Molecular aggregation by hydrogen bonding in cold-crystallization behavior of mixed nucleobases analyzed by temperature-controlled infrared spectroscopy

The cold-crystallization behaviors of dodecyl-substituted nucleobases (adenine, uracil, and thymine) were analyzed. The dodecyl derivative from uracil alone did not exhibit cold crystallization; however, a mixture of adenine and uracil derivatives at a molar ratio of 1 : 1 exhibited cold crystallization. These results are similar to the thermal behavior of dodecyl derivatives of adenine and thymine alone and in mixtures reported in a previous study. Temperature-controlled infrared spectroscopy was used to observe the molecular assembly states of the liquid, supercooled state, and cold-crystallized compounds. Hydrogen-bonded molecular pairs in the high-temperature liquid state, multiple hydrogen-bonded networks in the supercooled state, and reverse Hoogsteen-type complementary hydrogen bonds in cold-crystallized compounds were observed using infrared spectroscopy. The heterogeneity of the system, due to multiple types of hydrogen bonding, retarded the crystallization rate, resulting in supercooling and cold crystallization. Infrared spectroscopy, which can be used to measure the aggregation state of molecules, including the liquid and supercooled states, is an effective analytical method for clarifying the process of cold crystallization.

Cold crystallization and photo-induced thermal behavior of alkyl-derivatized diarylethene molecules

The thermal behavior of alkylated diarylethene molecules (2,3-bis(2,4,5-trimethyl-3-thienyl)-N-alkylmaleimides; DAE-Cn) was investigated. DAE-C1 and DAE-C2 exhibited cold crystallization, which is a heat-storage phenomenon. In addition, DAE-Cn showed photoisomerization; the open-ring isomer O-DAE-Cn was formed by visible light irradiation and transformed to the closed-ring isomer C-DAE-Cn by UV light irradiation. X-ray diffraction and optical microscopy analyses revealed that O-DAE-Cn exhibited cold crystallization and C-DAE-Cn showed poor crystallinity. UV irradiation (365 nm) inhibited cold crystallization, and visible light irradiation (525 nm) triggered cold crystallization, suggesting that heat storage by the cold crystallization of DAE-Cn can be photo-controlled.

The alkylated diarylethene molecules exhibited cold crystallization, and their thermal behavior can be photo-controlled.

Crystallization Propensity of Amorphous Pharmaceuticals: Kinetics and Thermodynamics

Four model compounds, nifedipine, indomethacin, felodipine, and ketoconazole, all with nearly identical glass transition temperatures, were chosen to study the effects of thermodynamics and molecular mobility on their crystallization propensities. The time and temperature dependence of the crystallization induction time of each compound was determined by differential scanning calorimetry (DSC) and enabled the generation of their time-temperature-transformation (TTT) diagrams. The relaxation times (τ_α) were measured by dielectric spectroscopy, and the Gibbs free energy (ΔG) and entropy (ΔS) difference between the crystalline and amorphous states were obtained by DSC. The temperature dependence of the crystallization induction time (τ₀(T)) is a function of the thermodynamic activation barrier and the frequency of "attempted jumps" (1/τ_α(T)) to overcome the barrier. Even though the four model compounds exhibited very similar molecular mobility (relaxation time) over a wide range of temperatures, their crystallization propensities were very different. The observed difference in crystallization propensity was explained in terms of the difference in the thermodynamic barrier, and it is correlated to the empirical relation (TΔS³)/ΔG².

Alkyl substituent-dependent systematic change in cold crystallization of azo molecules

The thermal behavior of alkyl-derivatized 1-(2,4-dimethylphenylazo)-4-naphthol and 1-(2,4-dimethylphenylazo)-2-naphthol (2,4-DM-4-Cn and 2,4-DM-2-Cn, respectively) was investigated. The change in the position of the alkyl substituent led to a variation in the thermal behavior, including the cold crystallization, which is a heat-storing phenomenon. In addition, a comprehensive study of the alkyl chain length revealed that 2,4-DM-4-Cn had better crystallinity and exhibited cold crystallization with short alkyl chains. The π–π, C–H⋯N, and C–H⋯π interactions stabilized the crystal structure of 2,4-DM-4-Cn. On the other hand, the polymorphism of 2,4-DM-2-Cn inhibited the formation of a uniform crystalline phase during cooling, which led to poor crystallinity. The only difference between the compounds, the position of the substituent, resulted in a clear variation in the cold crystallization and heat storage properties.

The change in alkyl substitution position and alkyl chain length of azo molecules led to the variation in cold crystallization behavior.

