l‐Cysteinium semioxalate

The title salt, C₃H₈NO₂⁺·C₂HO₄⁻, formed between l ‐cysteine and oxalic acid, was studied as part of a comparison of the structures and properties of pure amino acids and their cocrystals. The structure of the title salt is very different from that formed by oxalic acid and equivalent amounts of d ‐ and l ‐cysteine molecules. The asymmetric unit contains an l ‐cysteinium cation and a semioxalate anion. The oxalate anion is only singly deprotonated, in contrast with the double deprotonation in the crystal structure of bis(dl ‐cysteinium) oxalate. The oxalate anion is not planar. The conformation of the l ‐cysteinium cation differs from that of the neutral cysteine zwitterion in the monoclinic and orthorhombic polymorphs of l ‐cysteine, but is similar to that of the cysteinium cation in bis(dl ‐cysteinium) oxalate. The structure of the title salt can be described as a three‐dimensional framework formed by ions linked by strong O—H...O and N—H...O and weak S—H...O hydrogen bonds, with channels running along the crystallographic a axis containing the bulky –CH₂SH side chains of the cysteinium cations. The cations are only linked through hydrogen bonds via semioxalate anions. There are no direct cation–cation interactions via N—H...O hydrogen bonds between the ammonium and carboxylate groups, or via weaker S—H...S or S—H...O hydrogen bonds.     

dl‐Proline

In the structure of dl ‐proline, C₅H₉NO₂, the mol­ecules are connected via classical inter­molecular N—H⋯O hydrogen bonds involving the amine and carbox­yl groups [N⋯O = 2.7129 (15) and 2.8392 (16) Å], and form chains along the b‐axis direction and parallel to (01). The chains are linked into sheets via weak non‐classical hydrogen bonds. The conformation of the mol­ecule and its packing are notably different from the monohydrated dl ‐proline form.     

dl‐Alaninium semi‐oxalate monohydrate

The structure of the title compound, C₃H₈NO₂⁺·C₂HO₄⁻·H₂O, is formed by two chiral counterparts (l ‐ and d ‐alaninium cations), semi‐oxalate anions and water molecules, with a 1:1:1 cation–anion–water ratio. The structure is compared with that of the previously known anhydrous dl ‐alaninium semi‐oxalate [Subha Nandhini, Krishnakumar & Natarajan (2001). Acta Cryst. E 57 , o666–o668] in order to investigate the role of water molecules in the crystal packing. The structure of the hydrate resembles that of anhydrous alaninium semi‐oxalate, with the water molecule incorporated into the general three‐dimensional network of hydrogen bonds where it forms four hydrogen bonds with neighbours disposed tetrahedrally about it. Although the main structural motifs in the hydrate and in the anhydrous form are topologically similar, the incorporation of water molecules in the network results in significant geometric distortion. There are several types of hydrogen bond in the crystal structure of the hydrate, two of which (O—H...O bonds between the semi‐oxalate anions and O—H...O hydrogen bonds between water and alaninium cations) are very short. Such hydrogen bonds between semi‐oxalate anions are also present in the anhydrous form of this compound. Short distances between semi‐oxalate anions in neighbouring chains in the hydrate alternate with longer ones, whereas in the anhydrous structure they are equidistant. Despite the similarity of these compounds, dehydration of the hydrate on storage is not of a single‐crystal to single‐crystal type, but gives a polycrystalline pseudomorph, preserving the crystal habit. This transformation proceeds through the formation of an intermediate compound, presumably a hemihydrate.     

Anilinium mono­hydrogen dl‐malate

Crystals of the title salt, [(C₆H₅NH₃)]⁺·[(HOOC(CH₂)CH(OH)COO)]⁻ or C₆H₈N⁺·C₄H₅O₅⁻, are built up from protonated anilinium residues and monodissociated dl ‐malate ions. The NH₃⁺ group of the anilinium cation is ordered at room temperature. Rotation of the NH₃⁺ group along the C(aromatic)—Nsp³ bond (often observed at room temperature in other anilinium salts) is prevented by N—H⋯O hydrogen bonds between the NH₃⁺ group and the malate anions. The anions are connected by four O—H⋯O hydrogen bonds into two‐dimensional sheets parallel to the (001) plane. The charged moieties, i.e. the anilinium cations and the sheets of hydrogen‐bonded malate anions, form two‐dimensional layers in which the phenyl rings of the anilinium residues lie perpendicular to the malate‐ion sheets. The conformation of the monodissociated malate ion in the crystal is compared with that obtained from ab initio molecular‐orbital calculations.     

