Low-Frequency Vibrational Spectroscopy and Quantum Mechanical Simulations of the Crystalline Polymorphs of the Antiviral Drug Ribavirin

High-Pressure Polymorphism of Ribavirin

The effect of high pressure on ribavirin, a broad-spectrum antiviral consisting of ribofuranosyl triazole, and carboxamide moieties, has been studied up to ∼10 GPa. Three new high-pressure phases, designated V3, V4 and V5, have been obtained by compression of the ambient-pressure V2 form with structures refined up to 7.5 GPa. The new phases are formed at 5.3, 6.0, and 7.2 GPa, respectively, and crystallize in space group P212121 with Z' = 3, 1, and 1. They are distinguished by changes in the conformation of the ribofuranosyl moiety which impacts both the molecular geometry and the supramolecular structure.

Sofosbuvir Polymorphs Distinguished by Linearly and Circularly Polarized Raman Microscopy

Most currently marketed pharmaceuticals are manufactured in the solid state, where the bioavailability of the active pharmaceutical ingredient (API) can be optimized through different polymorphs, cocrystals, solvates, or salts. Efficient techniques are needed to monitor the structure of pharmaceuticals during production. Here, we explore the potential of linearly and circularly polarized Raman microscopy for distinguishing three polymorphs of sofosbuvir, an antiviral drug used to treat hepatitis C. Raman spectra were recorded on a Raman microscope for a polycrystalline API diluted in a KBr matrix. To understand spectral features including the low-frequency region, we simulated band frequencies and intensities using quantum-chemical computational strategies based on cluster and transfer approaches. Very good agreement was achieved between simulated and experimental intensities. The 20 to 200 cm-1 wavenumber region appeared particularly useful for polymorph discrimination already based on unpolarized measurements. The depolarization ratios obtained from linearly polarized Raman spectra made the distinction even more reliable. Moreover, circularly polarized Raman spectra and normalized degrees of circularity provided useful additional information and revealed several tentative markers of the different polymorphs of sofosbuvir. Although in some spectral regions the differences were less obvious, the results indicate that polarized Raman microscopy is a handy tool for discriminating between polymorphs of APIs and other compounds.

Quantum-level machine learning calculations of Levodopa

Many drug molecules contain functional groups, resulting in a torsional barrier corresponding to rotation around the bond linking the fragments. In medicinal chemistry and pharmaceutical sciences, inclusive of drug design studies, the exact calculation of the potential energy surface (PES) of these molecular torsions is extremely important and precious. Machine learning (ML), including deep learning (DL), is currently one of the most rapidly evolving tools in computer-aided drug discovery and molecular simulations. In this work, we used ANI-1x neural network potential as a quantum-level ML to predict the PESs of the L-3,4-dihydroxyphenylalanine (Levodopa) antiparkinsonian drug molecule. The electronic energies and structural parameters calculated by density functional theory (DFT) using the wB97X method and all possible Pople's basis sets indicated the 6-31G(d) basis set, when used with the wB97X functional, exhibits behavior similar to that of the ANI-1x model. The vibrational frequencies investigation showed a linear correlation between DFT and ML data. All ANI-1x calculations were completed quickly in a very short computing time. From this perspective, we expect the ANI-1x dataset applied in this work to be appreciably efficient and effective in computational structure-based drug design studies.

Where do the Fluorine Atoms Go in Inorganic-Oxide Fluorinations? A Fluorooxoborate Illustration under Terahertz Light

The substitution of fluorine atoms for oxygen atoms/hydroxyl groups has emerged as a promising strategy to enhance the physical and chemical properties of oxides/hydroxides in fluorine chemistry. However, distinguishing fluorine from oxygen/hydroxyl in the reaction products poses a significant challenge in existing characterization methods. In this study, we illustrate that terahertz (THz) spectroscopy provides a powerful tool for addressing this challenge. To this end, we investigated two fluorination reactions of boric acid, utilizing MHF2 (M=Na, C(NH2)3) as fluorine reagents. Through an interplay between THz spectroscopy and solid-state density functional theory, we have conclusively demonstrated that fluorine atoms exclusively bind with the sp3-boron but not with the sp2-boron in the reaction products of Na[B(OH)3][B3O3F2(OH)2] (NaBOFH) and [C(NH2)3]2B3O3F4OH (GBF2). Based on this evidence, we have proposed a reaction pathway for the fluorinations under investigation, a process previously hindered due to structural ambiguity. This work represents a step forward in gaining a deeper understanding of the precise structures and reaction mechanisms involved in the fluorination of oxides/hydroxides, illuminated by the insights provided by THz spectroscopy.

Crystallization Selectivity of Ribavirin Solution and Amorphous Phase

Crystallization selectivity is an important principle in polymorph control. Ribavirin Form I, Form II, DMSO solvate, and amorphous ribavirin are prepared, and the short-range order similarities between these solid forms and ribavirin aqueous solution and DMSO solution are compared via mid-frequency Raman difference spectra (MFRDS). The crystallization process from amorphous ribavirin to Form I and from solution to amorphous phase is explained. Reasons for the difficulty in preparing the DMSO solvate are proposed. The rationale provided for the crystallization selectivity provides a foundation for the synthesis of metastable phases with a robust and convenient method.

Low-Frequency Raman Spectroscopy of Pure and Cocrystallized Mycophenolic Acid

The aqueous solubility of solid-state pharmaceuticals can often be enhanced by cocrystallization with a coformer to create a binary cocrystal with preferred physical properties. Greater understanding of the internal and external forces that dictate molecular structure and intermolecular packing arrangements enables more efficient design of new cocrystals. Low-frequency (sub-200 cm^-1) Raman spectroscopy experiments and solid-state density functional theory simulations have been utilized together to investigate the crystal lattice vibrations of mycophenolic acid, an immunosuppressive drug, in its pure form and as a cocrystal with 2,2'-dipyridylamine. The lattice vibrations primarily consist of large-amplitude translations and rotations of the crystal components, thereby providing insights into the critical intermolecular forces governing cohesion of the molecular solids. The simulations reveal that despite mycophenolic acid having a significantly unfavorable conformation in the cocrystal as compared to the pure solid, the cocrystal exhibits greater thermodynamic stability over a wide temperature range. The energetic penalty due to the conformational strain is more than compensated for by the strong intermolecular forces between the drug and 2,2'-dipyridylamine. Quantifying the balance of internal and external energy factors in cocrystal formation indicates a path forward in the development of future mycophenolic acid cocrystals.