2,6-Bis(2,4-dimethyl­benzyl­idene)cyclo­hexa­none

In the crystal structure of the title compound, C24H6O, the mol­ecule exhibits point symmetry m but the mirror plane is not utilized as part of the space-group symmetry. The structure contains face-to-face inter­actions between the 2,4-dimethyl­benzyl­idene substituents in which the methyl groups lie directly above the centroids of adjacent benzene rings

Intermolecular interactions and unexpected isostructurality in the crystal structures of the dichlorobenzaldehyde isomers

The crystal structures of the six dichlorobenzaldehyde isomers, four of them newly determined, are analyzed in terms of the geometry and energies of their intermolecular interactions, quantified using the semi-classical density sums (SCDS-PIXEL) method. A consistent feature in all six structures is molecular stacks propagating along a short crystallographic axis of ca 3.8 Å. The stacks have a closely comparable geometry in each isomer, but the interaction energies between stacked molecules are variable on account of the differing relative positions of the Cl substituents. In the majority of the isomers the stacking interactions are the most stabilizing in the structure. Exceptions are the 2,4- and 3,5-isomers, where more stabilizing interactions are made between stacks. In general, the most stabilizing non-stacking intermolecular interactions in the structures are those involving C—H...O contacts. Observed motifs based on Cl...Cl interactions appear to be largely imposed by the constraints of other more stabilizing intermolecular interactions. The isomeric series displays the following noteworthy features: (i) the 2,3- and 2,6-isomers are isostructural despite having different orientations of the Cl and aldehyde functionalities; (ii) the 2,5-isomer exhibits whole-molecule disorder; (iii) the 2,5- and 3,5-isomers have more than one molecule in the crystallographic asymmetric unit (Z′ &gt; 1). These features in particular are considered on the basis of the intermolecular interaction energies.</jats:p

How cholesterol interacts with proteins and lipids during its intracellular transport

AbstractSterols, as cholesterol in mammalian cells and ergosterol in fungi, are indispensable molecules for proper functioning and nanoscale organization of the plasma membrane. Synthesis, uptake and efflux of cholesterol are regulated by a variety of protein–lipid and protein–protein interactions. Similarly, membrane lipids and their physico-chemical properties directly affect cholesterol partitioning and thereby contribute to the highly heterogeneous intracellular cholesterol distribution. Movement of cholesterol in cells is mediated by vesicle trafficking along the endocytic and secretory pathways as well as by non-vesicular sterol exchange between organelles. In this article, we will review recent progress in elucidating sterol–lipid and sterol–protein interactions contributing to proper sterol transport in living cells. We outline recent biophysical models of cholesterol distribution and dynamics in membranes and explain how such models are related to sterol flux between organelles. An overview of various sterol-transfer proteins is given, and the physico-chemical principles of their function in non-vesicular sterol transport are explained. We also discuss selected experimental approaches for characterization of sterol–protein interactions and for monitoring intracellular sterol transport. Finally, we review recent work on the molecular mechanisms underlying lipoprotein-mediated cholesterol import into mammalian cells and describe the process of cellular cholesterol efflux. Overall, we emphasize how specific protein–lipid and protein–protein interactions help overcoming the extremely low water solubility of cholesterol, thereby controlling intracellular cholesterol movement. This article is part of a Special Issue entitled: Lipid–protein interactions

2-[(2-Acetoxybenzoyl)oxy]benzoic acid

The title compound, C16H12O6, is a common impurity of ortho-acetylsalicylic acid (aspirin). The benzene rings form a dihedral angle of 81.9&amp;#8197;(1)&amp;#176; while the acetyl and carboxyl groups form dihedral angles of 74.0&amp;#8197;(1) and 26.4&amp;#8197;(2)&amp;#176;, respectively, with the benzene rings to which they are bound. In the crystal, molecules are linked by pairs of O&amp;#8212;H...O hydrogen bonds between the carboxyl groups, forming inversion dimers

Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation

Nanoindentation is applied to the two polymorphs of aspirin to examine and differentiate their interaction anisotropy and shear instability. Aspirin provides an excellent test system for the technique because: (i) polymorphs I and II exhibit structural similarity in two dimensions, thereby facilitating clear examination of the differences in mechanical response in relation to well-defined differences between the two crystal structures; (ii) single crystals of the metastable polymorph II have only recently become accessible; (iii) shear instability has been proposed for II. Different elastic moduli and hardness values determined for the two polymorphs are correlated with their crystal structures, and the interpretation is supported by measured thermal expansion coefficients. The stress-induced transformation of the metastable polymorph II to the stable polymorph I can be brought about rapidly by mechanical milling, and proceeds via a slip mechanism. This work establishes that nanoindentation provides "signature" responses for the two aspirin polymorphs, despite their very similar crystal structures. It also demonstrates the value of the technique to quantify stability relationships and phase transformations in molecular crystals, enabling a deeper understanding of polymorphism in the context of crystal engineering