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The effect of pressure on organic molecular crystals, in particular, on amino acid crystals, has been extensively studied in recent years. These studies are important for molecular materials science and pharmaceutical industry, in the context of geo-and cosmochemical problems, and for studying polypeptide (protein) folding (modification of the secondary and tertiary structure) [1].The search for new high-pressure polymorphs is of special interest. Pressure-induced phase transitions have been described for two polymorphs of glycine [2 − 8], which is not chiral, and for several L -amino acids, including, quite recently, L -serine [6][7][8][9][10]. To the best of our knowledge, comparison of the effects of pressure on the crystals of L -amino acids and corresponding racemates ( DL ) has not been carried out.In this work, we compared, for the first time, the effect of pressure on the vibrational spectra of L -serine and DL -serine and considered the factors that are responsible for the stability of one phase in DL -serine up to pressures of about 8 GPa and for two phase transitions in L -serine (at about 5 and 8 GPa).Serine C 3 H 7 NO 3 is one of the simplest optically active amino acids, which is prone to form clusters of molecules of the same chirality even in racemic mixtures. It is suggested that this property of serine is responsible for its specific role in life's origin [11]. DLserine crystals contain two-dimensional layers of molecules of the same chirality ( L or D ) [12], which renders comparison of the properties of L -and DL -serine crystals especially interesting.The experiments, at hydrostatic pressures up to 8.7 GPa, were carried out in diamond anvil cells (DACs) of the Merrill-Bassett type. A gasket was made of 70-µ m-thick steel foil with a 220-µ m hole. An ethanol-methanol mixture (1 : 4) was used as the pressuretransmitting medium. The pressure was estimated from the shift of the ruby luminescence line with an accuracy of ± 0.03 GPa at pressures under 1 GPa and with an accuracy of ± 0.05 GPa at higher pressures up to 8 GPa. Raman spectra were recorded on a Dilor OMARS 89 spectrophotometer equipped with an LN/CCD1100 PB multichannel detector (Princeton Ins.) and a holographic notch filter. Crystals were observed and photographed with a Carl Zeiss polarization microscope. The experimental technique was described in more detail in [13]. L -serine single crystals (needles up to ~0.4 × 0.4 × 2 mm 3 in size) were obtained by recrystallization of a commercially available reagent (ICN Biomedicals) from a water-alcohol mixture. Prismatic single crystals of DL -serine (up to ~0.3 × 0.2 × 1 mm 3 ) were grown by evaporation of an aqueous solution of a commercially available reagent (ICN Biomedicals) at room temperature. For DAC measurements, split-off crystalline blocks up to ~100 µ m in size were used. Raman spectroscopy and optical microscopy clearly showed a reversible phase transition with hysteresis in L -serine at 5.3 GPa on increasing pressure and at 4.0 GPa on decreasing pressure. The phase transition ...

The anisotropy of lattice strain in the crystals of L-serine (

SummaryThe anisotropy of lattice strain in the crystals of DL-serine (

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Protocyanin is a complex pigment extracted from flower petals of blue cornflower, Centaurea cyanus. The components of protocyanin were recently demonstrated to be anthocyanin (AN), flavone glycoside (FL), Fe 3+ , Mg 2+ and Ca 2+ ions [1]. For X-ray structure determination, protocyanin was reconstructed from the components and crystallized in space group P2 1 2 1 2 1 with unit cell dimensions of a = 29.7, b = 49.2 and c = 78.3Å. Two protocyanin molecules are contained in an asymmetric unit. Data were collected on the beam line 6A at Photon Factory KEK to1.05 Å resolution. The refined molecule has pseudo threefold symmetry and four metals align along the pseudo threefold axis in order of Ca 2+ , Fe 3+ , Mg 2+ and Ca 2+. The four metals are coordinated to six AN and six FL molecules. The inner Fe 3+ and Mg 2+ ions are each coordinated to three AN's, respectively, while the outer two Ca 2+ ions are each coordinated to three FL's. Both AN and FL molecules are self-associated with each other as AN-AN and FL-FL in pair and this hydrophobic association also exists between AN and FL molecules, building copigmentation stacks. Protocyanin is a tetra-metal (Fe 3+ , Mg 2+ , 2Ca 2+) nuclear complex, a new type of supramolecular pigment.

The crystal structures of the a-polymorph of glycylglycine (glygly) and of its hydrate (glygly  1.5 H 2 O) 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 H 2 O). 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 H 2 O. The contributions of the distortion of the intermolecular hydrogen bonds and of the conformational changes of the zwitterions to the lattice strain are considered.