Structural investigation of a 2:1 co-crystal between diflunisal and isonicotinamide based on terahertz and Raman spectroscopy

Analysis of Co-Crystallization Mechanism of Theophylline and Citric Acid from Raman Investigations in Pseudo Polymorphic Forms Obtained by Different Synthesis Methods

Designing co-crystals can be considered as a commonly used strategy to improve the bioavailability of many low molecular weight drug candidates. The present study has revealed the existence of three pseudo polymorphic forms of theophylline-citric acid (TP-CA) co-crystal obtained via different routes of synthesis. These forms are characterized by different degrees of stability in relation with the strength of intermolecular forces responsible for the co-crystalline cohesion. Combining low- and high-frequency Raman investigations made it possible to identify anhydrous and hydrate forms of theophylline-citric acid co-crystals depending on the preparation method. It was shown that the easiest form to synthesize (form 1'), by milling one hydrate with an anhydrous reactant, is very metastable, and transforms into the anhydrous form 1 upon heating or into the hydrated form 2 when it is exposed to humidity. Raman investigations performed in situ during the co-crystallization of forms 1 and 2 have shown that two different types of H-bonding ensure the co-crystalline cohesion depending on the presence of water. In the hydrated form 2, the cohesive forces are related to strong O-H … O H-bonds between water molecules and the reactants. In the anhydrous form 1, the co-crystalline cohesion is ensured by very weak H-bonds between the two anhydrous reactants, interpreted as corresponding to π-H-bonding. The very weak strength of the cohesive forces in form 1 explains the difficulty to directly synthesize the anhydrous co-crystal.

The fingerprints of nifedipine/isonicotinamide cocrystal polymorph studied by terahertz time-domain spectroscopy

Cocrystal is constructed to improve physicochemical properties of active pharmaceutical ingredient and prevent polymorphism via intermolecular interactions. However, recent examples on cocrystal polymorphs display significantly different properties. Even though some analytical techniques have been used to characterize the cocrystal polymorphic system, it remains unclear how intermolecular interactions drive and stabilize the structure. In this work, we study the cocrystal polymorphs of nifedipine (NFD) and isonicotinamide (INA) using terahertz (THz) spectroscopy. Form I and form II of NFD-INA cocrystals show spectral fingerprints in THz region. Temperature-dependent THz spectra display distinguished frequency shifts of each fingerprint. Combined with solid-state density functional theory (DFT) calculations, the experimental fingerprints and their distinct responses to temperature are elucidated by specific collective vibrational modes. The vibrations of hydrogen bonding between dihydropyridine ring of NFD and INA are generally distributed below 1.5 THz, which play important roles in stabilizing cocrystal and preventing the oxidation of NFD. The rotations of methyl group in NFD are widely distributed in the range of 1.5-4.0 THz, which helps the steric recognition. The results demonstrate that THz spectroscopy is a sensitive tool to discriminate cocrystal polymorphs. It has the potential to be used as a non-invasive technique for pharmaceutical screening.

Dual spectroscopic detection of THz energy modes of critical chemical compounds

Precise identification and sensing of organic and inorganic molecular systems are key factors in several applications in present industrial and scientific domains. While high energy modes, due to electronic interactions, are mostly impervious to the initial thermodynamic or chemical conditions, the low energy modes are sensitive to such alterations which makes them suitable for quality control purpose with sensitive spectral identification methods. Here we report for the first time, several low frequency peaks of specific nitrogen-based compounds and their derivatives, using the dual spectroscopic approach of Terahertz Time-Domain Spectroscopy (THz-TDS) and THz Raman Spectroscopy (THz-RS). Two different isomeric molecular systems have also been investigated to assess both the selectivity and specificity of low energy modes in their identification and spectral correlation in terms of molecular interactions. This information of low frequency modes can be utilized readily by pharmaceutical and agri-food industries, chemical engineering and crystal growth communities in identification, detection, quality control and industrial waste management.

The necessity of periodic boundary conditions for the accurate calculation of crystalline terahertz spectra

Terahertz vibrational spectroscopy has emerged as a powerful spectroscopic technique, providing valuable information regarding long-range interactions - and associated collective dynamics - occurring in solids. However, the terahertz sciences are relatively nascent, and there have been significant advances over the last several decades that have profoundly influenced the interpretation and assignment of experimental terahertz spectra. Specifically, because there do not exist any functional group or material-specific terahertz transitions, it is not possible to interpret experimental spectra without additional analysis, specifically, computational simulations. Over the years simulations utilizing periodic boundary conditions have proven to be most successful for reproducing experimental terahertz dynamics, due to the ability of the calculations to accurately take long-range forces into account. On the other hand, there are numerous reports in the literature that utilize gas phase cluster geometries, to varying levels of apparent success. This perspective will provide a concise introduction into the terahertz sciences, specifically terahertz spectroscopy, followed by an evaluation of gas phase and periodic simulations for the assignment of crystalline terahertz spectra, highlighting potential pitfalls and good practice for future endeavors.

