the syn- and anti-conformers (Figure 1). The two iopamidol conformational forms (namely, syn and anti) are present in aqueous solution in a ratio of 1:2 at room temperature with the hindered interconversion barrier of about 80 kJ/mol.8,13,14 Iopamidol is indeed characterized by interconversion half-lives of minutes, with a recently determined14 interconversion time placing this molecule in the bottom part of class 2 of the scheme given by LaPlante.15 Long interconversion times would enable the “atropisomeric forms” to behave as true thermodynamically distinct species, thus slowing down or even inhibiting precipitation at the formal saturation (the actual concentration of the form in equilibrium with the crystalline species would be below the saturation limit). Therefore, the kinetics aspect is an important issue for the conformational syn−anti equilibrium and stability of iopamidol in solution. The intrinsic interest in the putative conformational differences of iopamidol crystal forms lies in the existence of several isomers, some of them separated by high energy barrier (of the order of 80 kJ/mol), still interconverting in the solution state. Under these circumstances, not only are the structural details from crystallographic and spectroscopic studies necessary but also the interplay between intra- and intermolecular energetics is worthy of investigation.1 Although the correct definition of polymorphs excludes that this term can be attributed to compounds that differ in the chemical formulas (e.g., because of the presence of different amount and nature of lattice solvent molecules), here we specifically focus our attention on the molecular conformation of iopamidol, as found in several crystalline forms. By exploring the temperature-induced transformations, up to three different crystalline polymorphs, an anhydrous form, and other four solvates have been identified (Bellich et al. current submission to J. Therm. Anal. Calorim.), the conformational key being a fundamental parameter for their classification. In the specific case of iopamidol, nucleation and precipitation from an oversaturated aqueous solution at room temperature generate pentahydrate crystals, which are the less soluble form; another energetically stable solvate, a monohydrate form, has also been identified.16 While a third crystalline form (an anhydrous phase) has been reported and its structure determined,12 none of the two hydrates have been so far structurally characterized. In particular, it is not known whether crystallization of the pentahydrate form (the least soluble one) from a solution containing both the syn- and the anticonformations (in the ratio of 1:2) includes only one or both conformers and, if both are present, what their ratio is. In addition, a complete picture of the intermolecular interactions in the solid state would provide the relevant information on packing geometry leading to crystal formation. Furthermore, whether the soluble aggregates preceding nucleation and precipitation initially contain only the right conformer or the aggregation occurs indifferently between syn- and antimolecular conformers is still to be clarified. According to the literature,3,5 two main routes can be envisaged: in the first case, only one of the two forms in equilibrium, the anti-form or the syn-form, is able to aggregate in solution, giving rise to primary
Figure 1. Molecular structure of iopamidol with atom identification (top) and nomenclature for the atropisomeric forms generated by hindered rotations (bottom). 469
DOI: 10.1021/acs.molpharmaceut.6b00902 Mol. Pharmaceutics 2017, 14, 468−477
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Molecular Pharmaceutics
scaled with the SCALEPACK22 program. An empirical correction for the absorption effects has been applied with the software XABS2.23 Structure solution was performed by direct methods included in SHELXS,20 in the P21/c space group, and subsequent Fourier methods. The refinement has been carried out using the software SHELX9720 by full-matrix least-squares on F2. All non-hydrogen atoms, except a disordered oxygen atom and a disordered carbon atom (vide infra), were refined anisotropically. The hydrogen atoms of the iopamidol molecule were modeled by the riding mode; those of the water molecules have been refined adding constraints that take into account the molecule geometry, with restrained thermal parameters. During modeling of the crystal structures of the two hydrated forms (monohydrate and pentahydrate), we used the centrosymmetric P1̅ and P21/c space groups (instead of the non-centrosymmetric P1 and P21 ones), as the relative positions of one hydroxyl and of one methyl on the same side arm (those defining chirality) only marginally change the overall packing features, allowing an accurate structural model for the remaining part of the molecule to be firmly assessed. Thus, the structural models here proposed for the monohydrate and the pentahydrate forms were eventually refined with a 50:50 disorder (over two sites) for the CH3/OH residues of the 2-hydroxy-1-oxopropyl moieties and with a minimal increase of the agreement factors, with respect to those of the acentric models. Crystal data and other refinement parameters can be found in the CIF files deposited with the CCDC, Ref. No. 1483721 and 1025617, for the mono- and pentahydrate crystal phases, respectively. 