Monitoring the influence of additives on the crystallization processes of glycine with dynamic nuclear polarization solid-state NMR

Crystallization is fundamental in many domains, and the investigation of the sequence of solid phases produced as a function of crystallization time is thus key to understand and control crystallization processes. Here, we used a solid-state nuclear magnetic resonance strategy to monitor the crystallization process of glycine, which is a model compound in polymorphism, under the influence of crystallizing additives, such as methanol or sodium chloride. More specifically, our strategy is based on a combination of low-temperatures and dynamic nuclear polarization (DNP) to trap and detect transient crystallizing forms, which may be present only in low quantities. Interestingly, our results show that these additives yield valuable DNP signal enhancements even in the absence of glycerol within the crystallizing solution.

Mesoscale clusters of organic solutes in solution and their role in crystal nucleation

It is becoming evident that primary nucleation of crystals of organic molecules from solution is often anything but ‘classical’ in its complexity. It is also becoming increasingly clear that mesoscopic...

Pre-Nucleation Clusters Predict Crystal Structures in Models of Chiral Molecules

Kinetics can play an important role in the crystallization of molecules and can give rise to polymorphism, the tendency of molecules to form more than one crystal structure. Current computational methods of crystal structure prediction, however, focus almost exclusively on identifying the thermodynamically stable polymorph. Kinetic factors of nucleation and growth are often neglected because the underlying microscopic processes can be complex and accurate rate calculations are numerically cumbersome. In this work, we use molecular dynamics computer simulations to study simple molecular models that reproduce the crystallization behavior of real chiral molecules, including the formation of enantiopure and racemic crystals, as well as polymorphism. A significant fraction of these molecules forms crystals that do not have the lowest free energy. We demonstrate that at high supersaturation crystal formation can be accurately predicted by considering the similarities between oligomeric species in solution and molecular motifs in the crystal structure. For the case of racemic mixtures, we even find that knowledge of crystal free energies is not necessary and kinetic considerations are sufficient to determine if the system will undergo spontaneous chiral separation. Our results suggest conceptually simple ways of improving current crystal structure prediction methods.

Molecular aspects of glycine clustering and phase separation in an aqueous solution during anti-solvent crystallization

The anti-solvent crystallization behavior of the glycine aqueous and ethanol system was addressed through molecular dynamics simulation of a non-equilibrium state.

A Strategy for Probing the Evolution of Crystallization Processes by Low-Temperature Solid-State NMR and Dynamic Nuclear Polarization

Crystallization plays an important role in many areas, and to derive a fundamental understanding of crystallization processes, it is essential to understand the sequence of solid phases produced as a function of time. Here, we introduce a new NMR strategy for studying the time evolution of crystallization processes, in which the crystallizing system is quenched rapidly to low temperature at specific time points during crystallization. The crystallized phase present within the resultant "frozen solution" may be investigated in detail using a range of sophisticated NMR techniques. The low temperatures involved allow dynamic nuclear polarization (DNP) to be exploited to enhance the signal intensity in the solid-state NMR measurements, which is advantageous for detection and structural characterization of transient forms that are present only in small quantities. This work opens up the prospect of studying the very early stages of crystallization, at which the amount of solid phase present is intrinsically low.

`NMR Crystallization': in-situ NMR techniques for time-resolved monitoring of crystallization processes

Solid-state NMR spectroscopy is a well-established and versatile technique for studying the structural and dynamic properties of solids, and there is considerable potential to exploit the power and versatility of solid-state NMR for in-situ studies of chemical processes. However, a number of technical challenges are associated with adapting this technique for in-situ studies, depending on the process of interest. Recently, an in-situ solid-state NMR strategy for monitoring the evolution of crystallization processes has been developed and has proven to be a promising approach for identifying the sequence of distinct solid forms present as a function of time during crystallization from solution, and for the discovery of new polymorphs. The latest development of this technique, called `CLASSIC' NMR, allows the simultaneous measurement of both liquid-state and solid-state NMR spectra as a function of time, thus yielding complementary information on the evolution of both the liquid phase and the solid phase during crystallization from solution. This article gives an overview of the range of NMR strategies that are currently available for in-situ studies of crystallization processes, with examples of applications that highlight the potential of these strategies to deepen our understanding of crystallization phenomena.