Ternary Eutectic Ezetimibe-Simvastatin-Fenofibrate System and the Physical Stability of Its Amorphous Form

In this study, the phase diagram of the ternary system of ezetimibe–simvastatin–fenofibrate was established. It has been proven that the ternary composition recommended for the treatment of mixed hyperlipidemia forms a eutectic system. Since eutectic mixtures are characterized by greater solubility and dissolution rate, the obtained result can explain the marvelous medical effectiveness of combined therapy. Considering that another well-known method for improving the aqueous solubility is amorphization, the ternary system with eutectic concentration was converted into an amorphous form. Thermal properties, molecular dynamics, and physical stability of the obtained amorphous system were thoroughly investigated through various experimental techniques compared to both: neat amorphous active pharmaceutical ingredients (considered separately) and other representative concentrations of ternary mixture. The obtained results open up a new way of selecting the therapeutic concentrations for combined therapies, a path that considers one additional variable: eutecticity.

Importance of Mesoporous Silica Particle Size in the Stabilization of Amorphous Pharmaceuticals-The Case of Simvastatin

In this paper, the role of mesoporous silica (MS) particle size in the stabilization of amorphous simvastatin (SVT) is revealed. For inhibiting recrystallization of the supercooled drug, the two MS materials (Syloid® XDP 3050 and Syloid® 244 FP) were employed. The crystallization tendency of SVT alone and in mixture with the MS materials was investigated by Differential Scanning Calorimetry (DSC) and Broadband Dielectric Spectroscopy (BDS). Neither confinement of the SVT molecules inside the MS pores nor molecular interactions between functional groups of the SVT molecules and the surface of the stabilizing excipient could explain the observed stabilization effect. The stabilization effect might be correlated with diffusion length of the SVT molecules in the MS materials that depended on the particle size. Moreover, MS materials possessing different particle sizes could offer free spaces with different sizes, which might influence crystal growth of SVT. All of these factors must be considered when mesoporous materials are used for stabilizing pharmaceutical glasses.

Influence of Annealing in the Close Vicinity of Tg on the Reorganization within Dimers and Its Impact on the Crystallization Kinetics of Gemfibrozil

In this paper, broadband dielectric spectroscopy (BDS) has been applied to study the molecular dynamics and crystallization kinetics of the antihyperlipidemic active pharmaceutical ingredient (API), gemfibrozil (GEM), as well as its deuterated (dGEM) and methylated (metGEM) derivatives, characterized by different types and strengths of intermolecular interactions. Moreover, calorimetric and infrared measurements have been carried out to characterize the thermal properties of examined samples and to probe a change in the H-bonding pattern in GEM, respectively. We found that the dielectric spectra of all examined compounds, collected below the glass transition temperature (T_g), reveal the presence of two secondary relaxations (β, γ). According to the coupling model (CM) predictions, it was assumed that the slower process (β) is of JG type, whereas the faster one (γ) has an intramolecular origin. Interestingly, the extensive crystallization kinetics measurements performed after applying two paths, i.e., the standard procedure (cooling and subsequently heating up to the appropriate temperature, T_c), as well as annealing at two temperatures in the vicinity of T_g and further heating up to T_c, showed that the annealing increases the crystallization rate in the case of native API, while the thermal history of the sample has no significant impact on the pace of this process in the two derivatives of GEM. Analysis of the dielectric strength (Δε) of the α-process during annealing, together with the results of Fourier transform infrared spectroscopy (FTIR) measurements, suggested that the reorganization within dimeric structures formed between the GEM molecules is responsible for the observed behavior. Importantly, our results differ from those obtained by Tominaka et al. (Tominaka, S.; Kawakami, K.; Fukushima, M.; Miyazaki, A.Physical Stabilization of Pharmaceutical Glasses Based on Hydrogen Bond Reorganization under Sub-T_g Temperature Mol. Pharm. 2017 14 264 273 10.1021/acs.molpharmaceut.6b00866.), who demonstrated that the sub-T_g annealing of ritonavir (RTV), which is able to form extensive supramolecular hydrogen bonds, protects this active substance against crystallization. Therefore, based on these contradictory reports, one can hypothesize that materials forming H-bonded structures, characterized by varying architecture, may behave differently after annealing in the vicinity of the glass transition temperature.

Can Storage Time Improve the Physical Stability of Amorphous Pharmaceuticals with Tautomerization Ability Exposed to Compression? The Case of a Chloramphenicol Drug

In this article we thoroughly investigated the physical stability of the amorphous form of a chloramphenicol drug. The tendency toward recrystallization of this drug has been examined (i) at nonisothermal conditions by means of a DSC technique; (ii) at isothermal conditions and temperature close to T_room by means of dielectric spectroscopy; (iii) at isothermal conditions and elevated temperatures of T = 323 K and 338 K by dielectric spectroscopy; and (iv) at conditions imitating the manufacturing procedure (i.e., elevated temperature and compression procedure). Our investigations have shown that amorphous chloramphenicol, stored at both standard storage and elevated temperature conditions, does not reveal a tendency toward recrystallization. However, compression significantly changes this behavior and destabilizes the examined compound. We found that due to chemical equilibration of the sample, the elongation of the storage time before compression might improve the physical stability of the examined pharmaceutical exposed to compression 34-times.