Microbatch under-oil salt screening of organic cations: single-crystal growth of active pharmaceutical ingredients

High-throughput nanoscale crystallization of dihydropyridine active pharmaceutical ingredients

Single-crystal X-ray diffraction analysis of small molecule active pharmaceutical ingredients is a key technique in the confirmation of molecular connectivity, including absolute stereochemistry, as well as the solid-state form. However, accessing single crystals suitable for X-ray diffraction analysis of an active pharmaceutical ingredient can be experimentally laborious, especially considering the potential for multiple solid-state forms (solvates, hydrates and polymorphs). In recent years, methods for the exploration of experimental crystallization space of small molecules have undergone a `step-change', resulting in new high-throughput techniques becoming available. Here, the application of high-throughput encapsulated nanodroplet crystallization to a series of six dihydropyridines, calcium channel blockers used in the treatment of hypertension related diseases, is described. This approach allowed 288 individual crystallization experiments to be performed in parallel on each molecule, resulting in rapid access to crystals and subsequent crystal structures for all six dihydropyridines, as well as revealing a new solvate polymorph of nifedipine (1,4-dioxane solvate) and the first known solvate of nimodipine (DMSO solvate). This work further demonstrates the power of modern high-throughput crystallization methods in the exploration of the solid-state landscape of active pharmaceutical ingredients to facilitate crystal form discovery and structural analysis by single-crystal X-ray diffraction.

Advanced crystallisation methods for small organic molecules

Molecular materials based on small organic molecules often require advanced structural analysis, beyond the capability of spectroscopic techniques, to fully characterise them. In such cases, diffraction methods such as single crystal X-ray diffraction (SCXRD), are one of the most powerful tools available to researchers, providing molecular and structural elucidation at atomic level resolution, including absolute stereochemistry. However SCXRD, and related diffraction methods, are heavily dependent on the availability of suitable, high-quality crystals, thus crystallisation often becomes the major bottleneck in preparing samples. Following a summary of classical methods for the crystallisation of small organic molecules, this review will focus on a number of recently developed advanced methods for crystalline material sample preparation for SCXRD. This review will cover two main areas of modern small organic molecule crystallisation, namely the inclusion of molecules within host complexes (e.g., "crystalline sponge" and tetraaryladamantane based inclusion chaperones) and the use of high-throughput crystallisation, employing "under-oil" approaches (e.g., microbatch under-oil and ENaCt). Representative examples have been included for each technique, together with a discussion of their relative advantages and limitations to aid the reader in selecting the most appropriate technique to overcome a specific analytical challenge.

Crystalline Forms of Trazodone Dihydrates

In this study, treatment of anhydrous trazodone powder with ammonium carbamate in warm water crystallised two new polymorphs or dihydrates of trazodone after 5 h, whose structures were determined by X-ray single crystal diffraction. Each dihydrate contains infinite zigzag hydrogen-bonded chains of water molecules, which are stabilised by the N4 acceptor atom of the piperazine ring and the pendant carbonyl O1 atom of the triazole ring, as well as other water molecules. The strong dipole moment expected for the O1 atom makes it a good hydrogen bond acceptor for stabilising the chains of water molecules. Each molecule of trazodone has a similar conformation in both hydrates, except for the propyl chains, which adopt different conformations: anti-gauche in the β hydrate (triazole N-C-C-C and C-C-C-piperazine N) and anti-anti in the γ hydrate. Both piperazine rings adopt chair conformations, and the exocyclic N-C bonds are in equatorial orientations. The Hirshfeld surfaces and two-dimensional fingerprint plots for the polymorphs were calculated using CrystalExplorer17, which indicated contacts significantly shorter than the sum of the van der Waals radii in the vicinity of the piperazine N4 and triazole O1 atoms corresponding to the strong hydrogen bonds accepted by these atoms.

Studying the cellular distribution of highly phototoxic platinated metalloporphyrins using isotope labelling

Novel tetraplatinated metalloporphyrin-based photosensitizers (PSs) are reported, which show excellent phototoxic indexes (PIs) up to 5800 against HeLa cells, which is, to the best of our knowledge, the highest value reported for any porphyrin so far. Furthermore, 67Zn isotope labelling allowed the determination of the ratio of zinc to platinum inside the cells using ICP-MS.

New Crystalline Salts of Nicotinamide Riboside as Food Additives

NR⁺ is a highly effective vitamin B₃ type supplement due to its unique ability to replenish NAD⁺ levels. While NR⁺ chloride is already on the market as a nutritional supplement, its synthesis is challenging, expensive, and low yielding, making it cumbersome for large-scale industrial production. Here we report the novel crystalline NR⁺ salts, d/l/dl-hydrogen tartrate and d/l/dl-hydrogen malate. Their high-yielding, one-pot manufacture does not require specific equipment and is suitable for multi-ton scale production. These new NR⁺ salts seem ideal for nutritional applications due to their bio-equivalence compared to the approved NR⁺ chloride. In addition, the crystal structures of all stereoisomers of NR⁺ hydrogen tartrate and NR⁺ hydrogen malate and a comparison to the known NR⁺ halogenides are presented.

