CrystEngComm Blog

Drugs are often prepared in crystalline form.  However, the relative thermodynamic stability of crystalline compounds can limit water solubility and bioavailability in the body.  An alternative is to make the drugs in an amorphous form that is less stable and more soluble.  Unfortunately, this form tends to revert to the crystalline form during manufacture, processing or storage, which can limit its usefulness.   A new paper by Aiguo Zeng et al., aims to increase understanding of the crystallisation of amorphous drugs, using loratadine (shown below) as a model compound.

Loratadine is a widely-available, non-sedating anti-histamine, used to treat hay-fever and other allergies.  It has been on the market since 1993 and is included in the World Health Organisation’s Model List of Essential Medicines.

Four amorphous samples were prepared by quench-cooling – where molten compounds are rapidly cooled so they have insufficient time to arrange into a crystal lattice.  Study of the samples obtained by cooling at 298 K, 277 K, 253 K and 233 K showed no significant differences in the crystallisation mechanism with quenching temperature.

However, the tendency to crystallise increased with decreasing quench-cooling temperature.  Authors attribute this to the differences in molecular mobility and relaxation of the samples, especially the so-called Johari-Goldstein process of loratadine, involving motion of all atoms in the molecule. This finding will allow the preparation of  loratadine with a lower tendency to crystallise but which retains the beneficial properties of the amorphous form.   It will also inform studies of the amorphous form of other drug molecules.

For more details, read the full paper at:

Ruimiao Chang, Qiang Fu, Yong Li, Mingchan Wang, Wei Du, Chun Chang and Aiguo Zeng  

CrystEngComm, 2017, Advance Article
DOI: 10.1039/C6CE01645F, Paper
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Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors. She published a book, ‘Molecules, Medicines and Mischief’, in 2014, on some of the chemicals found in plants and is currently working on a follow-up.

Many potential applications of metal-organic frameworks (MOFs) rely on the size and nature of the available free volume or pores within the framework structure.  These include use for gas storage or capture and catalysis.   Tuning of the pores is typically achieved by variation of the metal ions or organic ligands.  Lengthening the organic chains can lead to increased pore size but is often limited by a decrease in stability of the framework. A new paper by Yan-Zhen Zheng and colleagues at Xi’an Jiaotong University and the University of Arizona reports a new method of structurally modifying MOFs by insertion of alkali metal ions.

In a series of experiments, a heterometallic MOF with O-containing ligands was modified by the insertion of alkali metal ions into the coordination environment formed by two bridged lanthanide centres.  Initially, 2D or 3D Cu-Pr MOFs were formed with bridging isonicotinate ligands, by reacting isonicotinic acid, CuI and Pr(NO₃)₃ in a range of organic solvents.  The reaction producing one of these MOFs, {[Pr₃(Cu₄I₄)3(ina)₉(DMF)₄](DMF)}n (where ina is isonicotinato and DMF is N,N-dimethylformamide), was then repeated with the addition of NaCl, KCl, RbCl  or CsCl.

In the NaCl and KCl reactions, new MOFs were produced incorporating Na⁺ or K⁺ ions, with void volumes of 53% and 61%, respectively (compared with 10% for the alkali metal ion free MOF).

Reaction involving the next largest ion, Rb⁺, produced an unstable MOF which could not be studied further.  However, reaction with the larger Cs⁺ ion produced a new MOF which didn’t contain this ion, but with a void volume of 59%.

The authors suggest that their method of inducing structural variation using appropriately-sized alkali metal ions should be extendable to other ligand systems for the production of a variety of MOFs with novel structures and functions.

For more information, read the full paper at:
An alkali-ion insertion approach to structurally transform metal–organic frameworks
Yue-Qiao Hu, Mu-Qing Li, Teng Li, Yan-Yan Wang, Zhiping Zheng and Yan-Zhen Zheng
CrystEngComm, 2016, Advance Article
DOI: 10.1039/C6CE00540C
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Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors. She published a book, ‘Molecules, Medicines and Mischief’, in 2014, on some of the chemicals found in plants and is currently working on a follow-up.

Nanolimes, alcoholic dispersions of colloidal Ca(OH)₂ nanoparticles, are commonly used for the conservation of porous materials such as stone and marble.  However, only the basics of the process they undergo – carbonation to produce CaCO₃ – are understood and this limits their potential use.

A new paper by Rodriguez-Navarro looks at the carbonation process in detail, with the aims of increasing the effectiveness of and understanding any limitations in the use of nanolimes in conservation.

Conservation of materials using nanolimes is typically carried out in humid air at room temperature.  Under these conditions, amorphous calcium carbonate (ACC) initially forms.  This can then transform into the metastable vaterite (up to 35 wt%) and a small amount of aragonite (up to 5%), but only in the presence of alcohol.  These polymorphs partially dissolve and the stable polymorph, calcite, precipitates.  Alternatively, calcite can form directly after dissolution of ACC.

Results of the kinetic studies show that the rate-limiting step in the production of calcite is the amount of unreacted Ca(OH)₂.  Although the formation of metastable states might be considered a limitation to the use of nanolimes in conservation, the fast kinetics of the vaterite to calcite conversion (72 % in 10 days) means that almost the full consolidation potential can be reached within weeks of application and it is only over very short time-scales that the performance might be sub-optimal.

These results may also have implications for the design of new CaCO₃ materials for other applications, using syntheses analogous to the multi-step crystallisation shown in the carbonation of nanolimes in the presence of alcohol.

For more information, read the full paper at:

Amorphous and crystalline calcium carbonate phases during carbonation of nanolimes: implications in heritage conservation

Carlos Rodriguez-Navarro, Kerstin Elert and Radek Ševčík

CrystEngComm, 2016, Advance Article
DOI: 10.1039/C6CE01202G, Paper

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Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors. She published a book, ‘Molecules, Medicines and Mischief’, in 2014, on some of the chemicals found in plants and is currently working on a follow-up.