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

Triglycine (GGG) Adopts a Polyproline II (pPII) Conformation in Its Hydrated Crystal Form: Revealing the Role of Water in Peptide Crystallization

Polyproline II (pPII) is a left-handed 31-helix conformation, which has been observed to be the most abundant secondary structure in unfolded peptides and proteins compared to α-helix and β-sheet. Although pPII has been reported as the most stable conformation for several unfolded short chain peptides in aqueous solution, it is rarely observed in their solid state. Here, we show for the first time a glycine homopeptide (gly-gly-gly) adopting the pPII conformation in its crystalline dihydrate structure. The single crystal X-ray structure with molecular dynamic simulation suggests that a network of water and the charged carboxylate group is critical in stabilizing the pPII conformation in solid state, offering an insight into the structures of unfolded regions of proteins and the role of water in peptide crystallization.

Phase transition in an organic ferroelectric: glycinium phosphite, with and without X-ray radiation damage

Thermal evolution of an organic ferroelectric, namely, glycinium phosphite, was probed by multi-temperature single-crystal diffraction using synchrotron radiation and also by a similar experiment with a laboratory X-ray diffractometer. Both series of measurements showed a transition from the paraelectric to the ferroelectric state at nearly the same temperature, T_c = 225 K. Temperature evolution of the unit-cell parameters and volume are drastically different for the synchrotron and laboratory data. The latter case corresponds to previous reports and shows an expected contraction of the cell on cooling. The data collected with the synchrotron beam show an abnormal nonlinear increase in volume on cooling. Structure analysis shows that this volume increase is accompanied by a suppression of scattering at high angles and an apparent increase of the anisotropic displacement parameters for all atoms; we therefore link these effects to radiation damage accumulated during consecutive data collections. The effects of radiation on the formation of the polar structure of ferroelectric glycinium phosphite is discussed together with the advantages and drawbacks of synchrotron experimentation with fine temperature sampling.

Supramolecular interactions in the solid state

In the last few decades, supramolecular chemistry has been at the forefront of chemical research, with the aim of understanding chemistry beyond the covalent bond. Since the long-range periodicity in crystals is a product of the directionally specific short-range intermolecular interactions that are responsible for molecular assembly, analysis of crystalline solids provides a primary means to investigate intermolecular interactions and recognition phenomena. This article discusses some areas of contemporary research involving supramolecular interactions in the solid state. The topics covered are: (1) an overview and historical review of halogen bonding; (2) exploring non-ambient conditions to investigate intermolecular interactions in crystals; (3) the role of intermolecular interactions in morphotropy, being the link between isostructurality and polymorphism; (4) strategic realisation of kinetic coordination polymers by exploiting multi-interactive linker molecules. The discussion touches upon many of the prerequisites for controlled preparation and characterization of crystalline materials.

Sarcosine and betaine crystals upon cooling: structural motifs unstable at high pressure become stable at low temperatures

The crystal structures of N-methyl derivatives of the simplest amino acid glycine, namely sarcosine (C3H7NO2) and betaine (C5H11NO2), were studied upon cooling by single-crystal X-ray diffraction and single-crystal polarized Raman spectroscopy. The effects of decreasing temperature and increasing hydrostatic pressure on the crystal structures were compared. In particular, we have studied the behavior upon cooling of those structural motifs in the crystals, which are involved in structural rearrangement during pressure-induced phase transitions. In contrast to their high sensitivity to hydrostatic compression, the crystals of both sarcosine and betaine are stable to cooling down to 5 K. Similarly to most α-amino acids, the crystal structures of the two compounds are most rigid upon cooling in the direction of the main structural motif, namely head-to-tail chains (linked via the strongest N-H···O hydrogen bonds and dipole-dipole interactions in the case of sarcosine, or exclusively by dipole-dipole interactions in the case of betaine). The anisotropy of linear strain in betaine does not differ much upon cooling and on hydrostatic compression, whereas this is not the case for sarcosine. Although the interactions between certain structural motifs in sarcosine and betaine weaken as a result of phase transitions induced by pressure, the same interactions strengthen when volume reduction results from cooling.

Can the thermal expansion be controlled by varying the hydrogen bond dimensionality in polymorphs?

A higher dimensional (1-D) hydrogen bonded form shows smaller thermal expansion than a lower dimensional (0-D) hydrogen bonded form of 2-butynoic acid.