Biological Activity of Ionic Liquids and Their Application in Pharmaceutics and Medicine

Ionic liquids are remarkable chemical compounds, which find applications in many areas of modern science. Because of their highly tunable nature and exceptional properties, ionic liquids have become essential players in the fields of synthesis and catalysis, extraction, electrochemistry, analytics, biotechnology, etc. Apart from physical and chemical features of ionic liquids, their high biological activity has been attracting significant attention from biochemists, ecologists, and medical scientists. This Review is dedicated to biological activities of ionic liquids, with a special emphasis on their potential employment in pharmaceutics and medicine. The accumulated data on the biological activity of ionic liquids, including their antimicrobial and cytotoxic properties, are discussed in view of possible applications in drug synthesis and drug delivery systems. Dedicated attention is given to a novel active pharmaceutical ingredient-ionic liquid (API-IL) concept, which suggests using traditional drugs in the form of ionic liquid species. The main aim of this Review is to attract a broad audience of chemical, biological, and medical scientists to study advantages of ionic liquid pharmaceutics. Overall, the discussed data highlight the importance of the research direction defined as "Ioliomics", studies of ions in liquids in modern chemistry, biology, and medicine.

Molecular dynamics simulations of aqueous glycine solutions

Pre-nucleation clusters of glycine are strongly hydrated dynamic solutes, which change size and shape within hundreds of picoseconds.

"Hydration Shells" of CH2 Groups of ω-Amino Acids as Studied by Deuteron NMR Relaxation

Hydration phenomena play a very important role in various processes, in particular in biological systems. Water molecules in aqueous solutions of organic compounds can be distributed among the following substructures: (i) hydration shells of hydrophilic functional groups of molecules, (ii) water in the environment of nonpolar moieties, and (iii) bulk water. Up to now, the values of hydration parameters suggested for the description of various solutions of organic compounds were not thoroughly analyzed in the aspect of the consideration of the total molecular composition. The temperature and concentration dependences of relaxation rates of water deuterons were studied in a wide range of concentration and temperature in aqueous (D2O) solutions of a set of ω-amino acids. Assuming the coordination number of the CH2 group equal to 7, which was determined from quantum-chemical calculations, it was found that the rotational correlation times of water molecules near the methylene group is 1.5-2 times greater than one for pure water. The average rotational mobility of water molecules in the hydration shells of hydrophilic groups of ω-amino acids is a bit slower than that in pure solvent at temperatures higher that 60 °C, but at lower temperatures, it is 0.8-1.0 of values of correlation times for bulk water. The technique suggested provides the basis for the characterization of different hydrophobic and hydrophilic species in the convenient terms of the rotational correlation times for the nearest water molecules.

Monitoring the evolution of crystallization processes by in-situ solid-state NMR spectroscopy

Crystallization processes play a crucial role in many aspects of biological and physical sciences. Progress in deepening our fundamental understanding of such processes relies, to a large extent, on the development and application of new experimental strategies that allow direct in-situ monitoring of the process. In this paper, we give an overview of an in-situ solid-state NMR strategy that we have developed in recent years for monitoring the time-evolution of different polymorphic forms (or other solid forms) that arise as the function of time during crystallization from solution. The background to the strategy is described and several examples of the application of the technique are highlighted, focusing on both the evolution of different polymorphs during crystallization and the discovery of new polymorphs.

New in situ solid-state NMR techniques for probing the evolution of crystallization processes: pre-nucleation, nucleation and growth

