Because of HNO's emerging role as an important effector molecule in biology, there is great current interest in the coordination chemistry of HNO and its deprotonated form, the nitroxyl anion (NO(-)), with hemes. Here we report the preparation of four new ferrous heme-nitroxyl model complexes, {FeNO}(8) in the Enemark-Feltham notation, using three electron-poor porphyrin ligands and the bis-picket fence porphyrin H2[3,5-Me-BAFP] (3,5-Me-BAFP(2-) = 3,5-methyl-bis(aryloxy)-fence porphyrin dianion). Electrochemical reduction of [Fe(3,5-Me-BAFP)(NO)] (1-NO) induces a shift of ν(N-O) from 1684 to 1466 cm(-1), indicative of formation of [Fe(3,5-Me-BAFP)(NO)](-) (1-NO(-)), and similar results are obtained with the electron-poor hemes. These results provide the basis to analyze general trends in the properties of ferrous heme-nitroxyl complexes for the first time. In particular, we found a strong correlation between the electronic structures of analogous {FeNO}(7) and {FeNO}(8) complexes, which we analyzed using density functional theory (DFT) calculations. To further study their reactivity, we have developed a new method for the preparation of bulk material of pure heme {FeNO}(8) complexes via corresponding [Fe(porphyrin)](-) species. Reaction of [Fe(To-F2PP)(NO)](-) (To-F2PP(2-) = tetra(ortho-difluorophenyl)porphyrin dianion) prepared this way with acetic acid generates the corresponding {FeNO}(7) complex along with the release of H2. Importantly, this disproportionation can be suppressed when the bis-picket fence porphyrin complex [Fe(3,5-Me-BAFP)(NO)](-) is used, and excitingly, with this system we were able to generate the first ferrous heme-NHO model complex reported to date. The picket fence of the porphyrin renders this HNO complex very stable, with a half-life of ~5 h at room temperature in solution. Finally, with analogous {FeNO}(8) and {FeNHO}(8) complexes in hand, their biologically relevant reactivity toward NO was then explored.

4,4-Diphenyl-2,5-cyclohexadienone (1) crystallized as four conformational polymorphs and a record number of 19 crystallographically independent molecules have been characterized by low-temperature X-ray diffraction: form A (P2(1), Z'=1), form B (P1, Z'=4), form C (P1, Z'=12), and form D (Pbca, Z'=2). We have now confirmed by variable-temperature powder X-ray diffraction that form A is the thermodynamic polymorph and B is the kinetic form of the enantiotropic system A-D. Differences in the packing of the molecules in these polymorphs result from different acidic C-H donors approaching the C=O acceptor in C-H...O chains and in synthons I-III, depending on the molecular conformation. The strength of the C-HO interaction in a particular structure correlates with the number of symmetry-independent conformations (Z') in that polymorph, that is, a short C-HO interaction leads to a high Z' value. Molecular conformation (Econf) and lattice energy (Ulatt) contributions compensate each other in crystal structures A, B, and D resulting in very similar total energies: Etotal of the stable form A=1.22 kcal mol(-1), the metastable form B=1.49 kcal mol(-1), and form D=1.98 kcal mol(-1). Disappeared polymorph C is postulated as a high-Z', high-energy precursor of kinetic form B. Thermodynamic form A matches with the third lowest energy frame based on the value of Ulatt determined in the crystal structure prediction (Cerius2, COMPASS) by full-body minimization. Re-ranking the calculated frames on consideration of both Econf (Spartan 04) and Ulatt energies gives a perfect match of frame #1 with stable structure A. Diphenylquinone 1 is an experimental benchmark used to validate accurate crystal structure energies of the kinetic and thermodynamic polymorphs separated by <0.3 kcal mol(-1) (approximately 1.3 kJ mol(-1)).

Close packing conflict in a metastable polymorph of the pure title host (Z' = 2, melt crystal m) is resolved in the stable form (Z' = 1, sublimed crystal s) as O-H...O hydrogen bond changes to O-H...pi interaction. Melt crystallization and sublimation show a greater percentage of high Z' structures in CSD statistics.

Characterization of crystalline polymorphs and their quantitation has become an integral part of the pre-clinical drug development process. Raman spectroscopy is a powerful technique for the rapid identification of phases of pharmaceuticals. In the present work we demonstrate the use of low wavenumber Raman vibrational spectroscopy (including phonon measurement) for discrimination among polymorphs. A total of 10 polymorphic pharmaceuticals were employed to conduct a critical assessment. Raman scattering in the low frequency region (10–400 cm−1), which includes crystal lattice vibrations, has been analyzed and the results indicate lattice phonon Raman scattering can be used for rapid discrimination of polymorphic phases with additional discriminating power compared to conventional collection strategies. Moreover structural insight and conformational changes can be detected with this approach.

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Five polymorphs of the antidepressant drug venlafaxine hydrochloride (VenHCl) are reported. Forms 1 and 2 are crystalline modifications, form 3 is obtained by melting, form 4 is a hydrate/alcoholate, and form 5 is an amorphous, glassy phase from sublimation. These five polymorphs of VenHCl are characterized by differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and additionally structures of polymorphs 1 and 2 are confirmed by single-crystal XRD (Pca21 and P21/n). The endotherms for the melting transition of forms 1−4 appear at slightly different temperatures in different patents. Moreover, DSC thermograms of forms 1 and 2 (recorded at 10 K/min) are single, sharp peaks, and there is no discussion of phase transition other than melting. Our DSC profiles (recorded @ 2 K/min) show that both forms 1 and 2 undergo a phase transition just after the melting event in the temperature range 210−220 °C. Form 1 transforms to form 3 (phase from melting) and form 5 (phase from sublimation), whereas form 2 converts completely to form 5 in DSC heat−cool−heat cycles. These transitions are also examined under a hot stage microscope. Form 5 is not very stable and converts to form 1 (inert conditions) or hydrate form 4 (open air) in laboratory experiments. In the heating cycle of DSC (30−300 °C), both polymorphs 1 and 2 undergo sublimation to form 5, as characterized by thermogravimetry-infrared (TG-IR) spectroscopy of the evolved vapor. There is no interconversion between polymorphs 1 and 2 in the ambient-to-melting temperature range. Crystal lattice energy of polymorph 2 is lower than form 1 by 2.0 kcal/mol. However, form 1 is stable to ball mill grinding, whereas form 2 partially transforms to hydrate phase 4, suggesting that form 1 is perhaps more suitable in pharmaceutical formulations. With the identification of at least five different solid-state modifications, VenHCl belongs to the category of highly polymorphic drug substances.

Crystal structures for the commercial monohydrate form and an anhydrate form of dasatinib, an oral anti-cancer agent, are presented along with characterization by Raman spectroscopy, powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. Solubility measurements conducted in water reveal the anhydrate has dramatically improved solubility compared to the commercial hydrate form. Finally, dasatinib is a rare example of a promiscuous solvate former and the basis for this behavior can now be understood by examining the poor packing efficiency in the unsolvated form.

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