N‐Methyl‐dl‐aspartic acid mono­hydrate

The title compound, C₅H₉NO₄·H₂O, has been synthesized and crystallized. It crystallizes in Cc with one mol­ecule in the asymmetric unit. The compound is found in its zwitterionic form. d and l forms of the compound are linked in the crystal via O—H?O and N—H?O hydrogen bonds, both directly between the aspartic acid‐derivative entities and to the crystal water mol­ecule. A weak intramolecular N—H?O interaction is found. The carbon skeleton is slightly twisted with C—C—C—C = 166.83 (11)°. A comparison with other derivatives of aspartic acid shows only two rotamers – one with a near planar carbon skeleton and one with a significantly twisted carbon skeleton.     

Bis(dl‐cysteinium) oxalate

In the title compound, 2C₃H₈NO₂S⁺·C₂O₄²⁻, the oxalate anion occupies an inversion centre and is coordinated to cysteine molecules of different chirality (l and d ) via O—H...O and N—H...O hydrogen bonds, the resulting cysteine–oxalate stoichiometry in the crystal structure being 2:1. The oxalate anion is completely deprotonated, whereas cysteine has a positively charged –NH₃⁺ group and a neutral protonated carboxyl group. The structure is built from infinite hydrogen‐bonded triple layers, consisting of an oxalate layer in the middle with layers of l ‐ and d ‐cysteine molecules on either side. The thiol groups are at the external sides of the layers and form S—H...O hydrogen bonds with the carboxyl groups of neighbouring cysteine molecules. An interesting feature of the structure is the occurrence of short S...S contacts between SH groups of molecules in neighbouring layers, which form not S—H...S but S—H...O intermolecular hydrogen bonds. Due to the effects of crystal packing and intermolecular hydrogen‐bond formation, the conformation of the cysteine cation in the title structure is different from that calculated theoretically for an individual cation, as well as from those of cysteine zwitterions in crystals of pure cysteine.     

Bis­(melaminium) dl‐malate tetrahydrate

The crystal structure of the title melaminium salt, bis(2,4,6‐tri­amino‐1,3,5‐triazin‐1‐ium) dl ‐malate tetrahydrate, 2C₃H₇N₆⁺·C₄H₄O₅²⁻·4H₂O, consists of singly protonated melaminium residues, dl ‐malate dianions and water mol­ecules. The melaminium residues are connected into chains by four N—H⃛N hydrogen bonds, and these chains form a stacking structure along the c axis. The dl ‐malate dianions form hydrogen‐bonded chains and, together with hydrogen‐bonded water mol­ecules, form a layer parallel to the (100) plane. The conformation of the malate ion is compared with an ab initio molecular‐orbital calculation. The oppositely charged moieties, i.e. the stacks of melaminium chains and hydrogen‐bonded dl ‐malate anions and water mol­ecules, form a three‐dimensional polymeric structure, in which N—H⃛O hydrogen bonds stabilize the stacking.     

dl‐Arginine monohydrate at 100 K

In the title compound, C₆H₁₄N₄O₂·H₂O, the α‐amino group is neutral. The molecular side chain including the guanidinium group is not fully extended, having a near gauche–gauche conformation [χ³ = 59.0 (1)°; χ⁴ = 72.8 (1)°]. The network of hydrogen bonds stabilizing the crystal lattice includes those formed between the deprotonated and negatively charged α‐carboxyl­ate groups and the positively charged amino groups of the guanidinium group of neighbouring mol­ecules. N—H?O=C and water‐mediated N—H?O hydrogen bonds link individual mol­ecules to produce pairs of spiral motifs laterally connected by N—H?O and C—H?O hydrogen bonds.     

