New hydrophobic L‐amino acid salts: maleates of L‐leucine,L‐isoleucine and L‐norvaline

Crystals of maleates of three amino acids with hydrophobic side chains [L‐leucenium hydrogen maleate, C₆H₁₄NO₂⁺·C₄H₃O₄⁻, (I), L‐isoleucenium hydrogen maleate hemihydrate, C₆H₁₄NO₂⁺·C₄H₃O₄⁻·0.5H₂O, (II), and L‐norvalinium hydrogen maleate–L‐norvaline (1/1), C₅H₁₁NO₂⁺·C₄H₃O₄⁻·C₅H₁₂NO₂, (III)], were obtained. The new structures contain C₂²(12) chains, or variants thereof, that are a common feature in the crystal structures of amino acid maleates. The L‐leucenium salt is remarkable due to a large number of symmetrically non‐equivalent units (Z′ = 3). The L‐isoleucenium salt is a hydrate despite the fact that L‐isoleucine is a nonpolar hydrophobic amino acid (previously known amino acid maleates formed hydrates only with lysine and histidine, which are polar and hydrophilic). The L‐norvalinium salt provides the first example where the dimeric cation L‐Nva...L‐NvaH⁺ was observed. All three compounds have layered noncentrosymmetric structures. Preliminary tests have shown the presence of the second harmonic generation (SGH) effect for all three compounds.     

The effect of amino acid backbone length on molecular packing: crystalline tartrates of glycine, β‐alanine, γ‐aminobutyric acid (GABA) and dl‐α‐aminobutyric acid (AABA)

We report a novel 1:1 cocrystal of β‐alanine with dl ‐tartaric acid, C₃H₇NO₂·C₄H₆O₆, (II), and three new molecular salts of dl ‐tartaric acid with β‐alanine {3‐azaniumylpropanoic acid–3‐azaniumylpropanoate dl ‐tartaric acid–dl ‐tartrate, [H(C₃H₇NO₂)₂]⁺·[H(C₄H₅O₆)₂]⁻, (III)}, γ‐aminobutyric acid [3‐carboxypropanaminium dl ‐tartrate, C₄H₁₀NO₂⁺·C₄H₅O₆⁻, (IV)] and dl ‐α‐aminobutyric acid {dl ‐2‐azaniumylbutanoic acid–dl ‐2‐azaniumylbutanoate dl ‐tartaric acid–dl ‐tartrate, [H(C₄H₉NO₂)₂]⁺·[H(C₄H₅O₆)₂]⁻, (V)}. The crystal structures of binary crystals of dl ‐tartaric acid with glycine, (I), β‐alanine, (II) and (III), GABA, (IV), and dl ‐AABA, (V), have similar molecular packing and crystallographic motifs. The shortest amino acid (i.e. glycine) forms a cocrystal, (I), with dl ‐tartaric acid, whereas the larger amino acids form molecular salts, viz. (IV) and (V). β‐Alanine is the only amino acid capable of forming both a cocrystal [i.e. (II)] and a molecular salt [i.e. (III)] with dl ‐tartaric acid. The cocrystals of glycine and β‐alanine with dl ‐tartaric acid, i.e. (I) and (II), respectively, contain chains of amino acid zwitterions, similar to the structure of pure glycine. In the structures of the molecular salts of amino acids, the amino acid cations form isolated dimers [of β‐alanine in (III), GABA in (IV) and dl ‐AABA in (V)], which are linked by strong O—H…O hydrogen bonds. Moreover, the three crystal structures comprise different types of dimeric cations, i.e. (A…A)⁺ in (III) and (V), and A⁺…A⁺ in (IV). Molecular salts (IV) and (V) are the first examples of molecular salts of GABA and dl ‐AABA that contain dimers of amino acid cations. The geometry of each investigated amino acid (except dl ‐AABA) correlates with the melting point of its mixed crystal.     