New Pharmaceutical Salts of Trazodone

New pharmaceutically acceptable salts of trazodone (trazodone hydrogen bromide and trazodone 1-hydroxy-2-naphthonic acid) for the treatment of central nervous system disorders are synthesized and described. Although trazodone salts are poorly crystalline, single-crystal X-ray diffraction data for trazodone 1-hydroxy-2-naphthonic acid were collected and analyzed as well as compared to the previously described crystal structure of commercially available trazodone hydrochloride. The powder samples of all new salts were characterized by Fourier transform infrared spectroscopy, X-ray diffraction and ¹³C solid-state nuclear magnetic resonance spectroscopy. Spectroscopic studies were supported by gauge including projector augmented wave (GIPAW) calculations of carbon chemical shielding constants. The main goal of our research was to find salts with better physicochemical properties and to make an attempt to associate them with both the anion structure and the most prominent interactions exhibited by the protonated trazodone cation. The dissolution profiles of trazodone from tablets prepared from various salts with lactose monohydrate were investigated. The studies revealed that salts with simple anions show a fast release of the drug while the presence of more complex anion, more strongly interacting with the cation, effects a slow-release profile of the active substance and can be used for the preparation of the tables with a delay or prolonged mode of action.

Encapsulated Nanodroplet Crystallization of Organic-Soluble Small Molecules

Single-crystal X-ray diffraction analysis (SCXRD) constitutes a universal approach for the elucidation of molecular structure and the study of crystalline forms. However, the discovery of viable crystallization conditions remains both experimentally challenging and resource intensive in both time and the quantity of analyte(s). We report a robot-assisted, high-throughput method for the crystallization of organic-soluble small molecules in which we employ only micrograms of analyte per experiment. This allows hundreds of crystallization conditions to be screened in parallel with minimal overall sample requirements. Crystals suitable for SCXRD are grown from nanoliter droplets of a solution of analyte in organic solvent(s), each of which is encapsulated within an inert oil to control the rate of solvent loss. This encapsulated nanodroplet crystallization methodology can also be used to search for new crystal forms, as exemplified through both our discovery of a new (13^th) polymorph of the olanzapine precursor ROY and SCXRD analysis of the “uncrystallizable” agrochemical dithianon.

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Single crystals of small molecules are grown from nanoscale droplets of organic solvent

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Discovery of the 13^th polymorph (R18) of olanzapine precursor ROY

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X-ray diffraction analysis of “uncrystallizable” agrochemical dithianon

Small molecules can form crystalline solids, in which individual molecules pack together into ordered three-dimensional arrays. Once a suitable crystal is grown, the packing and atomic connectivity of the constituent molecules can be studied by X-ray diffraction. However, the discovery of experimental conditions for successful crystal growth is often challenging. We have developed a nanoscale crystallization technique for organic-soluble small molecules by using high-throughput liquid-handling robotics to undertake multiple crystallization experiments simultaneously with minimal sample requirements and high success rates. We showcase our methodology through the crystallization of a diverse set of small molecules, including “uncrystallizables,” combined with structural analysis by X-ray diffraction. We anticipate that this rapid and reliable method for small-molecule crystallization will have far-reaching impact, facilitating academic and industrial research in the molecular sciences.

Single crystal growth of water-soluble metal complexes with the help of the nano-crystallization method

Let pipetting robots set up nano crystallization trials of water-soluble metal complexes in order to obtain single crystals!

Hydrophobic Interface-Assisted Protein Crystallization: Theory and Experiment

Macromolecular crystallization is crucial to a large number of scientific fields, including structural biology; drug design, formulation, and delivery; manufacture of biomaterials; and preparation of foodstuffs. The purpose of this study is to facilitate control of crystallization, by investigating hydrophobic interface-assisted protein crystallization both theoretically and experimentally. The application of hydrophobic liquids as nucleation promoters or suppressors has rarely been investigated, and provides an underused avenue to explore in protein crystallization. Theoretically, crystal nucleation is regarded as a two-step process, the first step being a local increase in protein concentration due to its adsorption on the hydrophobic surface. Subsequently, the protein is ordered in a crystal lattice. The energetic aspect of crystal nucleation on water/hydrophobic substance interfaces is approached by calculating the balance between the cohesive energy maintaining integrity of the two-dimensional crystal nucleus and the sum of destructive energies tending to tear up the crystal. This is achieved by comparing the number of bonds shared by the units forming the crystal and the number of unshared (dangling) bonds on the crystal surface pointing toward the solution. The same approach is extended to three-dimensional protein crystal nucleation at water/hydrophobic liquid interfaces. Experimentally, we studied protein crystallization over oils and other hydrophobic liquids (paraffin oil, FC-70 Fluorinert fluorinated oil, and three chlorinated hydrocarbons). Crystallizations of α-lactalbumin and lysozyme are compared, and additional information is acquired by studying α-crustacyanin, trypsin, an insulin analogue, and protein Lpg2936. Depending on the protein type, concentration, and the interface aging time, the proteins exhibit different crystallization propensities depending on the hydrophobic liquid used. Some hydrophobic liquids provoke an increase in the effective supersaturation, which translates to enhancement of crystal nucleation at their interface with the crystallization solution, leading to the formation of crystals.