T. N. Drebushchak

Polymorphism of glycine, Part II

The contribution summarizes the results of a systematic study of the three glycine polymorphs (a, b, g-forms), including: i) the controlled crystallization of a desirable form, ii) a comparative calorimetric study of the three forms in the temperature range between 5 K and the sublimation temperatures (»500 K).

A comparative study of the anisotropy of lattice strain induced in the crystals of L-serine by cooling down to 100 K or by increasing pressure up to 4.4 GPa

Abstract The anisotropy of lattice strain in the crystals of L-serine (P212121, at ambient conditions a = 5.615(1) Å, b = 8.589(2) Å, c = 9.346(2) Å) on cooling down to 100 K and with increasing hydrostatic pressure up to 4.4 GPa was compared with each other and also with the results previously obtained for the polymorphs of glycine. On cooling, the structure expanded slightly along the crystallographic a-direction, compression along the crystallographic b- and c-directions (normal to the chains of the serine zwitter-ions) was very similar. With increasing pressure, the same structure compressed in all the crystallographic directions, linear strain along c-axis was the largest, linear strain along a-axis — the smallest, linear compression along the b-axis with increasing pressure was slightly larger than that along the a-axis. The different anisotropy of lattice strain of the same structure on cooling and under pressure could be correlated with different response of intermolecular hydrogen bonds to these two scalar actions.

A comparative study of the anisotropy of lattice strain induced in the crystals of DL-serine by cooling down to 100 K, or by increasing pressure up to 8.6 GPa. A comparison with L-serine

Summary The anisotropy of lattice strain in the crystals of DL-serine (P21/n) on cooling down to 100 K and with increasing hydrostatic pressure up to 8.6 GPa was studied by single-crystal X-ray diffraction. In contrast to L-serine undergoing pressure-induced phase transitions at about 5 and 8 GPa, no phase transitions were observed in DL-serine at least up to 8.6 GPa (the highest pressure reached in the experiment). The anisotropy of strain in DL-serine on cooling was shown to be radically different from that with increasing pressure. The response of the crystal structure of DL-serine to cooling and to increasing pressure was considerably different from that of L-serine.

Variable temperature (100–360 K) single-crystal X-ray diffraction study of the orthorhombic polymorph of paracetamol (p-hydroxyacetanilide)

Abstract The crystal structure of the orthorhombic polymorph of paracetamol was refined in Pbca space group by single-crystal X-ray diffraction at 100, 200, 300, 360 K. Cell parameters were measured in the range between 100 and 360 K. The anisotropy of lattice strain on cooling was analysed and compared with that induced by increasing pressure. The data for the orthorhombic paracetamol were compared also with the results previously reported for the monoclinic form of the same compound. The bulk thermal expansion coefficient of the orthorhombic polymorph is larger than that for the monoclinic form. Maximum compression on cooling was observed in the direction normal to the molecular layers. The structure expanded slightly on cooling in the crystallographic a-direction. Relative compressibility of OH…O and NH…O intermolecular hydrogen bonds on cooling, as well as the changes in the intramolecular geometry of paracetamol molecules and the atomic displacement parameters were measured and analysed.

Structural distortion of the α, β, and γ polymorphs of glycine on cooling

Abstract The effect of cooling down to 150 K on the structures of α- (s.g. P21/n), β- (s.g. P21), and γ- (s.g. P31) polymorphs of glycine was studied by X-ray single-crystal diffraction. No polymorphic transformations were detected. Relative volume changes and the anisotropy of structural distortion of the three polymorphs of glycine were compared. Maximum contraction of structures was measured in the b-direction for the α- and β-polymorphs, and in the a-direction for the γ-polymorph. Minimum contraction in the γ-polymorph was measured in c-direction. The structures of the α- and the β-polymorphs slightly expanded on cooling in the directions close to c. The directions of maximum and minimum lattice strain were related to the directions of weak and strong hydrogen bonds in the structures. Crystal structures for the three polymorphs were refined at 294 K and at 150 K. The effects of cooling on the intramolecular geometry of glycine molecules and on the intermolecular hydrogen bonds in all the three polymorphs were compared. Structural distortion on cooling was compared with that on increasing hydrostatic pressure.