Racemic dipeptide glycyl‐dl‐leucine at 120 K

The structure of glycyl‐dl ‐leucine, C₈H₁₆N₂O₃, has been determined at 120 K by single‐crystal X‐ray diffraction. In addition to three N—H?O‐type hydrogen bonds of the positively charged RNH₃⁺ group of the zwitterionic mol­ecule, an intermolecular N—H?O contact exists between the peptide bond and the carboxyl­ate group. Four hydrogen‐bond cycles were identified, giving a complex pattern.     

The low‐temperature study of d‐ and dl‐camphoric anhydride

The low‐temperature crystal stuctures of d ‐ and dl ‐camphoric anhydride, C₁₀H₁₄O₃, have been determined by X‐ray diffraction methods. Although the two enantiomers crystallize in different space groups, the cell volumes and densities are essentially the same. The six‐membered rings deviate significantly from planarity, both exhibiting half‐boat conformations. The dihedral angle between the six‐ and five‐membered rings is 80.3 (1)° in both cases. The main difference in the molecular stuctures can be described by two torsion angles associated with the H atoms of the methyl substituents. The packing of the racemic and chiral structures are essentially the same.     

Dichlorobis(dl‐proline‐κO)zinc(II)

The title compound, [ZnCl₂(C₅H₉NO₂)₂], crystallizes in the centrosymmetric space group C2/c with the Zn atom on a twofold axis. The two proline residues in any one complex thus have the same absolute configuration. Hydrogen bonding links the mol­ecules into linear chains, which run in the crystallographic b direction. The proline residues within any one chain also have an identical absolute configuration.     

Bis[2,4‐diamino‐5‐(3,4,5‐trimethoxybenzyl)pyrimidin‐1‐ium] dl‐malate

<!?tpct=1pt>Racemic malic acid and trimethoprim [5‐(3,4,5‐trimethoxybenzyl)pyrimidine‐2,4‐diamine] form a 1:2 salt (monoclinic, P2₁/c), 2C₁₄H₁₉N₄O₃⁺·C₄H₄O₅²⁻, in which the malate component is disordered across a centre of inversion. The crystal structure of the salt consists of protonated trimethoprim residues and a malate dianion. The carboxylate group of the malate ion interacts with the trimethoprim cation in a linear fashion through pairs of N—H...O hydrogen bonds to form a cyclic hydrogen‐bonded motif. This is similar to the carboxylate–trimethoprim cation interaction observed earlier in the complex of dihydrofolate reductase with trimethoprim. The structure of the salt of trimethoprim with racemic dl ‐malic acid reported here is the first of its kind. The present study investigates the conformations and the hydrogen‐bonding interactions, which are very important for biological functions. The pyrimidine plane makes a dihedral angle of 78.08 (7)° with the benzene ring of the trimethoprim cation. The cyclic hydrogen‐bonded motif observed in this structure is self‐organized, leading to novel types of hydrogen‐bonding motifs in supramolecular patterns.     

dl‐Aspartic Acid‐Mediated Hydrothermal Synthesis of β‐ZnS Microspheres and Their Optical Properties

ZnS hollow microspheres were synthesized by a dl ‐aspartic acid mediated hydrothermal route. dl ‐aspartic acid plays an important role as crystal growth soft template, which regulates the release of Zn²⁺ ions for the formation of ZnS hollow spheres. The formation of these hollow spheres was mainly attributed to an Ostwald ripening process. The products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), electron diffraction (ED), UV/Vis spectroscopy (UV), and photoluminescence (PL). The shells of the microspheres were composed of ZnS quantum dots (QDs) with the average size of 2.31 nm. The average microspheres diameter is 0.5–3.5 μm. The shell thickness of the hollow sphere is ≈?300 nm. The optical bandgap energy increased significantly compared to the bulk ZnS material due to the strong quantum confinement effect. Two strong emissions at ≈?425 nm and ≈?472 nm in the photoluminescence (PL) spectrum of ZnS hollow microspheres indicate strong quantum confinement because of the presence of QDs.     

