C-11/C-9 Helices in Crystals of alpha beta Hybrid Peptides and Switching Structures between Helix Types by Variation in the alpha-Residue

Close-packed helices with mixed hydrogen bond directionality are unprecedented in the structural chemistry of alpha-polypeptides. While NMR studies in solution state provide strong evidence for the occurrence of mixed helices in (beta beta)(n) and (alpha beta)(n) sequences, limited information is currently available in crystals. The peptide structures presented show the occurrence of C-11/C-9 helices in (alpha beta)(n) peptides. Transitions between C-11 and C-11/C-9 helices are observed upon varying the alpha-amino acid residue.

Glass, Gel, and Liquid Crystals: Arrested States of Graphene Oxide Aqueous Dispersions

Colloidal systems with competing interactions are known to exhibit a range of dynamically arrested states because of the systems' inability to reach its underlying equilibrium state due to intrinsic frustration. Graphene oxide (GO) aqueous dispersions constitute a class of 2D-anisotropic colloids with competing interactions long-range electrostatic repulsion, originating from ionized groups located on the rim of the sheets, and weak dispersive attractive interactions originating from the unoxidized graphitic domains. We show here that aqueous dispersions of GO exhibit a range of arrested states, encompassing fluid, glass, and gels that coexist with liquid-crystalline order with increasing volume fraction. These states can be accessed by varying the relative magnitudes of the repulsive and attractive forces. This can be realized by changing the ionic strength of the medium. We observe at low salt concentrations, where long-range electrostatic repulsion dominates, the formation of a repulsive Wigner glass, while at high salt concentrations, when attractive forces dominate, the formation of gels exhibits a nematic to columnar liquid-crystalline transition. The present work highlights how the chemical structure of GO hydrophilic ionizable groups and hydrophobic graphitic domains coexisting on a single sheet gives rise to a rich and complex array of arrested states.

Finite element simulations of notch tip fields in magnesium single crystals

Recent experiments using three point bend specimens of Mg single crystals have revealed that tensile twins of {10 (1) over bar2}-type form profusely near a notch tip and enhance the fracture toughness through large plastic dissipation. In this work, 3D finite element simulations of these experiments are carried out using a crystal plasticity framework which includes slip and twinning to gain insights on the mechanics of fracture. The predicted load-displacement curves, slip and tensile twinning activities from finite element analysis corroborate well with the experimental observations. The numerical results are used to explore the 3D nature of the crack tip stress, plastic slip and twin volume fraction distributions near the notch root. The occurrence of tensile twinning is rationalized from the variation of normal stress ahead of the notch tip. Further, deflection of the crack path at twin-twin intersections observed in the experiments is examined from an energy standpoint by modeling discrete twins close to the notch root.

Studying Microstructure in Molecular Crystals With Nanoindentation: Intergrowth Polymorphism in Felodipine

Intergrowth polymorphism refers to the existence of distinct structural domains within a single crystal of a compound. The phenomenon is exhibited by form II of the active pharmaceutical ingredient felodipine, and the associated microstructure is a significant feature of the compound's structural identity. Employing the technique of nanoindentation on form II reveals a bimodal mechanical response on specific single-crystal faces, demonstrating distinct properties for two polymorphic forms within the same crystal.

Herringbone to cofacial solid state packing via H-bonding in diketopyrrolopyrrole (DPP) based molecular crystals: influence on charge transport

Themono-alkylation of DPP derivatives leads to cofacial pi-pi stacking via H-bonding unlike their di-alkylated counterparts, which exhibit a classical herringbone packing pattern. Single crystal organic field-effect transistor (OFET) measurements reveal a significant enhancement of charge carrier mobility for mono-hexyl DPP derivatives.