Salts of maleic and fumaric acids with oxine: the role of isomeric acids in hydrogen‐bonding patterns

Both maleic and fumaric acid readily form adducts or complexes with other organic molecules. The 1:1 adduct formed by quinolin‐8‐ol (oxine) with maleic and fumaric acid are salts, namely 8‐hydroxyquinolinium hydrogen maleate, C₉H₈NO⁺·C₄H₃O₄⁻, (I), and 8‐hydroxyquinolinium hydrogen fumarate, C₉H₈NO⁺·C₄H₃O₄⁻, (II). The cations and anions of both salts are linked by ionic N⁺—H...O⁻ hydrogen bonds. The maleate salt crystallizes in the space group P2₁2₁2₁, while the fumarate salt crystallizes in P. The maleic and fumaric acids in their complex forms exist as semimaleate and semifumarate ions (mono‐ionized state), respectively. Classical N—H...O and O—H...O hydrogen bonds, together with short C—H...O contacts, generate an extensive hydrogen‐bonding network. The crystal structures of the maleate and fumarate salts of oxine have been elucidated to study the importance of noncovalent interactions in the aggregation and interaction patterns of biological molecules. The structures of the salts of the Z and E isomers of butenedioic acid (maleic and fumaric acid, respectively) with quinolin‐8‐ol are compared.     

Molecular aggregation in selected crystalline 1:1 complexes of hydrophobic d‐ and l‐amino acids. IV. The l‐phenylalanine series

The amino acid l ‐phenylalanine has been cocrystallized with d ‐2‐aminobutyric acid, C₉H₁₁NO₂·C₄H₉NO₂, d ‐norvaline, C₉H₁₁NO₂·C₅H₁₁NO₂, and d ‐methionine, C₉H₁₁NO₂·C₅H₁₁NO₂S, with linear side chains, as well as with d ‐leucine, C₉H₁₁NO₂·C₆H₁₃NO₂, d ‐isoleucine, C₉H₁₁NO₂·C₆H₁₃NO₂, and d ‐allo‐isoleucine, C₉H₁₁NO₂·C₆H₁₃NO₂, with branched side chains. The structures of these 1:1 complexes fall into two classes based on the observed hydrogen‐bonding pattern. From a comparison with other l :d complexes involving hydrophobic amino acids and regular racemates, it is shown that the structure‐directing properties of phenylalanine closely parallel those of valine and isoleucine but not those of leucine, which shares side‐chain branching at C^γ with phenylalanine and is normally considered to be the most closely related non‐aromatic amino acid.     

Semi‐maleate salts of L‐ and DL‐serinium: the first example of chiral and racemic serinium salts with the same composition and stoichiometry

L‐Serinium semi‐maleate, (I), and DL‐serinium semi‐maleate, (II), both C₃H₈NO₃⁺·C₄H₃O₄⁻, provide the first example of chiral and racemic anhydrous serine salts with the same organic anion. A comparison of their crystal structures with each other, with the structures of the pure components (L‐serine polymorphs, DL‐serine and maleic acid) and with other amino acid maleates is important for understanding the formation of the crystal structures, their response to variations in temperature and pressure, and structure–property relationships. As in other known crystal structures of amino acid maleates, there are no direct links between the semi‐maleate anions in the two new structures. The serinium cations have different conformations in (I) and (II). In (I), they are linked into infinite chains via hydrogen bonds between carboxylic acid and hydroxy groups. In (II), there are no such chains formed by the serinium cations. In both (I) and (II), there are C₂²(12) chains consisting of alternating semi‐maleate anions and serinium cations. Two types of such chains are present in (I) and (II), termed C₂²(12) and C₂²(12)′. In (I), these chains, lying in the same plane, are further linked to each other via hydrogen bonds, whereas in (II) they are not.     

Bis(glycinium) oxalate: evidence of strong hydrogen bonding

In the title 2:1 salt, 2C₂H₆NO₂⁺·C₂O₄²⁻, the glycine mol­ecule is in the cationic form with a positively charged amino group and an uncharged carboxylic acid group. The doubly charged oxalate anion lies across a crystallographic inversion centre. One of the reasons why the 1:1 glycinium oxalate salt has a higher melting point than the title compound may be the difference in their hydrogen‐bonding patterns. A database search for salts formed between amino acids or substituted amino acids and oxalic acid revealed that, in most of the structures, the conformation about the O=C—OH bond is synplanar. d ‐Tryptophan oxalate is the only example where the OH group of a semi‐oxalate adopts an anti­planar conformation. The 2:1 stoichiometry seen in the present salt is observed only in the salts of dl ‐serine, dl ‐aspartic acid and betaine with oxalic acid.     

