Apigenin Cocrystals: From Computational Prescreening to Physicochemical Property Characterization

Apigenin (4′,5,7-trihydroxyflavone, APG) has many potential therapeutic benefits; however, its poor aqueous solubility has limited its clinical applications. In this work, a large scale cocrystal screening has been conducted, aiming to discover potential APG cocrystals for enhancement of its solubility and dissolution rate. In order to reduce the number of the experimental screening tests, three computational prescreening tools, i.e., molecular complementarity (MC), hydrogen bond propensity (HBP), and hydrogen bond energy (HBE), were used to provide an initial selection of 47 coformer candidates, leading to the discovery of seven APG cocrystals. Among them, six APG cocrystal structures have been determined by successful growth of single crystals, i.e., apigenin–carbamazepine hydrate 1:1:1 cocrystal, apigenin–1,2-di(pyridin-4-yl)ethane hydrate 1:1:1 cocrystal, apigenin–valerolactam 1:2 cocrystal, apigenin-(dl) proline 1:2 cocrystal, apigenin-(d) proline/(l) proline 1:1 cocrystal. All of the APG cocrystals showed improved dissolution performances with the potential to be formulated into drug products

Crystal or Glass? Chemical and Crystallographic Factors in the Amorphization of Molecular Materials

The creation of long-lived amorphous phases has potential applications in numerous fields; for example, the instability of the amorphous phase leads to higher solubility of pharmaceutical phases, often leading to higher bioavailability. The rate of recrystallization of an amorphous phase poses a significant limitation to the application of many such phases; however, understanding the energetic and structural factors that control the stability of molecular amorphous phases is limited by empirical classifications based on thermal analysis used to identify materials. From a set of molecularly related benzanilides, examples of all three classes have been identified, allowing use of crystal structural analysis, Raman spectroscopy, and energetic calculations to determine the structural factors playing a role in the different stabilities. While the behavior of most systems reflects the relative energy of the crystalline phase to the amorphous phase, kinetic factors based on whether a NH···OC hydrogen bond is present in the crystalline phase play a key role in stabilizing the amorphous phase as the loss of this bond reduces the conversion rate. In contrast, systems without this bond display fast recrystallization due to the greater structural similarity between the amorphous and crystalline phases

Solution mediated phase transformations between co-crystals

A solution mediated transformation between two co-crystal phases has been observed for the p-5 toluensulfonamide/triphenylphosphine oxide co-crystal system. This system has two known co-crystals with 1:1 and 3:2 stoichiometry respectively, and the ternary phase diagram (TPD) for the system has been determined in acetonitrile previously. By manipulating the solution composition in this 10 solvent to a region of the TPD where the 1:1 co-crystal is stable, the 3:2 co-crystal could be observed to convert to the 1:1 co-crystal. The corresponding transformation was true for the 1:1 co-crystal in a region of the TPD where the 3:2 co-crystal is stable; the 1:1 co-crystal converted to the 3:2 co-15 crystal

Investigation into solid and solution properties of quinizarin

Polymorphism, crystal shape and solubility of 1,4-dihydroxyanthraquinone (quinizarin) have been investigated in acetic acid, acetone, acetonitrile, n-butanol and toluene. The solubility of FI and FII from 20 degrees C to 45 degrees C has been determined by a gravimetric method. By slow evaporation, pure FI was obtained from n-butanol and toluene, pure FII was obtained from acetone, while either a mixture of the two forms or pure FI was obtained from acetic acid and acetonitrile. Slurry conversion experiments have established an enantiotropic relationship between the two polymorphs and that the commercially available FI is actually a metastable polymorph of quinizarin under ambient conditions. However, in the absence of FII, FI is kinetically stable for many days over the temperature range and in the solvents investigated. FI and FII have been characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission and ordinary powder X-ray diffraction (PXRD) at different temperatures. The crystal structure of FII has been determined by single-crystal XRD. DSC and high-temperature PXRD have shown that both FI and FII will transform into a not previously reported hightemperature form (FIII) around 185 degrees C before this form melts at 200-202 degrees C. By indexing FIII PXRD data, a triclinic P (1) over bar cell was assigned to FIII. The solubility of quinizarin FI and FII in the pure organic solvents used in the present work is below 2.5% by weight and decreases in the order: toluene, acetone, acetic acid, acetonitrile and n-butanol. The crystal shapes obtained in different solvents range from thin rods to flat plates or very flat leaves, with no clear principal difference observed between FI and FII