2.2.2. FTIR Experiments. Infrared spectra were obtained using a FTIR Bruker spectrometer (mod. Tensor 27) equipped with a deuterium triglycine sulfate (DTGS) detector. FTIR spectra of solid samples dispersed in KBr pellets were recorded averaging over 50 scans with 2 cm−1 resolution. The spectrum of the iopamidol solution was obtained in transmission mode with a cell equipped with CaF2 windows and reported after subtraction of the spectrum of pure water. 2.2.3. Raman Scattering Experiments. Iopamidol crystalline powders (anhydrous, monohydrate, and pentahydrate) and amorphous glass were investigated by Raman scattering experiments carried out at the beamline BL10.2−IUVS at the Elettra Synchrotron Laboratory in Trieste. A full description of the experimental setup can be found in ref 24. The Raman spectra were acquired at fixed wavelength λ = 532 nm and collected, at room temperature, in backscattering geometry, using a triple stage spectrometer (Trivista Princeton Instrument). The overall frequency ranged from 300 to 3100 cm−1, split in 10 different frequency windows. The acquisition time for each spectral window was 5 min. The experimental resolution was set to 5 cm−1, in order to achieve a good compromise with the achievable count rate. 2.2.4. Quantum Chemical Computations. Simulations of vibrational spectra were carried out with Gaussian 09 software package, by using the supercomputer cluster PLX of the CINECA computing facilities. Geometry optimization and vibrational frequencies for iopamidol crystals were calculated with the density functional theory employing the B3LYP exchange-correlation function together with a 6-31+G* basis set for C, H, O, and N atoms and the LANL2DZ basis for I atoms. The crystallographic coordinates for the three crystals,
nuclei that further grow in crystals; the second hypothesis is that the aggregation is not selective toward a specific conformation and the change in conformation arises in a second step as a result of the different mean field surrounding the aggregate. Since a deeper understanding of the connection between the behavior in solution and the related solid state is helpful in the design of a stable formulation of the solution, this paper reports on the structural, energetic, and spectroscopic characterization of the solid forms of iopamidol, i.e., the three crystal forms and the amorphous glass phase. In this investigation X-ray diffraction, FT-IR absorption, and Raman scattering experiments were carried out with the support of quantum chemistry computation, employed in simulating the vibrational spectra. Taking advantage of the previous evidence on isomeric equilibria in solution, our results are discussed with the aim of identifying a consistent protocol for studying the conformational isomerism and polymorphism of active pharmaceutical ingredients. This study is indeed a key step in our long-term characterization of iopamidol and related molecules, targeted to clarifying the role of the quasi-atropisomerism during the crystallization process.
2. MATERIALS AND METHODS 2.1. Materials. Iopamidol anhydrous and monohydrate polycrystalline powders were supplied by Bracco Imaging SpA, Italy. Iopamidol pentahydrate was obtained by inducing precipitation from aqueous solution at room temperature. An aqueous solution of iopamidol at molality (m) concentration of 1.5 m was prepared from the anhydrous iopamidol powder. 2.2. Methods. 2.2.1. Single-Crystal X-ray Diffraction Measurements. Crystals of iopamidol monohydrate suitable for X-ray single-crystal analysis were grown from aqueous solutions of iopamidol by slow crystallization, on standing for weeks at 60 °C. A good-looking crystal was placed on a glass fiber with an epoxy glue and mounted on the goniometric head of an Enraf-Nonius CAD4 diffractometer, equipped with an Xray tube [λ(Mo Kα) = 0.71073 Å] operating at 50 kV and 25 mA (University of Insubria). Diffraction data were collected at room temperature in the ω scan mode. The raw data have been analyzed using the CAD4 suite of programs,17 enabling also an empirical (ψ-scan) correction for the absorption effects.18 Structure solution was performed by direct methods SIR92,19 in the P1̅ space group, and subsequent Fourier methods. The refinement has been carried out using the software SHELX9720 included in the WINGX suite21 by full-matrix least-squares on F2. All non-hydrogen atoms, except a disordered oxygen atom (vide infra), were refined anisotropically. The hydrogen atoms of the iopamidol molecule were modeled by the riding mode; those of the water molecules were refined adding constraints that take into account the molecule geometry, with restrained thermal parameters. Crystals of iopamidol pentahydrate were grown from aqueous solutions of iopamidol by slow evaporation at room temperature, reaching their full size within few weeks. A goodlooking crystal was placed on a glass fiber with an epoxy glue and mounted on the goniometric head of a Bruker Nonius FR590 diffractometer, equipped with an X-ray tube (λ(Mo Kα) = 0.71073 Å) operating at 50 kV and 32 mA (Center of Excellence in Biocrystallography, University of Trieste). Diffraction data were collected at room temperature in the oscillation mode (ϕ scans, 3°, speed 0.13° min−1). The raw data were analyzed using the DENZO-SMN22 software and 470