Pharmaceutical salts of emoxypine with dicarboxylic acids

New salt forms of the antioxidant drug emoxypine (EMX, 2‐ethyl‐6‐methylpyridin‐3‐ol) with pharmaceutically acceptable maleic (Mlt), malonic (Mln) and adipic (Adp) acids were obtained {emoxypinium maleate, C₈H₁₂NO⁺·C₄H₃O₄⁻, [EMX+Mlt], emoxypinium malonate, C₈H₁₂NO⁺·C₃H₃O₄⁻, [EMX+Mln], and emoxypinium adipate, C₈H₁₂NO⁺·C₆H₉O₄⁻, [EMX+Adp]} and their crystal structures determined. The molecular packing in the three EMX salts was studied by means of solid‐state density functional theory (DFT), followed by QTAIMC (quantum theory of atoms in molecules and crystals) analysis. It was found that the major contribution to the packing energy comes from pyridine–carboxylate and hydroxy–carboxylate heterosynthons forming infinite one‐dimensional ribbons, with [EMX+Adp] additionally stabilized by hydrogen‐bonded C(9) chains of Adp⁻ ions. The melting processes of the [EMX+Mlt] (1:1), [EMX+Mln] (1:1) and [EMX+Adp] (1:1) salts were studied and the fusion enthalpy was found to increase with the increase of the calculated lattice energy. The dissolution process of the EMX salts in buffer (pH 7.4) was also studied. It was found that the formation of binary crystals of EMX with dicarboxylic acids increases the EMX solubility by more than 30 times compared to its pure form.     

Cocrystals of the antimalarial drug 11‐azaartemisinin with three alkenoic acids of 1:1 or 2:1 stoichiometry

The stoichiometry, X‐ray structures and stability of four pharmaceutical cocrystals previously identified from liquid‐assisted grinding (LAG) of 11‐azaartemisinin (11‐Aza; systematic name: 1,5,9‐trimethyl‐14,15,16‐trioxa‐11‐azatetracyclo[10.3.1.0^4,13.0^8,13]hexadecan‐10‐one) with trans‐cinnamic (Cin), maleic (Mal) and fumaric (Fum) acids are herein reported. trans‐Cinnamic acid, a mono acid, forms 1:1 cocrystal 11‐Aza:Cin ( 1 , C₁₅H₂₃NO₄·C₉H₈O₂). Maleic acid forms both 1:1 cocrystal 11‐Aza:Mal ( 2 , C₁₅H₂₃NO₄·C₄H₄O₄), in which one COOH group is involved in self‐catenation, and 2:1 cocrystal 11‐Aza₂:Mal ( 3 , 2C₁₅H₂₃NO₄·C₄H₄O₄). Its isomer, fumaric acid, only affords 2:1 cocrystal 11‐Aza₂:Fum ( 4 ). All cocrystal formation appears driven by acid–lactam R₂²(8) heterosynthons with short O—H…O=C hydrogen bonds [O…O = 2.56 (2) Å], augmented by weaker C=O…H—N contacts. Despite a better packing efficiency, cocrystal 3 is metastable with respect to 2 , probably due to a higher conformational energy for the maleic acid molecule in its structure. In each case, the microcrystalline powders from LAG were useful in providing seeding for the single‐crystal growth.     

A phase transition from monoclinic C2 with Z′ = 1 to triclinic P1 with Z′ = 4 for the quasiracemate l‐2‐aminobutyric acid–d‐methionine (1/1)

Racemates of hydrophobic amino acids with linear side chains are known to undergo a unique series of solid‐state phase transitions that involve sliding of molecular bilayers upon heating or cooling. Recently, this behaviour was shown to extend also to quasiracemates of two different amino acids with opposite handedness [Görbitz & Karen (2015). J. Phys. Chem. B, 119 , 4975–4984]. Previous investigations are here extended to an l ‐2‐aminobutyric acid–d ‐methionine (1/1) co‐crystal, C₄H₉NO₂·C₅H₁₁NO₂S. The significant difference in size between the –CH₂CH₃ and –CH₂CH₂SCH₃ side chains leads to extensive disorder at room temperature, which is essentially resolved after a phase transition at 229 K to an unprecedented triclinic form where all four d ‐methionine molecules in the asymmetric unit have different side‐chain conformations and all three side‐chain rotamers are used for the four partner l ‐2‐aminobutyric acid molecules.     

