Dyanne L. Cruickshank

Enantiotropically related albendazole polymorphs.

In the present study we report the solid-state properties of albendazole (ABZ) re-crystallized from different solvents for comparison with the commercially available form. Crystalline phases were characterized as to thermal behavior, X-ray diffractometry, both on powder and single crystal, and solubility in methanol or 0.1 N HCl. The relevant thermodynamic parameters were calculated from solubility measurements at different temperatures. The re-crystallization of ABZ both from methanol and N,N-dimethylformamide afforded a new stable polymorph form (Form II) enantiotropically related to the commercially available ABZ (Form I), the latter being the metastable form at ambient temperature. Both forms proved to be physically quite stable, likely due to a high-energy barrier for the activation of the interconversion. ABZ in the solid state represents a rather complex system in which the molecular structural differences that could be associated with the polymorphism are of at least four possible types, or combinations of these: (a) tautomeric; (b) different conformations of either or both of the side-chains attached to the bicyclic ring system; (c) the occurrence of molecular disorder or its absence; (d) no essential difference in molecular structure but different hydrogen bonding arrangements in the two polymorphs.

Absorbate-induced piezochromism in a porous molecular crystal.

Atmospherically stable porous frameworks and materials are interesting for heterogeneous solid-gas applications. One motivation is the direct and selective uptake of pollutant/hazardous gases, where the material produces a measurable response in the presence of the analyte. In this report, we present a combined experimental and theoretical rationalization for the piezochromic response of a robust and porous molecular crystal built from an extensively fluorinated trispyrazole. The electronic response of the material is directly determined by analyte uptake, which provokes a subtle lattice contraction and an observable bathochromic shift in the optical absorption onset. Selectivity for fluorinated absorbates is demonstrated, and toluene is also found to crystallize within the pore. Furthermore, we demonstrate the application of electronic structure calculations to predict a physicochemical response, providing the foundations for the design of electronically tunable porous solids with the chemical properties required for development of novel gas-uptake media.

Inclusion of trans-resveratrol in methylated cyclodextrins: synthesis and solid-state structures

Summary The phytoalexin trans-resveratrol, 5-[(1E)-2-(4-hydroxyphenyl)ethenyl]-1,3-benzenediol, is a well-known, potent antioxidant having a variety of possible biomedical applications. However, its adverse physicochemical properties (low stability, poor aqueous solubility) limit such applications and its inclusion in cyclodextrins (CDs) has potential for addressing these shortcomings. Here, various methods of the attempted synthesis of inclusion complexes between trans-resveratrol and three methylated cyclodextrins (permethylated α-CD, permethylated β-CD and 2,6-dimethylated β-CD) are described. Isolation of the corresponding crystalline 1:1 inclusion compounds enabled their full structure determination by X-ray analysis for the first time, revealing a variety of guest inclusion modes and unique supramolecular crystal packing motifs. The three crystalline inclusion complexes were also fully characterized by thermal analysis (hot stage microscopy, thermogravimetric analysis and differential scanning calorimetry). To complement the solid-state data, phase-solubility studies were conducted using a series of CDs (native and variously derivatised) to establish their effect on the aqueous solubility of trans-resveratrol and to estimate association constants for complex formation.

Effect of cyclodextrins on the reactivity of fenitrothion

The hydrolysis reaction of fenitrothion was studied in water containing 2% dioxane and in the presence of native cyclodextrins (α-, β- and γ-CD) and two commercially available modified derivatives, namely, permethylated β- and α-cyclodextrin (TRIMEB and TRIMEA, respectively). The kinetics of the reaction in the presence of TRIMEA could not be measured because the complex formed is insoluble and precipitated even at low concentration. On the other hand, the reaction is only weakly affected by the presence of α-CD. The hydrolysis reaction is inhibited by all the other cyclodextrins. From the kinetic data the association equilibrium constants for the formation of the 1:1 inclusion complexes were determined as 417, 511 and 99M(-1) for β-CD, TRIMEB and γ-CD, respectively. Despite the differences in the association constants for β- and γ-CD, the observed inhibition effect is about the same and this is due to the fact that the rate of hydrolysis in the cavity of γ-CD is smaller than that in the cavity of β-CD. The strongest inhibitor is TRIMEB and this result is consistent with the known structure of the complex in the solid state.

Solid-state structures and thermal properties of inclusion complexes of the organophosphate insecticide fenitrothion with permethylated cyclodextrins

The X-ray crystal structures and thermal stabilities of the inclusion complexes formed between the organophosphate insecticide fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate] and the host compounds TRIMEA and TRIMEB (permethylated alpha- and beta-cyclodextrins, respectively) are reported. In the complex (TRIMEA)(2).fenitrothion 1, the guest phosphate ester group is disordered and the molecule is fully encapsulated within a novel TRIMEA dimer in which the secondary rims of the two host molecules are in close contact. In contrast, the complex TRIMEB.fenitrothion 2 is monomeric and the guest molecule is statistically disordered over two positions, with the phosphate group inserted in the host cavity in both cases. Thermal analysis indicated gradual and partial loss of the guest in 1 during heating between 130 degrees C and the melting point of the complex (approximately 200 degrees C), whereas complex 2 displayed significant mass loss only after fusion of the complex at 161 degrees C.