DOI: 10.1021/acs.molpharmaceut.6b00902 Mol. Pharmaceutics 2017, 14, 468−477
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Figure 2. Perspective views of iopamidol molecules in the anhydrous (syn/ANTI), monohydrate (syn/ANTI), and pentahydrate (anti/---) forms (a, b, and c panels, respectively), drawn by SCHAKAL.27 Color codes: I (magenta); C (gray); O (red); N (blue). Hydrogen atoms have been omitted for clarity. Panel d shows the “reference” scorpion stereochemistry, here in the syn (large claws, Ar−CO−NHR) and SYN (tail, Ar−NH−COR) disposition. In panel c, the labeling and positioning of the CH3/OH residues on the anilido branch are arbitrary, as only two disordered atoms with no unique element assignment are shown.
unavoidable numerical correlations occurring while attempting a full refinement in the acentric analogues, doubling the number of the crystallographically independent molecules. The possibility of such an occurrence has nicely been commented on in the seminal review paper by Flack et al.26 As indicated above by the Z′ = 2 value, crystals of the monohydrate phase contain two crystallographically independent iopamidol molecules, both in the same conformation. In the following text and figures, for the sake of simplicity, we refer to only one of them (the first in the list of the atomic coordinates deposited in the CSD database and provided in the Supporting Information). On the contrary, the pentahydrate form possesses a single crystallographically independent molecule, thus clearing any source of ambiguity. 3.2. Comparative Crystal Chemistry. In Figure 2 the conformations of the iopamidol molecules in the three crystal forms are compared. As expected, all three molecules show a substantial overlapping of the core, their main differences appearing in the relative orientations of the arms. With reference to the sketch shown in Figure 2, iopamidol in the anhydrous and monohydrate forms (all together, three crystallographically independent molecules) shows the Ar− CO−NHR residues in syn disposition, while the anti one is present in the pentahydrate phase. In addition, less relevant conformational differences are also evident in the terminal hydroxyl-methyl group of these branches. The scorpion tail (see Figure 2d), i.e., the anilido residue (which, as described above, suffers from a statistical disorder imposed by the adoption of a centrosymmetric space group), possesses a well-defined relative disposition, ANTI to the syn
as obtained by XRD investigations, were used as input for the optimization. Raman intensity at T = 298 K and λ = 532 nm was calculated from Raman activity following the theory of Raman scattering.25 No scale factor was applied to the calculated vibrational shift. Profiles of the simulated Raman and IR lines were obtained by applying the convolution with a Lorentzian band shape function, 5 cm−1 fwhm.
3. RESULTS AND DISCUSSION 3.1. Crystal Structures of Iopamidol Crystal Phases. Iopamidol is known to be an enantiomerically pure compound. Accordingly, it is expected to orderly pack, in the solid, in one of the 65 Sohncke (“chiral”) space groups. However, since only a minor pendant arm, the terminal portion of the (structurally less relevant) 2-hydroxy-1-oxopropyl moiety, bears the chiral information, crystal structures have been modeled, and described, in centrosymmetric space groups. This strategy has already been adopted, in the long known single-crystal structure characterization of the anhydrous form12 [triclinic, P1̅], and used throughout these studies, where the P1̅ (Z′ = 2, instead of P1, Z′ = 4) and P21/c (Z′ = 1, instead of P21, Z′ = 2) were chosen for the mono- and pentahydrate forms, respectively. This occurrence is not new, since pure enantiomers can easily pack in “pseudo-centrosymmetric” crystals, provided that steric influences or other specific intermolecular contacts are not at work. The choice of the centrosymmetric space groups, in the presence of an extremely large centrosymmetric portion of the crystal (including the three heavy scatterers, the iodine atoms), is thus largely justified by Occam’s razor, and by the 471
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Figure 3. Hydration scheme of the iopamidol molecule in its monohydrate crystal form. The two crystallographically independent iopamidol molecules are shown with different colors, while symmetry equivalents possess the same color. All water oxygen atoms are drawn in red; small blue/ green dots are used to indicate interactions with atoms of symmetrically related iopamidol molecules, not shown in the picture.
