Structural systematics is the comparison of sets of chemically related crystal structures with the aim to establish and describe relevant similarities and relationships. An important topic in this context is the comparison of hydrogen-bonded structures (HBSs) and their representation by suitable descriptors. Three different description methods for HBSs are proposed, a graphical representation, a symbolic representation and connectivity tables. The most comprehensive description is provided by a modified graph of the underlying net topology of an HBS which contains information on the multiplicity of links, the directionality and chemical connectivity of hydrogen bonds and on symmetry relations. By contrast, the alternative symbolic representation is restricted to essential properties of an HBS, i.e. its dimensionality, topology type and selected connectivity characteristics of nodes. A comparison of their connectivity tables readily identifies differences and similarities between crystal structures with respect to the intermolecular interaction modes adopted by their functional groups. The application of these methods to the known polymorphs of sulfathiazole and sulfapyridine is demonstrated and it is shown that they enable the rationalisation of previously reported and intricate relationships. The proposed methods facilitate the comprehensive description of the most important relevant aspects of an HBS, including its chemical connectivity, net topology and symmetry characteristics, and they represent a new way to recognise similarities and relationships in organic crystal structures.

Mycosporine-like amino acids (MAAs) are water-soluble metabolites, reported to exhibit strong UV-absorbing properties. They have been found in a wide range of marine organisms, especially those that are exposed to extreme levels of sunlight, to protect them against solar radiation. In the present study, the absolute configuration of 14 mycosporine-like-amino acids was determined by combining the results of electronic circular dichroism (ECD) experiments and that of advanced Marfey’s method using LC-MS. The crystal structure of a shinorine hydrate was determined from single crystal X-ray diffraction data and its absolute configuration was established from anomalous-dispersion effects. Furthermore, the anti-aging and wound-healing properties of these metabolites were evaluated in three different assays namely the inhibition of collagenase, inhibition of advanced glycation end products (AGEs) and wound healing assay (scratch assay).

Four polymorphic forms of methyl paraben (methyl 4-hydroxybenzoate, 1), denoted 1-I (melting point 126 °C), 1-III (109 °C), 1-107 (107 °C), and 1-112 (112 °C), have been investigated by thermomicroscopy, infrared spectroscopy, and X-ray crystallography. The crystal structures of the metastable forms 1-III, 1-107, and 1-112 have been determined. All polymorphs contain the same O–H···O═C connected catemer motif, but the geometry of the resulting H-bonded chain is different in each form. The Z′ = 3 structure of 1-I (stable form; space group Cc) contains local symmetry elements. The crystal packing of each of the four known crystal structures of 1 was compared with the crystal structures of 12 chemical analogues. Close two-dimensional relationships exist between 1-112 and a form of methyl 4-aminobenzoate and between 1-107 and dimethyl terephthalate. The lattice energies of the four methyl paraben structures have been calculated with a range of methods based on ab initio electronic calculations on either the crystal or single molecule. This shows that the differences in the induction energy of the different hydrogen-bonded chain geometries have a significant effect on relative lattice energies, but that conformational energy, repulsion, dispersion, and electrostatic also contribute.

An isomeric series of six linear, dinuclear, gold(I) complexes (AuCl)₂{P,P-C₆H₄N(CH₂PPh₂)₂(CO₂H)(OH)} 6a–6f have been synthesised in high yields (>90%) from AuCl(tht) (tht = tetrahydrothiophene) and the appropriate building blocks C₆H₄N(CH₂PPh₂)₂(CO₂H)(OH) 3a–3f in CH₂Cl₂. The diphosphine ligands C₆H₄N(CH₂PPh₂)₂1, 4-CO₂HC₆H₄N(CH₂PPh₂)₂2 and their corresponding digold(I) complexes 4 and 5 have also been prepared. Solution NMR (³¹P{¹H}, ¹H), FT-IR and microanalytical data are in full agreement with the proposed structures. Single crystal X-ray studies confirm that in each case, compounds 4, 5, 6a–6e are linear, with typical Au–P [2.227(4)–2.2349(17) Å]/Au–Cl [2.271(2)–2.325(4) Å] bond lengths and P–Au–Cl [169.37(1)–179.05(4)°] bond angles. Furthermore these studies demonstrate the relationship between regioselectivity, with respect to the –N(CH₂PPh₂)₂, –CO₂H and –OH functionalities around the benzene core, and solid state structures of the gold(I) complexes 6a–6e. In 6a, molecules are associated into pairs through classical intermolecular O–H⋯O hydrogen bonding, whilst in 6b and 6c, dimer pairs are formed using intermolecular O–H⋯Cl hydrogen bonds. 1-D chains of alternate 20-/22- or 8-/18-membered hydrogen bonded rings are propagated using O–H⋯Cl and/or O–H⋯O hydrogen bonds in compounds 6d and 6e. The Au⋯Au separation decreases from 6.179(3) Å (for 6a) to 3.0722(9) Å (for 6e) suggesting that aurophilicity increases within this series. The X-ray structure of a representative diphosphine 3a is also presented.