Charge‐assisted hydrogen bonding in salts of 2‐amino‐1H‐benzimidazole with 3‐phenylpropynoic acid and oct‐2‐ynoic acid

The 100 K structures of two salts, namely 2‐amino‐1H‐benzimidazolium 3‐phenylpropynoate, C₇H₈N₃⁺·C₉H₅O₂⁻, (I), and 2‐amino‐1H‐benzimidazolium oct‐2‐ynoate, C₇H₈N₃⁺·C₈H₁₁O₂⁻, (II), both have monoclinic symmetry (space group P2₁/c) and display N—H...O hydrogen bonding. Both structures show packing with corrugated sheets of hydrogen‐bonded molecules lying parallel to the [001] direction. Two hydrogen‐bonded ring motifs can be identified and described with graph sets R²₂(8) and R⁴₄(16), respectively, in both (I) and (II). Computational chemistry calculations performed on both compounds show that the hydrogen‐bonded ion pairs are more energetically favourable in the crystal structure than their hydrogen–bonded neutral molecule counterparts.     

The conformational analyses of 2‐amino‐N‐[2‐(dimethylphenoxy)ethyl]propan‐1‐ol derivatives in different environments

Four crystal structures of 2‐amino‐N‐(dimethylphenoxyethyl)propan‐1‐ol derivatives, characterized by X‐ray diffraction analysis, are reported. The free base (R,S)‐2‐amino‐N‐[2‐(2,3‐dimethylphenoxy)ethyl]propan‐1‐ol, C₁₃H₂₁NO₂, 1 , crystallizes in the space group P2₁/n, with two independent molecules in the asymmetric unit. The hydrochloride, (S)‐N‐[2‐(2,6‐dimethylphenoxy)ethyl]‐1‐hydroxypropan‐2‐aminium chloride, C₁₃H₂₂NO₂⁺·Cl^?, 2c , crystallizes in the space group P2₁, with one cation and one chloride anion in the asymmetric unit. The asymmetric unit of two salts of 2‐picolinic acid, namely, (R,S)‐N‐[2‐(2,3‐dimethylphenoxy)ethyl]‐1‐hydroxypropan‐2‐aminium pyridine‐2‐carboxylate, C₁₃H₂₂NO₂⁺·C₆H₄NO₂^?, 1p , and (R)‐N‐[2‐(2,6‐dimethylphenoxy)ethyl]‐1‐hydroxypropan‐2‐aminium pyridine‐2‐carboxylate, C₁₃H₂₂NO₂⁺·C₆H₄NO₂^?, 2p , consists of one cation and one 2‐picolinate anion. Salt 1p crystallizes in the triclinic centrosymmetric space group P, while salt 2p crystallizes in the space group P4₁2₁2. The conformations of the amine fragments are contrasted and that of 2p is found to have an unusual antiperiplanar arrangement about the ether group. The crystal packing of 1 and 2c is dominated by hydrogen‐bonded chains, while the structures of the 2‐picolinate salts have hydrogen‐bonded rings as the major features. In both salts with 2‐picolinic acid, the specific R₁²(5) hydrogen‐bonding motif is observed. Structural studies have been enriched by the generation of fingerprint plots derived from Hirshfeld surfaces.     

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.     

Cyclic heterotetrameric and low‐dimensional hydrogen‐bonded polymeric structures in the morpholinium salts of ring‐substituted benzoic acid analogues