Figure 4. Hydration scheme of the iopamidol molecule in its pentahydrate crystal form. The five crystallographically independent water molecules are shown with different colors, while symmetry equivalents possess the same color.
intermolecular halogen-bond interaction]. Iopamidol pentahydrate contains, in the asymmetric unit, five distinct water molecules, forming hydrogen bonds with the organic molecule as well as with each other (see Figure 4). Thus, a tight threedimensional lattice is formed by this extensive network of Hbond interactions; accordingly, the structure does not contain significant cavities, and a high packing coefficient can be computed (0.67). From Figure 4 it is evident that two water molecules (yellow and blue spheres in Figure 4) are involved in hydrogen bonds connecting the CO in the anilido side arm (C10) and one CO in the two carbonyl side arms (C7); more specifically the carbonyl side arm involved in such hydrogen bonding has the same spatial orientation (i.e., lies on the same side of the triiodoaryl plane) of the anilido side arm. This is possible only in the anti form of the iopamidol and in the never detected syn/ SYN conformer. The other three water molecules interact with an iodine atom and two OH groups, one from the anilido tail and one from one arylcarboxamido claw. Thus, two different sets of water molecules are present, which could be lost, upon heating, in a multiple-step event. The occurrence of different (sequential) water loss processes is indeed confirmed by thermogravimetric analyses14 and in the DSC studies reported elsewhere (Bellich et al. current submission to J. Therm. Anal. Calorim.). Finally, the absence of extra voids in the structures is nicely confirmed by the linear relationship between the experimentally determined molar volumes and the hydration level, depicted and commented on in Figure S3. Additionally, in Figure S4 we supply the X-ray powder diffraction traces, which can be
Ar−CO−NHR claws disposition (Figures 2a, 2b). For obvious symmetry reasons, in the case of the anti disposition of the pentahydrate, the SYN/ANTI labeling vanishes. 3.3. Intermolecular Networks. The different crystal packing features of the three forms are highlighted in Figure S1. In the anhydrous form and in the monohydrate, pairing of triiodoaromatic moieties is present in the crystal lattice (as a nearly continuous dimeric stacking in the anhydrous form and as a fragmented stacking of iopamidol dimers in the monohydrate), while the presence of many water molecules causes an almost complete iopamidol segregation in the pentahydrate. Iopamidol monohydrate contains two crystallographically independent water molecules, forming hydrogen bonds with the organic molecule, but not with each other (see Figure 3). Here, a tight three-dimensional lattice is formed by this extensive network of H-bond interactions; accordingly, the structure does not contain significant cavities, and a high packing coefficient can be computed (0.67). Within this crystal packing network, one crystallographic independent water molecule (O1w) shows four links (two H-bond donors and two H-bond acceptors), while the other (O2w) has only three H-bond contacts; both water molecules share a H-bond interaction with the O9 atom of the same iopamidol molecule (see Figure 1 and Figure S2). Similarly to what was found in the anhydrous form,12 in the monohydrate two neighboring iopamidol molecules show a short I···π contact of 3.52 Å (distance taken from I6c to the closest ring center), with most inter- and intramolecular I···I distances falling above 6 Å and the notable exception of a strong I6a···I6c contact of only 3.94 Å [too long, however, to be interpreted as a definite 472
DOI: 10.1021/acs.molpharmaceut.6b00902 Mol. Pharmaceutics 2017, 14, 468−477
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Figure 5. Comparison of iopamidol molecules before (gray shadow) and after geometry optimization (colored atoms) carried out with Gaussian; the molecules have been superimposed considering only the ring atoms.
