Crystal structure and Hirshfeld surface analysis of a third polymorph of 2,6-dimethoxybenzoic acid

A third crystalline form of the title compound, C9H10O4, crystallizing in the centrosymmetric monoclinic space group P21/c, has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent molecule with a synplanar conformation of the OH group. The sterically bulky o-methoxy substituents force the carboxy group to be twisted away from the plane of the benzene ring by 74.10 (6)°. The carboxy group exhibits the acidic H atom disordered over two sites between two O atoms. A similar situation has been found for the second tetragonal polymorph reported [Portalone (2011). Acta Cryst. E67, o3394–o3395], in which molecules with the OH group in a synplanar conformation form dimeric units via strong O—H...O hydrogen bonds. In contrast, in the first orthorhombic form reported [Swaminathan et al. (1976). Acta Cryst. B32, 1897–1900; Bryan & White (1982). Acta Cryst. B38, 1014–1016; Portalone (2009). Acta Cryst. E65, o327–o328], the molecular components do not form conventional dimeric units, as an antiplanar conformation adopted by the OH group favors the association of molecules in chains stabilized by linear O—H...O hydrogen bonds.

How do extracellular membrane-associated proteins stand erect and signal?

Even after decades of research, a comprehensive mechanism that elucidates the underpinnings of signalling through the cell membrane is still elusive. Here, we address a simple question “how do the ectodomain of a membrane-associated protein consisting of multiple domains and connected by flexible linkers stand ‘upright’ on the membrane?”. Our analysis based on large amount of available functional and structural data, looking for a pattern of association of these molecules in the crystal structures and with the concept that ‘random things seldom repeat’ lead to a surprisingly interesting and consistent observation that the weak cis interaction mediated ordered array of constitutively expressed signalling molecules not only support their ‘upright’ orientation but also bury their ligand-binding surface to avoid spurious signaling in the non-signalling resting state. With CD4, pMHCII, CD2 and TNFR1 as examples, we show that the observed association of molecules also correlate well with their functional role. Further, our model reconciles the long-standing controversies related to these molecules and appear to be generic enough to be applied to other signalling molecules

The equivalence of quasi-chemical and statistical description of adsorbate molecule association in a localised adsorption monolayer on a homogeneous solid surface

The analysis of Berezin and Kiselev’s concept assuming linear association of molecules in a localised adsorption monolayer on the homogeneous surface of a solid adsorbent was carried out. An inconsistency due to the lack of consideration of equilibrium concentration of free adsorption sites in the formulation of the expression for the association constant has been pointed out. It was shown that the correct form of this function leads to the final adsorption equation being identical to the specific case of the Fowler-Guggenheim equation. The obtained result has been generalised to cases of branched adsorbateadsorbate associations. A new adsorption equation limiting the association to at most the dimers has been introduced. Critical conditions for the two-dimensional condensation of the adsorption layer were determined. By applying the formalism of canonical ensemble, full equivalence of the phenomenological and statistical methods was demonstrated in the description of the intermolecular interactions in the localised adsorption monolayer.