Tuning Mechanical Properties of Pharmaceutical Crystals with Multicomponent Crystals: Voriconazole as a Case Study

Crystals of voriconazole, an antifungal drug, are soft in nature, and this is disadvantageous during compaction studies where pressure is applied on the solid. Crystal engineering is used to make cocrystals and salts with modified mechanical properties (e.g., hardness). Cocrystals with biologically safe coformers such as fumaric acid, 4-hydroxybenzoic acid, and 4-aminobenzoic acid and salts with hydrochloric acid and oxalic acid are prepared through solvent assisted grinding. The presence (salt) or absence (cocrystal) of proton transfer in these multicomponent crystals is unambiguously confirmed with single crystal X-ray diffraction. All the cocrystals have 1:1 stoichiometry, whereas salts exhibit variable stoichiometries such as HCl salt (1:2) and oxalate salts (1:1.5 and 1:1). The nanoindentation technique was applied on single crystals of the salts and cocrystals. The salts exhibit better hardness than the drug and cocrystals in the order salts drug cocrystals. The molecular origin of this mechanical modulation is explained on the basis of slip planes in the crystal structure and relative orientations of the molecules with respect to the nanoindentation direction. The hydrochloride salt is the hardest solid in this family. This may be useful for tableting of the drug during formulation and in drug development.

Designing Elastic Organic Crystals: Highly Flexible Polyhalogenated N-Benzylideneanilines

The intermolecular interactions and structural features in crystals of seven halogenated N-benzylideneanilines (Schiff bases), all of which exhibit remarkable flexibility, were examined to identify the common packing features that are the raison d'etre for the observed elasticity. The following two features, in part related, were identified as essential to obtain elastic organic crystals: 1)A multitude of weak and dispersive interactions, including halogen bonds, which may act as structural buffers for deformation through easy rupture and reformation during bending; and 2)corrugated packing patterns that would get interlocked and, in the process, prevent long-range sliding of molecular planes.

Low temperature giant magnetocaloric effect in multiferroic GdMnO3 single crystals

Magnetocaloric (MC) properties of GdMnO3 single crystals are investigated using magnetic and magneto-thermal measurements. GdMnO3 exhibits a giant MC effect (isothermal change in magnetic entropy (-Delta S-M) similar to 31 J (kg K)(-1) at 7 K and adiabatic change in temperature similar to 10 K at 19 K for magnetic field variation 0-80 kOe). Complex interactions between 3d and 4f magnetic sublattices influence MC properties. The rare-earth antiferromagnetic ordering induces an inverse MC effect (positive Delta S-M) along `a' and `c' axes whereas it's not seen along the `b' axis, revealing complex anisotropic magnetic ordering. The antiferromagnetic ordering possibly changes to ferromagnetic ordering at higher fields.

Temperature Dependence of Mechanical Properties in Molecular Crystals

Quantitative evaluation of the mechanical behavior of molecular materials by a nanoindentation technique has gained prominence recently. However, all the reported data have been on room-temperature properties despite many interesting phenomena observed in them with variations in temperature. In this paper, we report the results of nanoindentation experiments conducted as a function of temperature, T, between 283 and 343 K, on the major faces of three organic crystals: saccharin, sulfathiazole (form 2), and L-alanine, which are distinct in terms of the number and strength of intermolecular interactions in them. Results show that elastic modulus, E, and hardness, H, decrease markedly with increasing T. While E decreases linearly with T, the variations in H with T are not so, and were observed to drop by similar to 50% over the range of T investigated. The slope of the linear fits to E vs T for the organic crystals was found to be around 1, which is considerably higher than the values of 0.3-0.5 reported in the literature for metallic, ionic, and covalently bonded crystalline materials. Possible implications of the observed remarkable changes in H for pharmaceutical manufacturing are highlighted.

Investigations on growth, structure, optical properties and laser damage threshold of organic nonlinear optical crystals of Guanidinium L-Ascorbate

Single crystals of Guanidinium L-Ascorbate (GuLA) were grown and crystal structure was determined by direct methods. GuLA crystallizes in orthorhombic, non-centrosymmetric space group P2(1)2(1)2(1). The UV-cutoff was determined as 325 nm. The morphology was generated and the interplanar angles estimated and compared with experimental values. Second harmonic generation conversion efficiency was measured and compared with other salts of L-Ascorbic acid. Surface laser damage threshold was calculated as 11.3GW/cm(2) for a single shot of laser of 1064 nm wavelength.