Figure 6. Experimental IR (left) and Raman (right) spectra of iopamidol crystals (anhydrous is in black, monohydrate is in green, and pentahydrate is in red; amorphous glass (light blue); water solution at concentration 1.5 m (in gray, dotted line)). More detailed lists of peak frequencies and assignments are provided in the Supporting Information.
fruitfully used for fingerprinting. As the Rietveld plots provided in the Supporting Information clearly manifest a good match between the structural models obtained from single-crystal analyses and the powder diffraction traces collected on the polycrystalline samples, this also witnesses the structural and phase identity of the selected single-crystal specimens and their bulk counterparts, thus granting the monophasic nature of all samples.
the periodic repetition of crystal cells, but it only considers one isolated molecule. Therefore, the molecular geometry of the output molecule does not reflect the constraints of the crystalline state. Figure 5 reports the difference occurring before and after geometry optimization in case of the anhydrous (left), monohydrate (center), and pentahydrate (right) crystals of iopamidol. Indeed, for anhydrous and monohydrate crystals only minor changes occur in the side arm linked to the aromatic ring via NH−CO, while for pentahydrate changes are also seen for the other arms, and a slight bending of two iodine atoms with respect to the plane of the aromatic ring is observed (see data sheets in Supporting Information for more details).
4. GEOMETRY STRUCTURE OPTIMIZATION BY QUANTUM CHEMICAL COMPUTATIONS Quantum chemical computations were carried out for the geometry structure optimization and the assignment of molecular vibrations of both IR and Raman experimental spectra. The common procedure with the software Gaussian is to start from the crystallographic coordinates and to achieve an optimization of the molecular structure, then simulate its vibrational spectra. This approach does not take into account
5. IR AND RAMAN SPECTRA Vibrational assignments for the crystals have been carried out by comparison of the simulated and experimental IR and Raman spectra. Details of the vibrational assignments for the 473
DOI: 10.1021/acs.molpharmaceut.6b00902 Mol. Pharmaceutics 2017, 14, 468−477
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information derived by the calculations carried on the simple model systems. The signals observed at 1600−1700 cm−1 of both FTIR and Raman spectra can be assigned to the amide I modes,29,30 which are mainly CO stretching coupled with N−H bending vibrations of NH−CO groups.28 These modes should be strongly affected by the formation of H-bonds. As a matter of fact the amide I bands are considerably red-shifted in the experimental spectra compared to the corresponding simulated ones (see Figure S5, S6, and S7). This behavior can be ascribed to the effects induced by the strong participation of CO groups in H-bonding interactions with OH or NH groups, that are not taken into account in the simulation. These H-bonds decrease the force constant of the involved CO groups, thus leading to a shift toward lower values of their vibrational frequency.28 Simulations for the three crystalline species indicate the presence of three amide I modes related to the presence of three distinct CO−NH groups in the molecule, one in each side arm. While the resulting vibrational frequencies of these modes are practically coincident for the anhydrous monomer, a considerable red-shift is observed for the hydrated species owing to the formation of CO···HOH H-bonds. These effects are particularly important in the pentahydrate complex where one mode shows the most significant red shift. This mode corresponds to the vibration of the carbonyl side arm that has the same spatial orientation of the anilido side arm in anti configuration. This red-shift is the result of H-bond occurring with a water molecule, as depicted in the hydration scheme for iopamidol pentahydrate (see Figure 4). Figure 6 shows that the amide I peak shifts toward lower frequencies following the order amorphous > monohydrate ∼ solution > pentahydrate; this suggests that H-bonds involving the CO groups (i.e., CO···H−) in the pentahydrate are the strongest ones. On the other hand, peculiar features appear in the case of the IR and Raman spectra of the anhydrous form, with five different components, likely due to intermolecular interactions and vibrational couplings, specifically related to its crystalline organization. Thus, for this crystal, it is not safe to interpret the observed spectral features in terms of H-bonding properties only. Simulations indicate that the signals in the range 1500−1600 cm−1 originate from modes involving N−H bending coupled with C−N stretching vibrations of CO−NH groups (the socalled amide II mode) mixed with stretching of the aromatic ring and C−H bending vibrations. Several modes are detected in this range, and typically those with a larger contribution from the ring vibrations have a relatively larger Raman activity. In this respect