Crystallisation and physicochemical property characterisation of conformationally-locked co-crystals of fenamic acid derivatives

Polymorphism in drug compounds can cause significant problems for industrial-scale production and so a method for restricting the conformational freedom of the target compound whilst retaining desired chemical properties is highly beneficial to the pharmaceutical industry. Co-crystallisation is commonly used to alter the structure of an active pharmaceutical ingredient (API) without affecting its activity. A comprehensive co-crystal screen of four fenamic acid derivatives affords a strictly limited number of co-crystals. These show no evidence of polymorphism, although some of the parent APIs exhibit significant polymorphism. Two of these co-crystals, of mefenamic acid and tolfenamic acid with 4,4′-bipyridine, were previously unknown and are studied using X-ray diffraction. Co-crystals from this screen are fully characterised and display comparable solubility and stability with respect to the parent APIs; no phase transformations have been identified. A range of crystallisation techniques, including cooling and grinding methods, are shown to afford single polymorphic forms for each of the co-crystals.

Lisinopril Dihydrate: Single-Crystal X-Ray Structure and Physicochemical Characterization of Derived Solid Forms

Screening for new solid forms of the antihypertensive lisinopril was performed by recrystallization of the commercial form, lisinopril dihydrate, from various solvents and by exposing the product of its dehydration to a series of vapors under controlled conditions. Modifications other than the dihydrate encountered in the study included new anhydrous and amorphous forms, with intrinsic dissolution rates significantly greater than that of the dihydrate. Further physicochemical characterization included constant and programmed temperature powder X-ray diffraction, differential scanning calorimetry, thermogravimetry, and Fourier transform infrared spectroscopy. In the course of this study, the single-crystal X-ray structure of lisinopril dihydrate, [a = 14.550(2), b = 5.8917(8), c = 14.238(2) Å, β = 112.832(3)° at T = 173(2) K, space group P21 , Z = 2], was determined for the first time, revealing its double zwitterionic character in the solid state.

Polymorphism of the azobenzene dye compound methyl yellow

The crystal structure and polymorphism of the well-known aminoazobenzene, N,N-dimethyl-4-aminoazobenzene (DAB) are studied; a second polymorph of DAB reported and the relationship between the two polymorphs studied using differential scanning calorimetry and by evaluating their lattice energies and absorption spectra. Without significantly strong intermolecular interactions present in the two forms of DAB, a balance between conformational strain and a cumulative π-orbital overlapping effect results in the existence of the enantiotropically related polymorphs and their alternative packing arrangements. The UV/vis spectra of the two polymorphs are different, thus illustrating the significance of small structural changes on the colour of a material, which could behave differently under particular sensing conditions. DAB has been used primarily as an acid/base indicator solution and a dye molecule and the implications of the polymorphic relationships established here are discussed with respect to the range of applications this molecule finds as a sensor and for its incorporation into various functional films and devices.

Enclathration of bases by a fluorenyl host: structure, stability and selectivity

The host compound 9,9′-(1,4-phenylene)-bis-(fluoren-9-ol) forms inclusion compounds with pyridine, the isomers of picoline and morpholine. The structures of the inclusion compounds are stabilised by (Host)O–H⋯N(Guest) hydrogen bonds. Unusually, the structure of the host containing 4-picoline and morpholine displays both cis- and trans-conformation. The enthalpy of enclathration of these guests was estimated by packed-column chromatography.

Alternative solid-state forms of a potent antimalarial aminopyridine: X-ray crystallographic, thermal and solubility aspects

3-(6-Methoxypyridin-3-yl)-5-(4-methylsulfonyl phenyl)-pyridin-2-amine (MMP) is a member of a novel class of orally active antimalarial drugs. This aminopyridine molecule has shown potent in vitro antiplasmodial activity and in vivo antimalarial activity in Plasmodium berghei-infected mice. The aqueous solubility of this molecule is, however, limited. Thus investigations aimed at improving the physicochemical properties, including solubility, of MMP were accordingly conducted. Five salts of MMP were formed with co-former molecules saccharin, salicylic acid, fumaric acid, oxalic acid and suberic acid, but a cocrystal was obtained when the co-former adipic acid was employed. All these new multi-component systems have been fully characterised using X-ray diffraction and thermal methods. Semi-quantitative, turbidimetric solubility tests in a phosphate-buffered saline solution at a pH of 7.4 were performed on the salts and the cocrystal of MMP. The saccharinate salt, fumarate salt and the cocrystal of MMP proved to have greater solubility than MMP itself. This work illustrates the importance of screening and modifying candidate drug compounds in their preliminary stages of development.