The application of in situ techniques for investigating crystallization processes promises to yield significant new insights into fundamental aspects of crystallization science. With this motivation, we recently developed a new in situ solid-state NMR technique that exploits the ability of NMR to selectively detect the solid phase in heterogeneous solid–liquid systems (of the type that exist during crystallization from solution), with the liquid phase “invisible” to the measurement. As a consequence, the technique allows the first solid particles produced during crystallization to be observed and identified, and allows the evolution of different solid phases (e.g., polymorphs) present during the crystallization process to be monitored as a function of time. This in situ solid-state NMR strategy has been demonstrated to be a powerful approach for establishing the sequence of solid phases produced during crystallization and for the discovery of new polymorphs. The most recent advance of the in situ NMR methodology has been the development of a strategy (named “CLASSIC NMR”) that allows both solid-state NMR and liquid-state NMR spectra to be measured (essentially simultaneously) during the crystallization process, yielding information on the complementary changes that occur in both the solid and liquid phases as a function of time. In this article, we present new results that highlight the application of our in situ NMR techniques to successfully unravel different aspects of crystallization processes, focusing on: (i) the application of a CLASSIC NMR approach to monitor competitive inclusion processes in solid urea inclusion compounds, (ii) exploiting liquid-state NMR to gain insights into co-crystal formation between benzoic acid and pentafluorobenzoic acid, and (iii) applications of in situ solid-state NMR for the discovery of new solid forms of trimethylphosphine oxide and l-phenylalanine. Finally, the article discusses a number of important fundamental issues relating to practical aspects, the interpretation of results and the future scope of these techniques, including: (i) an assessment of the smallest size of solid particle that can be detected in in situ solid-state NMR studies of crystallization, (ii) an appraisal of whether the rapid sample spinning required by the NMR measurement technique may actually influence or perturb the crystallization behaviour, and (iii) a discussion of factors that influence the sensitivity and time-resolution of in situ solid-state NMR experiments.

The reluctant polymorph: investigation into the effect of self-association on the solvent mediated phase transformation and nucleation of theophylline

Evidence that theophylline forms aggregates in H-bond donor solvents, and the presence of these aggregates hinders the nucleation and phase transformation to form IV.

"CLASSIC NMR": an in-situ NMR strategy for mapping the time-evolution of crystallization processes by combined liquid-state and solid-state measurements

A new in-situ NMR strategy (termed CLASSIC NMR) for mapping the evolution of crystallization processes is reported, involving simultaneous measurement of both liquid-state and solid-state NMR spectra as a function of time. This combined strategy allows complementary information to be obtained on the evolution of both the solid and liquid phases during the crystallization process. In particular, as crystallization proceeds (monitored by solid-state NMR), the solution state becomes more dilute, leading to changes in solution-state speciation and the modes of molecular aggregation in solution, which are monitored by liquid-state NMR. The CLASSIC NMR experiment is applied here to yield new insights into the crystallization of m-aminobenzoic acid.

Pre-nucleation clusters as solute precursors in crystallisation

Crystallisation is at the heart of various scientific disciplines, but still the understanding of the molecular mechanisms underlying phase separation and the formation of the first solid particles in aqueous solution is rather limited. In this review, classical nucleation theory, as well as established concepts of spinodal decomposition and liquid-liquid demixing, is introduced together with a description of the recently proposed pre-nucleation cluster pathway. The features of pre-nucleation clusters are presented and discussed in relation to recent modifications of the classical and established models for phase separation, together with a review of experimental work and computer simulations on the characteristics of pre-nucleation clusters of calcium phosphate, calcium carbonate, iron(oxy)(hydr)oxide, silica, and also amino acids as an example of small organic molecules. The role of pre-nucleation clusters as solute precursors in the emergence of a new phase is summarized, and the link between the chemical speciation of homogeneous solutions and the process of phase separation via pre-nucleation clusters is highlighted.

On the study of crystal growth via interfacial analysis and string optimization

Mathematical ‘strings’ can be used with computer simulations and statistical mechanics to calculate the fraction of growth units and activation energies of flexible molecules present at the crystal–solution interface.

Polymorph formation and nucleation mechanism of tolfenamic acid in solution: an investigation of pre-nucleation solute association

PURPOSE

Crystallization from solution involves nucleation and growth; growth conditions greatly influence self-association behaviors of solute molecules in these steps, affecting crystal packing of organic molecules. We examined the role of pre-nucleation association to provide insights into the mutual influence between molecular conformation in solution and packing in the solid state.

METHODS

Crystallization experiments of tolfenamic acid were conducted in ethanol under different supersaturation conditions. UV spectroscopy was performed to study self-association of solute molecules in ethanol as a function of concentration. Intermolecular interaction energies of tolfenamic acid dimers were calculated with quantum mechanical methods.

