Volume 231, Issue 11

Pr 3 Rh 4 Ge 4 was synthesized by melting of the elements in an arc-melting furnace followed by annealing in a sealed tantalum ampoule in a muffle furnace. Structure refinement was based on temperature dependent single-crystal X-ray diffractometer data. At ambient temperature Pr 3 Rh 4 Ge 4 adopts the U 3 Ni 4 Si 4 type structure with strongly enhanced anisotropic displacement parameters for Rh1. Below 230 K additional reflections start to appear and at 110 K the structure could be described with the (3+1)D superspace group I 2/ m (α0γ)00; α=1/2, γ=1/2; ( Z =2). This commensurately modulated structure could be refined with 1448 F 2 values, 39 variables and residuals of wR =0.0417 for the main reflections and wR =0.1520 for the satellites of 1 st order, [ a =408.36(2), b =421.12(3) and c =2504.4(2) pm]. The commensurate description could be transformed to a 3D supercell with space group A 2/ m and Z =4: a =816.72(2), b =421.12(3), c =2537.5(1) pm, β =99.26(1)°, 1448 F 2 values, 69 variables and wR =0.0499. The relation of the U 3 Ni 4 Si 4 type structure, the (3+1)D modulated and the 3D supercell is discussed on the basis of a group–subgroup scheme. Temperature dependent magnetic susceptibility data reveal Curie–Weiss paramagnetism with an experimental moment of 3.72(2) μ B /Pr atom and a Weiss constant of –12.6(5) K. No magnetic ordering is evident down to 3 K.

A conformational polymorph, form β, for Ph 3 PAu[SC(OEt)=NPh] has been characterised. Like the original structure (form α), the molecule features a linear P–Au–S geometry. The difference between molecular structures rests with the relative disposition of the thiolate ligand which places the N-bound aryl ring in close proximity to the gold centre (form β) rather than the oxygen atom (form α). Density functional theory calculations show the molecule with the Au···π interaction is more stable by 5.2 kcal/mol than the one with the Au···O contact. The molecular packing of both forms are stabilised by C–H···O and C–H···π interactions which make approximately the same contribution to the overall Hirshfeld surfaces. However, key indicators, e.g. crystal packing efficiency and density, and the computational results suggest form β is the thermodynamically favoured form.

The crystal structure analysis of the biologically-relevant title compound ( 1 ) shows the carbonyl-O2 and amide-H atoms to be anti, and perpendicular relationships between the carbamate residue and the pyridyl ring [dihedral angle=84.60(10)°] and between the carbamate and aryl ring [74.84(11)°]; the rings are approximately co-planar [12.07(17)°]. An intramolecular hydroxyl-O–H···N(pyridyl) hydrogen bond that closes a S(7) loop is noted. Of interest is the observation that this hydrogen bond is not found in the structure of the pyrimidinyl analogue ( 2 ) which was characterised as a monohydrate, i.e. 2 ·H 2 O, in an earlier study. Density-functional theory calculations show the observed conformation in 1 is 2.0 kcal/mol more stable than the conformation where the intramolecular hydrogen bond is absent. This energy difference reduces to ca 0.5 kcal/mol in the case of 2 . The differences in molecular conformations found for 1 and 2 are therefore ascribed to the dictates of overall molecular packing, in particular due to the influence of lattice water in 2 ·H 2 O.

Biominerals have attracted multidiscipline interest (e.g. in material, medical and bio-geosciences) due to their unique organic–inorganic microtexture. In this work, electron backscatter diffraction (EBSD) and topological analysis of pole figures show that aragonite nacreous layers of Pinctada martensii (marine bivalve) shells have a mesocrystalline organization, which consists of at least a two-level domain structure. Primary domains are related by twin laws. The twinning between the primary domains is described as “mesotwinning.” Each domain is twinned by (110) planes. Secondary domains inside the primary domains are submicrometre units, ranging from several to hundreds of microns in dimension. The secondary domains are separated by low-angle grain boundaries. Angles between primary domains are ca. 63.5° or 52.5°; angles between secondary domains range from 5° to 12°. The twin relationship is quantified and twin boundary patterns are discussed.

Since it has been observed that low temperature protein crystal structures may differ from room temperature structures, it is necessary to compare systematically the protein hydration structure in low and room protein crystal structures. High quality data sets of protein structures were built in an extremely rigorous manner and crystal symmetry was included in the identification of four types of water molecules (buried in the protein core, deeply inserted into crevices at the protein surface, first and second hydration layers). More water molecules are observed at low temperature only if the resolution is better than 2.1–2.3 Å. At worse resolution, temperature does not play any role. The numerous water molecules that become detectable at low temperature and at higher resolution are more mobile, relative to the protein average flexibility. Despite that, the occupancy does not depend on temperature. It can be hypothesized that water structure and around proteins and hydrogen bond network do not depend on the temperature, at least in the temperature range examined here. At low temperature more water molecules are detected because the average flexibility of all the atoms decreases, so that also water molecules that are considerably more mobile than the average atoms become observable in the electron density maps.