Terahertz and Raman Spectroscopic Investigation of Monohydrate Cocrystal of Antitubercular Isoniazid with Protocatechuic Acid

Pharmaceutical cocrystal provides an alternative modification strategy for the formulation development of drugs owning to their potential ability to improve the physicochemical properties of active pharmaceutical ingredients (APIs) efficiently by changing inter-molecular interactions between raw materials. Isoniazid (INH) is an indispensable main drug for the treatment of tuberculosis, but its tablet formulation is unstable and prone to degradation. In the present study, the monohydrate cocrystal of INH and protocatechuic acid (PA) was prepared by solvent evaporation using PA as cocrystal former to optimize the properties of INH. The parent materials and corresponding 1:1 molar ratio INH-PA monohydrate cocrystal have been characterized by the terahertz time-domain (THz-TDS) and Raman spectroscopy. The THz absorption spectra displayed that there were obvious differences between the peaks of experimental cocrystal and the parent materials, and the same situation was found in Raman vibrational spectra. In addition, density functional theory (DFT) was applied to simulating and optimizing the structure of INH-PA monohydrate cocrystal and supplied corresponding vibrational modes. Our results provided a unique method to characterize the formation of INH-PA monohydrate cocrystal at the molecular-level and a lot of information about cocrystal structure and intra-molecular and/or inter-molecular hydrogen bond interactions in the emerging pharmaceutical cocrystal fields.

Vibrational spectral and structural characterization of multicomponent ternary co-crystal formation within acetazolamide, nicotinamide and 2-pyridone

Ternary co-crystal, as a novel co-crystal design strategy developed on the basis of binary co-crystal, could be used to improve the physicochemical properties of active pharmaceutical ingredients (APIs) efficiently. However, it is difficult to obtain specific ternary co-crystals since such ternary one involves complex assembly of three different molecules. There are few reports on the micro-molecular structure respect of specific ternary co-crystal systems. In present work, 1:1:1 ternary co-crystal between acetazolamide (ACZ), nicotinamide (NAM) and 2-pyridone (2HP) has been synthesized successfully by mechanical grinding approach, and their structures are investigated by terahertz time-domain spectroscopy (THz-TDS) and Raman spectroscopy combined with theoretical calculation at the molecular level. The experimental THz spectral results showed that ACZ-NAM-2HP ternary co-crystal and the starting parent materials exhibited a few distinct spectral features in frequency-domain absorption spectra. Likewise, the Raman spectral result also shows some difference between the co-crystal and starting raw materials. Through density functional theory (DFT) calculations, the theoretical THz/Raman spectra and vibrational modes of two kind of possible ternary co-crystal theoretical forms (form I and II) between ACZ, NAM and 2HP were obtained. By comparing experimental and theoretical spectral results, the most suitable structure and vibrational modes of ACZ-NAM-2HP ternary co-crystal were determined. These results provide a wealth of information and unique method for studying molecular assembly and also inter-molecular interactions in specific ternary co-crystals at the molecular level in the emerging pharmaceutical co-crystal fields.

Unprecedented morphology control of gas phase cocrystal growth using multi zone heating and tailor made additives

Herein, it is shown for the first time that cocrystals can be grown by cosublimation with unprecedented morphology control using tailor made additives. Multicrystalline sea-urchin motifs and solids that look like candy-floss are transformed into block-like and needle-like single crystals using additives.

Raman and Terahertz Spectroscopic Characterization of Solid-state Cocrystal Formation within Specific Active Pharmaceutical Ingredients

Cocrystallization of specific active pharmaceutical ingredients (APIs) in the solid-state phase is becoming a feasible way to improve their corresponding physicochemical properties and ultimate bioavailability without making and breaking any covalent bonds within them. Many recent reports deal with the characterization and analysis topics of pharmaceutical APIs-based cocrystals. In this mini-review, we will focus on the recent steady-state and time-dependent spectroscopic investigation into the cocrystallization of specific APIs based on both Raman and emerging terahertz spectroscopy in pharmaceutical fields. Distinctive spectral, structural and also kinetic information of pharmaceutical APIs-based cocrystals are obtained and discussed, which would highlight the potential of vibrational spectroscopy as an attractive technique for various drug research and development during cocrystallization of specific APIs.

Vibrational spectra of pyrazinamide and isoniazid studied by terahertz spectroscopy and density functional theory

Pyrazinamide and isoniazid, as two first-line anti-tuberculosis drugs, are investigated by terahertz time-domain spectroscopy (THz-TDS). Both pyrazinamide and isoniazid have three absorption peaks, at 0.50, 0.71, 1.42 THz and 1.16, 1.46, 1.56 THz, respectively, which can be used as the basis for qualitative identification of these two drugs. In order to gain insight into the origin of the characteristic absorption peaks, density functional theory (DFT) based on single molecular, dimer, and crystalline structures of pyrazinamide and isoniazid are performed. The purpose of the calculation based on the single molecular structure is to understand the intramolecular interaction, while those based on the dimer and crystalline structures are to investigate the intermolecular interactions in PNZ and INZ. Comparing the theoretical results of the dimer and crystalline based structures reveals that the crystalline structure leads to vibrational spectra that are closer to the experimental values in terms of the number of absorption peaks and the positions of the absorption peaks. Vibrational mode assignments can be summarized as that the characteristic absorption peaks of pyrazinamide mainly come from intermolecular interaction, and the characteristic absorption peaks of isoniazid originate from both the intramolecular and intermolecular interactions. Our experimental and theoretical results indicate that the combination of THz-TDS with DFT is an effective approach for identification of molecules with pharmaceutical significance.