it can be argued that the band observed in the experimental IR and Raman spectra can be essentially assigned to amide II and ring modes, respectively. Concerning the IR spectrum, the main band at ∼1550 cm−1 shifts toward higher frequencies in the order monohydrate < amorphous < pentahydrate < solution. For a bending mode this order should be indicative of a strengthening of H-bond interaction involving the N−H group. Likely, in solution the formation of relatively strong iopamidol−water H-bonding of the type N−H···OH2 is favored. On the other hand, in the Raman spectrum a peak is detected at ∼1520 cm−1 for all the samples; this is mainly ascribed to the ring modes, which will be less affected by Hbonding. In the case of the anhydrous crystal a second peak of comparable intensity is found at ∼1540 cm−1; this Raman
peaks of the IR and Raman spectra are reported in the Supporting Information. Generally speaking, the vibrational spectrum is expected to depend on both molecular symmetry (i.e., molecular conformation) and intermolecular interactions (i.e., the crystal lattice), thus, the number and intensity of the bands in the IR and Raman spectra also depend on both these topological properties. For the sake of completeness, the spectra of a glassy form of iopamidol are also reported (see Supporting Information for additional data on this amorphous sample). Given the absence of periodicity in the amorphous material, the interest in the glass relies on the possibility to average out strong intermolecular interactions, thus providing a “crystal-lattice-free” molecular conformation that may better resemble that of the solution state. Therefore, the IR and Raman spectra of iopamidol in the glass phase become useful also for the interpretation of the main intramolecular vibrations occurring in the crystal where additional effects related to the specific intermolecular organization might be important. Figure 6 reports the experimental IR (left panel) and Raman (right panel) spectra for the three crystal forms of iopamidol (anhydrous in black, monohydrate in green, and pentahydrate in red), for the glass (in light blue), and for a water solution at concentration 1.5 m (in gray, dotted curve); the attention has been focused in the range 1000−1800 cm−1 where several bands can be observed. In this frequency range the spectra of the three crystal forms show basically the same main components, even if spectral features that monitor specific structural and/or conformational differences can be clearly observed. The overall molecular character of the spectra is emphasized by the fact that similar principal contributions can be also observed for the iopamidol solution. It is worthy of note that both FTIR and Raman spectra of the anhydrous crystal seem to be somewhat more resolved, showing a larger number of distinct features when compared to the others. This might suggest that in this case local field effects and intermolecular vibrational coupling phenomena specific to this crystal structure are relatively more important. On the other hand, IR and Raman spectra of the amorphous sample are similar to those of the aqueous solution, as expected considering the absence of long-range ordering in these systems. From a general point of view, spectral calculations clearly evidence that, in the considered frequency range, often normal modes are strongly delocalized, involving vibrational contributions arising from different molecular portions. Moreover, a number of distinct modes are found to occur at similar wavenumbers showing significantly different IR and Raman activities. This emphasizes that the molecular origin of a given signal in the two spectra might not be the same, even if it appears at corresponding frequencies. The presence of crystallographic water molecules has been taken into account in the calculation of monohydrate and pentahydrate crystal (one and five water molecules, respectively) and is expected to affect the observed spectral features in terms of peak frequencies and intensities.28 A comparison between simulated spectra corresponding to the anhydrous and monohydrated species (which basically share the same conformation) nicely highlights this sensitivity, showing how the interaction with a water molecule alone might substantially influence the entire spectral pattern. Thus, it is clear that performing a detailed spectral assignment is a very challenging task for these complex molecular crystals and is beyond the scope of the present study. Instead, we seek to evidence the existence of possible structural and/or conformational spectral markers, also with the helpful 474
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Table 1. Solubility, Osmotic Coefficient, and Calorimetric Data for Iopamidol Solid Forms and Their Related Solutions at Saturationa form
solubility molality, m (25 °C)b
osmotic coeff ϕsatc
ΔdilH msat → m = 0 (kJ/mol)d
ΔISH solid → m = 0.01 (kJ/mol) (new data)
ΔDSH solid → m = msat (kJ/mol) (calculated)
dm/dT
anhydrous monohydrate pentahydrate amorphous
1.12 0.796 0.194 >10e
0.562 0.611 0.805