Do carboximide-carboxylic acid combinations form co-crystals? The role of hydroxyl substitution on the formation of co-crystals and eutectics

Carboxylic acids, amides and imides are key organic systems which provide understanding of molecular recognition and binding phenomena important in biological and pharmaceutical settings. In this context, studies of their mutual interactions and compatibility through co-crystallization may pave the way for greater understanding and new applications of their combinations. Extensive co-crystallization studies are available for carboxylic acid/amide combinations, but only a few examples of carboxylic acid/imide co-crystals are currently observed in the literature. The non-formation of co-crystals for carboxylic acid/imide combinations has previously been rationalized, based on steric and computed stability factors. In the light of the growing awareness of eutectic mixtures as an alternative outcome in co-crystallization experiments, the nature of various benzoic acid/cyclic imide combinations is established in this paper. Since an additional functional group can provide sites for new intermolecular interactions and, potentially, promote supramolecular growth into a co-crystal, benzoic acids decorated with one or more hydroxyl groups have been systematically screened for co-crystallization with one unsaturated and two saturated cyclic imides. The facile formation of an abundant number of hydroxybenzoic acid/cyclic carboximide co-crystals is reported, including polymorphic and variable stoichiometry co-crystals. In the cases where co-crystals did not form, the combinations are shown invariably to result in eutectics. The presence or absence and geometric disposition of hydroxyl functionality on benzoic acid is thus found to drive the formation of co- crystals or eutectics for the studied carboxylic acid/imide combinations.

Anisotropy in the mechanical properties of organic crystals: temperature dependence

The nanoindentation technique has recently been utilized for quantitative evaluation of the mechanical properties of molecular materials successfully, including their temperature (T) dependence. In this paper, we examine how the mechanical anisotropy varies with T in saccharin and L-alanine single crystals. Our results show that elastic modulus (E) decreases linearly in all the cases examined, with the T-dependence of E being anisotropic. Correspondence between directional dependence of the slopes of the E vs. T plots and the linear thermal expansion coefficients was found. The T-dependence of hardness (H), on the other hand, was found to be nonlinear and significant when (100) of saccharin and (001) of L-alanine are indented. While the anisotropies in E and H of saccharin and E of L-alanine enhance with T, the anisotropy in H of L-alanine was found to reduce with T. Possible mechanistic origins of these variations are discussed.

Structure-mechanical property correlations in mechanochromic luminescent crystals of boron difluoride dibenzoylmethane derivatives

The structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF(2)dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF(2)dbm(Bu-t)(2) can be bent permanently, whereas those of BF(2)dbm(OMe)(2) exhibit an inhomogeneous shearing mode of deformation, and finally BF(2)dbmOMe crystals are brittle. Quantitative mechanical analysis by nanoindentation on the major facets of the crystals shows that BF(2)dbm(Bu-t)(2) is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF(2)dbm(OMe)(2) and BF(2)dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF(2)dbm(Bu-t)(2), whereas BF(2)dbm(OMe)(2) exhibits only a moderate ML and BF(2)dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles.

Microhardness Studies of Vapour Grown Tin (II) Sulfide Single Crystals

Earth abundant tin sulfide (SnS) has attracted considerable attention as a possible absorber material for low-cost solar cells due to its favourable optoelectronic properties. Single crystals of SnS were grown by physical vapour deposition (PVD) technique. Microindentation studies were carried out on the cleaved surfaces of the crystals to understand their mechanical behaviour. Microhardness increased initially with the load, giving sharp maximum at 15 g. Quenching effect has increased the microhardness, while annealing reduced the microhardness of grown crystals. The hardness values of as-grown, annealed and quenched samples at 15 g load are computed to be 99.69, 44.52 and 106.29 kg/mm(2) respectively. The microhardness of PVD grown crystals are high compared to CdTe, a leading low-cost PV material. The as-grown faces are found to be fracture resistant.