RESULTS

As supersaturation increased, growth of the most stable polymorph outpaced the metastable one, contradicting Ostwald's Rule of Stages. UV spectroscopy measurement suggests solute molecules exist as hydrogen-bonded dimers and more dimers form as total concentration increases. Hydrogen bonding in the most stable form is significantly stronger than that in the metastable form.

CONCLUSIONS

With the fact that molecular conformation is different in the two polymorphs, as concentration increases, solute molecules rearrange their conformations to form stronger hydrogen-bonded dimers in solution, resulting in nucleation of the most stable form.

Microscopic mechanism of nanocrystal formation from solution by cluster aggregation and coalescence

Solute-cluster aggregation and particle fusion have recently been suggested as alternative routes to the classical mechanism of nucleation from solution. The role of both processes in the crystallization of an aqueous electrolyte under controlled salt addition is here elucidated by molecular dynamics simulation. The time scale of the simulation allows direct observation of the entire crystallization pathway, from early events in the prenucleation stage to the formation of a nanocrystal in equilibrium with concentrated solution. The precursor originates in a small amorphous aggregate stabilized by hydration forces. The core of the nucleus becomes crystalline over time and grows by coalescence of the amorphous phase deposited at the surface. Imperfections of ion packing during coalescence promote growth of two conjoint crystallites. A parameter of order and calculated cohesive energies reflect the increasing crystalline order and stress relief at the grain boundary. Cluster aggregation plays a major role both in the formation of the nucleus and in the early stages of postnucleation growth. The mechanism identified shares common features with nucleation of solids from the melt and of liquid droplets from the vapor.

A computational study of the mechanism of the selective crystallization of α- and β-glycine from water and methanol-water mixture

Understanding the control of polymorphism in organic crystals is of paramount importance to the pharmaceutical, chemical, and food industries. In this work, we investigated two mechanisms described in the literature about the selective crystallization of α- and β-glycine from water and from mixtures of water and methanol using molecular simulations. The link hypothesis (J. Phys. Chem. B 2008, 112, 7794; Cryst. Growth Des. 2006, 6, 1788; J. Inclusion Phenom. Mol. Recognit. Chem. 1990, 8, 395; J. Am. Chem. Soc. 1986, 108, 5871.), which tries to relate the structure of the polymorph obtained from crystallization to the structure of the prenucleation aggregates in the solutions, says the abundance of glycine cyclic dimers in aqueous solutions leads to the crystallization of α-glycine, the polymorph using cyclic dimers as the packing units. This hypothesis was studied first. We revisited the self-assembly of glycine molecules in solution using molecular dynamics to address the debate (Phys. Rev. Lett. 2007, 99, 115702; J. Phys. Chem. B 2008, 112, 7280; J. Am. Chem. Soc. 2008, 130, 13973.) about which is the dominating species in the glycine aqueous solutions and whether there is a link between the solution chemistry and the polymorphic outcome of crystallization. The structures of the glycine clusters were characterized using a structural parameter called cyclic dimer fraction. The glycine clusters in methanol-water mixtures have higher cyclic dimer compositions than those in the pure aqueous solutions. Moreover, the glycine open-chain dimer is more stable than the cyclic dimer regardless of the presence of methanol. All these suggest that the link hypothesis does not work for the polymorphic system of glycine, and the selective crystallization of α- and β-glycine from water and methanol-water mixture, respectively, is not due to the abundance of glycine aggregates in the solution phase with a similar structure to the crystallizing solid form. The hypothesis of the methanol inhibition on the growth of α-glycine {010} and {010} faces, proposed by Weissbuch (Angew. Chem., Int. Ed. 2005, 44, 3226.), was also studied. The interfaces between the {010} and {010} faces of both crystal forms (α and β) and both solvents (water and methanol-water 3:7 mixture) were studied using molecular simulation. No strong binding of methanol onto the {010} and {010} faces of both crystal forms was observed, and the addition of methanol dilutes the crystal-solvent interactions on all faces. Therefore, the selective crystallization of β and α-glycine with and without methanol does not follow either of the two mechanisms in the literature.