The morpholinium (tetrahydro‐2H‐1,4‐oxazin‐4‐ium) cation has been used as a counter‐ion in both inorganic and organic salt formation and particularly in metal complex stabilization. To examine the influence of interactive substituent groups in the aromatic rings of benzoic acids upon secondary structure generation, the anhydrous salts of morpholine with salicylic acid, C₄H₁₀NO⁺·C₇H₅O₃⁻, (I), 3,5‐dinitrosalicylic acid, C₄H₁₀NO⁺·C₇H₃N₂O₇⁻, (II), 3,5‐dinitrobenzoic acid, C₄H₁₀NO⁺·C₇H₃N₂O₆⁻, (III), and 4‐nitroanthranilic acid, C₄H₁₀NO⁺·C₇H₅N₂O₄⁻, (IV), have been prepared and their hydrogen‐bonded crystal structures are described. In the crystal structures of (I), (III) and (IV), the cations and anions are linked by moderately strong N—H…O_carboxyl hydrogen bonds, but the secondary structure propagation differs among the three, viz. one‐dimensional chains extending along [010] in (I), a discrete cyclic heterotetramer in (III), and in (IV), a heterotetramer with amine N—H…O hydrogen‐bond extensions along b, giving a two‐layered ribbon structure. With the heterotetramers in both (III) and (IV), the ion pairs are linked though inversion‐related N—H…O_carboxylate hydrogen bonds, giving cyclic R₄⁴(12) motifs. With (II), in which the anion is a phenolate rather than a carboxylate, the stronger assocation is through a symmetric lateral three‐centre cyclic R₁²(6) N—H…(O,O′) hydrogen‐bonding linkage involving the phenolate and nitro O‐atom acceptors of the anion, with extension through a weaker O—H…O_carboxyl hydrogen bond. This results in a one‐dimensional chain structure extending along [100]. In the structures of two of the salts [i.e. (II) and (IV)], there are also π–π ring interactions, with ring‐centroid separations of 3.5516 (9) and 3.7700 (9) Å in (II), and 3.7340 (9) Å in (IV).     

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.     

The dehydration process in the dl‐phenylglycinium trifluoromethanesulfonate monohydrate crystal revealed by XRD,vibrational and DSC studies

Thermal analysis, X‐ray diffraction and temperature‐dependent IR spectroscopy were used to study the dehydration process of crystalline dl ‐phenylglycinium trifluoromethanesulfonate monohydrate (PGTFH), C₈H₁₀NO₂⁺·CF₃SO₃^?·H₂O. PGTFH dehydrates in one step centred at 353 K and crystallizes in the monoclinic space group C2/c, whereas the anhydrous compound (PGTF) crystallizes in the triclinic space group P. The dehydration process in PGTFH is preceded by a weakening of both the noncovalent aromatic–aromatic interactions and the packing contacts. This process is accompanied by the breakage of medium‐strength O—H…O hydrogen bonds between ions inside layers and a reorganization of the ions within the layers. This reorganization results in the formation of two different ion pairs (dl ‐phenylglycinium trifluoromethanesulfonate) and the formation of a new hydrogen‐bond network. The dehydration process does not destroy the nature of the crystal structure. Both crystals, i.e. hydrated and anhydrous, have a layered structure, although the layers of each crystal are arranged somewhat differently.     

l‐Methioninium chloride and l‐seleno­methioninium chloride at 103 K

The crystal structures of the title compounds, (S)‐1‐carboxy‐3‐(methyl­sulfanyl)­propanaminium chloride, C₅H₁₂NO₂S⁺·Cl⁻, and (S)‐1‐carboxy‐3‐(methyl­selanyl)­propanaminium chloride, C₅H₁₂NO₂Se⁺·Cl⁻, are isomorphous. The proton­ated l ‐methionine and l ‐seleno­methionine mol­ecules have almost identical conformations and create very similar contacts with the Cl⁻ anions in the crystal structures of both compounds. The amino acid cations and the Cl⁻ anions are linked viaN—H⋯Cl⁻ and O—H⋯Cl⁻ hydrogen bonds.     

Supramolecular architectures of succinates of 1‐hydroxypropan‐2‐aminium derivatives

