Encapsulated Nanodroplet Crystallization of Organic-Soluble Small Molecules

Summary

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.

Graphical Abstract

Highlights

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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

The Bigger Picture

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.

Abstract

The routine crystallization of small molecules for single-crystal X-ray analysis remains a considerable experimental challenge. We report a general method for the high-throughput nanoscale crystallization of organic-soluble small molecules: encapsulated nanodroplet crystallization (ENaCt). ENaCt provides crystals suitable for X-ray analysis, allowing structural and de novo absolute stereochemical assignment for a diverse and challenging range of small molecules (bioactives, natural products, organometallics, etc.), as well as acting as a tool for new polymorph discovery.