Phase Transitions in the Crystals of l- and dl-Cysteine on Cooling: Intermolecular Hydrogen Bonds Distortions and the Side-Chain Motions of Thiol-Groups. 1. l-Cysteine

The role of the distortion of the hydrogen bond network and of the motions of the -CH 2SH side chains in the phase transition in the orthorhombic L-cysteine ( (+)NH 3-CH(CH 2SH)-COO (-)) on cooling and the reverse transformation on heating is discussed. The extended character of the phase transition, which was recently discovered by adiabatic calorimetry [ J. Phys. Chem. B 2007, 111, 9186 ], and its very high sensitivity to the thermal prehistory of the sample could be interpreted based on the changes in the polarized Raman spectra measured for the single-crystals in several orientations in the temperature range 3-300 K and precise diffraction data on the changes in intramolecular conformations and intermolecular hydrogen bonding. In the low-temperature phase the SH...S hydrogen bonds dominate as compared to the weaker SH...O contacts, and at ambient temperature the situation is inverse. The transition from one phase to another goes via a series of states differing in conformations of the cysteine zwitterions and the intermolecular contacts of the thiol-group. Motions of different molecular fragments (NH 3 (+), CH 2, CH, SH) are activated at different temperatures. Structural strain on cooling involves several dynamic processes, such as a rigid rotation of the molecule in the lattice, a rigid rotation of the NH 3 group with respect to NH 3-CH bond, and the rotation of the thiol side chain resulting in the switching of S-H hydrogen bonding from one type to another. Different NH...O hydrogen bonds forming the framework in the L-cysteine crystal structure are distorted to a different extent, and this provokes the rotation of the -CH 2SH side chains within the cavities of this framework resulting in a change in the coordination from SH...O to SH...S at low temperatures. The results are interesting for understanding the polymorphism of molecular crystals and the factors determining their dynamics and structural instability, and also for biophysical chemistry, since the properties of the hydrogen bonded thiole-groups in biomolecules can be mimicked using L-cysteine in the crystalline state, variations in temperature and pressure serving as powerful tools, to modify the intramolecular conformations and the intermolecular hydrogen bonding.

The investigation of ancient pottery

SummaryAncient ceramic samples (single fragments and different parts of pots, unbroken and repaired; total about 180 samples) dated from the transitional period of late Bronze to early Iron Age (VIII-VI centuries BC) and early Iron Age (VII-IV centuries BC) were investigated by thermal analysis, X-ray powder diffraction, petrography, and scanning electron microscopy equipped with the energy-dispersive X-ray analyzer. In addition to that, to identify the clay sources for the ceramic manufacturing, about 15 samples of clays and soils found near archeological digs and taken from the mineralogical museum were investigated. We found out that the calcite content of ceramics is a very informative parameter for the identification of the clay source for the pottery manufactured at low technological level (low-temperature firing).

Pressure-induced phase transitions in organic molecular crystals: a combination of X-ray single-crystal and powder diffraction, Raman and IR-spectroscopy

The contribution summarizes the results of recent studies of phase transitions induced by high pressure in a number of molecular organic crystals, such as polymorphs of paracetamol, chlorpropamide, polymorphs of glycine, L- and DL-serine, β-alanine. The main attention is paid to the following topics: (1) Reversible / irreversible transformations; (2) Different behavior of single crystals / powders; (3) The role of pressure-transmitting liquid; (4) The role of the kinetic factors: phase transitions on decompression, or after a long storage at a selected pressure; (5) Isosymmetric phase transitions; (6) The role of the changes in the hydrogen bond networks / intramolecular conformational changes in the phase transitions; (7) Superstructures / nanostructures formed as a result of pressure-induced phase transitions.

Variable temperature (100—295 K) single-crystal X-ray diffraction study of the α-polymorph of glycylglycine and a glycylglycine hydrate

Summary The crystal structures of the α-polymorph of glycylglycine (glygly) and of its hydrate (glygly × 1.5 H2O) were refined by single-crystal X-ray diffraction at 100, 150, 220, and 295 K (glygly) and at 100, 200, 295 K (glygly × 1.5 H2O). The values of the volume thermal expansions of glygly and its hydrate were shown to be larger than for the three polymorphs of glycine. The anisotropy of strain on cooling was analyzed. Despite a smaller bulk thermal expansion measured for glygly, linear strain (both compression and expansion) along the axes of the strain ellipsoids was larger for the structure of glygly, than for the structure of glygly × 1.5 H2O. The contributions of the distortion of the intermolecular hydrogen bonds and of the conformational changes of the zwitter-ions to the lattice strain are considered.

Thermoanalytical investigation of drug-excipient interaction. Part I. Piroxicam, cellulose and chitosan as starting materials

SummaryCellulose, chitosan and piroxicam were investigated by TG and DSC at heating up to 215°C, and by X-ray powder diffraction before and after the heating. Dehydration of cellulose and chitosan comes to the end near 160°C. Thermal decomposition of chitosan starts at the final stage of its dehydration, and the mass losses after these two reactions overlap with one another. Enthalpy of dehydration is 47.1±2.4 kJ mol-1of water for cellulose and 46.2±2.0 kJ mol-1for chitosan. Thermal decomposition of chitosan is an exothermic process. Crystal structure of cellulose after heating remains unchanged, but that of chitosan contracts. Piroxicam melts at 200.7°C with the enthalpy of melting 35 kJ mol-1. Heat capacity of the liquid phase is greater than that of the solid phase by approximately 100 J mol-1 K-1. Cooled back to ambient temperature, piroxicam remains glassy for a long time, crystallizing slowly back into the starting polymorph.