Aminoalkanol and aroxyalkyl derivatives are known as potential anticonvulsants. Two new salts, namely bis{(R,S)‐N‐[2‐(2,6‐dimethylphenoxy)ethyl]‐1‐hydroxypropan‐2‐aminium} succinate ( 1s ), C₁₃H₂₂NO₂⁺·0.5C₄H₄O₄²⁻, and bis{(S)‐(+)‐N‐[2‐(2,6‐dimethylphenoxy)ethyl]‐1‐hydroxypropan‐2‐aminium} succinate ( 2s ), C₁₃H₂₂NO₂⁺·0.5C₄H₄O₄²⁻, have been prepared and characterized by single‐crystal X‐ray diffraction. The N atoms are protonated by proton transfer from succinic acid. Salt 1s crystallizes in the space group P2₁/n with one cation and half an anion in the asymmetric unit across an inversion centre, while ( 2s ) crystallizes in the space group P2₁ with four cations and two anions in the asymmetric unit. The hydroxy group of the cation of 1s is observed in two R/S disorder positions. The crystals of these two salts display similar supramolecular architectures (i.e. two‐dimensional networks), built mainly by intermolecular N⁺—H…O^δ− and O—H…O^δ− hydrogen bonds, where `δ−' represents a partial charge. The succinate anions are engaged in hydrogen bonds, not only with protonated N atoms, but also with hydroxy groups.     

Concomitant cocrystal and salt: no interconversion in the solid state

A cocrystal and a molecular salt of β‐alanine and dl ‐tartaric acid, C₃H₈NO₂⁺·C₄H₄O₆^?, of the same chemical composition, were studied over a wide temperature range by single‐crystal and powder X‐ray diffraction. Neither the interconversion between the two phases nor any polymorphic transitions were observed in the temperature range from 100 K to the melting points. This contrasts with the solvent‐mediated phase transition from the salt to the cocrystal in a slurry that has been documented earlier.     

Influence of protonation on the geometry of 2-{[(2,6-dimethylphenoxy)ethyl]amino}-1-phenylethan-1-ol: crystal structures of the free base and of its chloride and 3-hydroxybenzoate salt forms

The aroxyalkylaminoalcohol derivatives are a group of compounds known for their pharmacological action. The crystal structures of four new xylenoxyaminoalcohol derivatives having anticonvulsant activity are reported, namely, 2-{[2-(2,6-dimethylphenoxy)ethyl]amino}-1-phenylethan-1-ol, C₁₈H₂₃NO₂, 1 , the salt N-[2-(2,6-dimethylphenoxy)ethyl]-1-hydroxy-1-phenylethan-2-aminium 3-hydroxybenzoate, C₁₈H₂₄NO₂⁺·C₇H₅O₃^?, 2 , and two polymorphs of the salt (R)-N-[2-(2,6-dimethylphenoxy)ethyl]-1-hydroxy-1-phenylethan-2-aminium chloride, C₁₈H₂₄NO₂⁺·Cl^?, 3 and 3p . Both polymorphs crystallize in the space group P2₁2₁2 and each has two cations and two anions in the asymmetric unit (Z′ = 2). The molecules in the polymorphs show differences in their molecular conformations and intermolecular interactions. The crystal packing of neutral 1 is dominated by intermolecular O—H…N hydrogen bonds, resulting in the formation of one-dimensional chains. In the crystal structures of the salt forms ( 2 , 3 and 3p ), each protonated N atom is engaged in a charge-assisted hydrogen bond with the corresponding anion. The protonation of the N atom also influences the conformation of the molecular linker between the two aromatic rings and changes the orientation of the rings. The crystal packing of the salt forms is dominated by intermolecular O—H…O hydrogen bonds, resulting in the creation of chains and rings. Structural studies have been enriched by the calculation of Hirshfeld surfaces and the corresponding fingerprint plots.     

X‐ray studies of crystalline complexes involving amino acids and peptides. XLIII. Adipic acid complexes of l‐ and dl‐lysine

The asymmetric unit of the dl ‐lysine complex of adipic acid [bis­(dl ‐lysinium) adipate], 2C₆H₁₅N₂O₂⁺·C₆H₈O₄²⁻, contains a zwitterionic singly charged lysinium cation and half a doubly charged adipate anion (the complete anion has inversion symmetry). That of the l ‐lysine complex (lysinium hydrogen adipate), C₆H₁₅N₂O₂⁺·C₆H₉O₄⁻, consists of a lysinium cation and a singly charged hydrogen adipate anion. In both structures, the lysinium cations organize into layers inter­connected by adipate or hydrogen adipate anions. However, the arrangement of the mol­ecular ions in the layer is profoundly different in the dl ‐ and l ‐lysine complexes. The hydrogen adipate anions in the l ‐lysine complex form linear arrays in which adjacent ions are inter­connected by a symmetric O⋯H⋯O hydrogen bond.