Investigation of the solid-state polymorphic transformations of piracetam

The solid-state polymorphic transformations of 2-oxo-1-pyrrolidine acetamide (Piracetam) were investigated using a combination of off-line and on-line techniques; Differential Scanning Calorimetry (DSC), High Temperature X-ray Diffraction (HT-XRD), thermal analysis and Hot Stage Optical Microscopy. Form II and Form III were each observed to transform directly to Form I upon heating, with Form II transforming at a slightly lower temperature. The transformation of both polymorphs to Form I was observed to cause physical cracking of the crystals as well as changing the optical properties. Form I consistently transformed to Form II when cooled. The molecular rearrangements required for the transformation from Form I to Form II were found to be more energetically favourable than those required for the transformation to Form III. The transformation from the metastable Form II to the stable Form III was not observed in the solid state, while the Form II – Form III transition temperature was found to be higher than the transition temperature of both polymorphs to Form I

Insight into the Mechanism of Formation of Channel Hydrates via Templating

Cocrystallization of modafinil, <b>1</b>, and 1,4-diiodotetrafluorobenzene, <b>2</b>, in toluene leads to the formation of a metastable modafinil channel hydrate containing an unusual hydrogen bonded dimer motif involving the modafinil molecules that is not seen in anhydrous forms of modafinil. Computational methodologies utilizing bias drift-free differential evolution optimization have been developed and applied to a series of molecular clusters and multicomponent crystals in the modafinil/water and modafinil/water/additive systems for the additive molecules <b>2</b> or toluene. These calculations show the channel hydrate is less energetically stable than the anhydrous modafinil but more stable than a cocrystal involving <b>1</b> and <b>2</b>. This provides theoretical evidence for the observed instability of the channel hydrate. The mechanism for formation of the channel hydrate is found to proceed via templating of the modafinil molecules with the planar additive molecules, allowing the formation of the unusual hydrogen-bonded modafinil dimer. It is envisaged that the additive is then replaced by water molecules to form the channel hydrate. The formation of the channel hydrate is more likely in the presence of <b>2</b> compared to toluene due to the destabilizing effect of the larger iodine molecules protruding into neighboring modafinil clusters

Tuning Proton Disorder in 3,5-Dinitrobenzoic Acid Dimers: the Effect of Local Environment

The carboxylic acid dimer is a frequently observed intermolecular association used in crystal engineering and design, which can show proton disorder across its hydrogen bonds. Proton disorder in benzoic acid dimers is a dynamic, temperature-dependent process whose reported occurrence is still relatively rare. A combination of variable temperature X-ray and neutron diffraction has been applied to demonstrate the effect of local crystalline environment on both the degree and onset of proton disorder in 3,5-dinitrobenzoic acid dimers. Dimers which have significantly asymmetric local intermolecular interactions are found to have a higher onset temperature for occupation of a second hydrogen atom site to be observed, indicating a greater energy asymmetry between the two configurations. Direct visualization of the electron density of hydrogen atoms within these dimers using high resolution X-ray diffraction data to characterize this disorder is shown to provide remarkably good agreement with that derived from neutron data

Tuning Proton Disorder in 3,5-Dinitrobenzoic Acid Dimers: the Effect of Local Environment

The carboxylic acid dimer is a frequently observed intermolecular association used in crystal engineering and design, which can show proton disorder across its hydrogen bonds. Proton disorder in benzoic acid dimers is a dynamic, temperature-dependent process whose reported occurrence is still relatively rare. A combination of variable temperature X-ray and neutron diffraction has been applied to demonstrate the effect of local crystalline environment on both the degree and onset of proton disorder in 3,5-dinitrobenzoic acid dimers. Dimers which have significantly asymmetric local intermolecular interactions are found to have a higher onset temperature for occupation of a second hydrogen atom site to be observed, indicating a greater energy asymmetry between the two configurations. Direct visualization of the electron density of hydrogen atoms within these dimers using high resolution X-ray diffraction data to characterize this disorder is shown to provide remarkably good agreement with that derived from neutron data