Crystallisation from a Water‐in‐Oil Emulsion as a Route to Enantiomer Separation: The Case of dl‐Threonine

The use of crystalliation as a means of separating enantiomers is well known. The utility of commonly applied seeding approaches is limited by the ultimate crystallisation of the antipode. Here we demonstrate how the combination of colloid science and crystal chemistry can lead to an emulsion based process yielding robust separation of a purified solid and impure liquid phases with ultimate product ee of up to 90 %. Threonine is used as a model to demonstrate the viability of the method but it is clear that extension to include, for example, simultaneous racemisation within the disperse phase is easily possible and would transform this from a separation to a preparation process.     

Label‐free aptamer‐based partial filling technique for enantioseparation and determination of dl‐tryptophan with micellar electrokinetic chromatography

In this study, a simple and reproducible method for enantioseparation and determination of dl ‐tryptophan (dl ‐T rp) was developed by using a partial filling technique in combination with MEKC . The corresponding l ‐T rp specific DNA aptamer was used as a chiral selector. Sodium cholate was used to form the chiral micelles and to enhance the enantioseparation of the enantiomers. Effects of aptamer concentration, filling time, buffer composition, and separation voltage on the enantioseparation were evaluated. The M g²⁺ and Na⁺ concentration in separation buffer was found to effectively affect the separation efficiency and reproducibility. Under the optimal conditions, d ‐ and l ‐T rp were completely enantioseparated in less than 9 min. This aptamer‐based partial‐filling approach has the potential to be extended to the separation of other enantiomers after the replacement of corresponding specific aptamers.     

Supramolecular hydrogen‐bonded networks in adeninediium hemioxalate chloride and adeninium semioxalate hemi(oxalic acid) monohydrate

In 9H‐adenine‐1,7‐diium hemioxalate chloride, C₅H₇N₅²⁺·0.5C₂O₄²⁻·Cl⁻, (I), adenine is doubly protonated, while in 7H‐adenin‐1‐ium semioxalate hemi(oxalic acid) monohydrate, C₅H₆N₅⁺·C₂HO₄⁻·0.5C₂H₂O₄·H₂O, (II), adenine and one oxalate anion are both monoprotonated. In (I), the adeninium cation forms R₂²(8) and R₁²(5) hydrogen‐bonding motifs with the centrosymmetric oxalate anion, while in (II), the cation forms R₂¹(6) and R₁²(5) motifs with the centrosymmetric oxalic acid molecule and R₁²(5)and R₂²(9) motifs with the monoprotonated oxalate anion. Linear hydrogen‐bonded trimers are observed in (I) and (II). In both structures, the hydrogen bonds lead to the formation of two‐dimensional supramolecular hydrogen‐bonded sheets in the crystal packing. The significance of this study lies in the analysis of the interactions occurring via hydrogen bonds and the diversity seen in the supramolecular hydrogen‐bonded networks as a result of such interactions.     

Hydrogen‐bonded networks in 5‐chloropyridin‐2‐amine–fumaric acid (2/1) and 2‐aminopyridinium dl‐malate

Crystals of 5‐chloropyridin‐2‐amine–(2E)‐but‐2‐enedioate (2/1), 2C₅H₅ClN₂·C₄H₄O₄, (I), and 2‐aminopyridinium dl ‐3‐carboxy‐2‐hydroxypropanoate, C₅H₇N₂⁺·C₄H₅O₅⁻, (II), are built from the neutral 5‐chloropyridin‐2‐amine molecule and fumaric acid in the case of (I) and from ring‐N‐protonated 2‐aminopyridinium cations and malate anions in (II). The fumaric acid molecule lies on an inversion centre. In (I), the neutral 5‐chloropyridin‐2‐amine and fumaric acid molecules interact via hydrogen bonds, forming two‐dimensional layers parallel to the (100) plane, whereas in (II), oppositely charged units interact via ionic and hydrogen bonds, forming a three‐dimensional network.