N‐(tert‐Butoxycarbonyl)‐L‐valyl‐L‐valine methyl ester: a twisted parallel β‐sheet in the crystal structure of a protected dipeptide

The title compound, C₁₆H₃₀N₂O₅, crystallizes with three molecules in the asymmetric unit, each adopting a β‐strand/polyproline II backbone conformation. The main‐chain functional groups are hydrogen bonded into tapes having the characteristics of parallel β‐sheets. Each tape has a left‐handed twist and thus forms a helix, with six peptide molecules needed to complete a full 360° rotation. A comparison of hydrogen‐bond lengths and twisting modes is made with other related structures of protected dipeptides and with a hexapeptide derived from amyloid‐β containing the Val–Val segment. Additionally, a comparison of the backbone conformation is made with that of the Val141–Val142 segment of the water channel aquaporin‐4 (AQP4).     

catena‐Poly[copper(II)‐μ‐l‐tyrosyl‐l‐leucinato]

In the title compound, [Cu(C₁₅H₂₀N₂O₄)]_n, the copper(II) coordination is square planar. The anionic l ‐tyrosyl‐l ‐leucinate ligand binds in an N,N′,O‐tridentate mode to one Cu^II cation on one side and in an O‐monodentate mode to a second Cu^II cation on the other side, thus defining –Cu—O—C—O—Cu′– chains which run along the a axis. These chains are held together by a strong hydrogen bond involving the hydroxy H atom.     

Conformational studies of hydantoin‐5‐acetic acid and orotic acid

Hydantoin‐5‐acetic acid [2‐(2,5‐dioxoimidazolidin‐4‐yl)acetic acid] and orotic acid (2,6‐dioxo‐1,2,3,6‐tetrahydropyrimidine‐4‐carboxylic acid) each contain one rigid acceptor–donor–acceptor hydrogen‐bonding site and a flexible side chain, which can adopt different conformations. Since both compounds may be used as coformers for supramolecular complexes, they have been crystallized in order to examine their conformational preferences, giving solvent‐free hydantoin‐5‐acetic acid, C₅H₆N₂O₄, (I), and three crystals containing orotic acid, namely, orotic acid dimethyl sulfoxide monosolvate, C₅H₄N₂O₄·C₂H₆OS, (IIa), dimethylammonium orotate–orotic acid (1/1), C₂H₈N⁺·C₅H₃N₂O₄⁻·C₅H₄N₂O₄, (IIb), and dimethylammonium orotate–orotic acid (3/1), 3C₂H₈N⁺·3C₅H₃N₂O₄⁻·C₅H₄N₂O₄, (IIc). The crystal structure of (I) shows a three‐dimensional network, with the acid function located perpendicular to the ring. Interestingly, the hydroxy O atom acts as an acceptor, even though the carbonyl O atom is not involved in any hydrogen bonds. However, in (IIa), (IIb) and (IIc), the acid functions are only slightly twisted out of the ring planes. All H atoms of the acidic functions are directed away from the rings and, with respect to the carbonyl O atoms, they show an antiperiplanar conformation in (I) and synperiplanar conformations in (IIa), (IIb) and (IIc). Furthermore, in (IIa), (IIb) and (IIc), different conformations of the acid O=C—C—N torsion angle are observed, leading to different hydrogen‐bonding arrangements depending on their conformation and composition.     

A three‐dimensional homochiral metal–organic framework constructed from manganese(II) with S‐carboxymethyl‐N‐(p‐tosyl)‐l‐cysteine and 4,4′‐bipyridine

In the chiral polymeric title compound, poly[aqua(4,4′‐bipyridine)[μ₃‐S‐carboxylatomethyl‐N‐(p‐tosyl)‐l ‐cysteinato]manganese(II)], [Mn(C₁₂H₁₃NO₆S₂)(C₁₀H₈N₂)(H₂O)]_n, the Mn^II ion is coordinated in a distorted octahedral geometry by one water molecule, three carboxylate O atoms from three S‐carboxyatomethyl‐N‐(p‐tosyl)‐l ‐cysteinate (Ts‐cmc) ligands and two N atoms from two 4,4′‐bipyridine molecules. Each Ts‐cmc ligand behaves as a chiral μ₃‐linker connecting three Mn^II ions. The two‐dimensional frameworks thus formed are further connected by 4,4′‐bipyridine ligands into a three‐dimensional homochiral metal–organic framework. This is a rare case of a homochiral metal–organic framework with a flexible chiral ligand as linker, and this result demonstrates the important role of noncovalent interactions in stabilizing such assemblies.     

Efficient Access to Enantiopure γ4‐Amino Acids with Proteinogenic Side‐Chains and Structural Investigation of γ4‐Asn and γ4‐Ser in Hybrid Peptide Helices

Hybrid peptides composed of α‐ and β‐amino acids have recently emerged as new class of peptide foldamers. Comparatively, γ‐ and hybrid γ‐peptides composed of γ⁴‐amino acids are less studied than their β‐counterparts. However, recent investigations reveal that γ⁴‐amino acids have a higher propensity to fold into ordered helical structures. As amino acid side‐chain functional groups play a crucial role in the biological context, the objective of this study was to investigate efficient synthesis of γ⁴‐residues with functional proteinogenic side‐chains and their structural analysis in hybrid‐peptide sequences. Here, the efficient and enantiopure synthesis of various N‐ and C‐terminal free‐γ⁴‐residues, starting from the benzyl esters (COOBzl) of N‐Cbz‐protected (E)‐α,β‐unsaturated γ‐amino acids through multiple hydrogenolysis and double‐bond reduction in a single‐pot catalytic hydrogenation is reported. The crystal conformations of eight unprotected γ⁴‐amino acids (γ⁴‐Val, γ⁴‐Leu, γ⁴‐Ile, γ⁴‐Thr(OtBu), γ⁴‐Tyr, γ⁴‐Asp(OtBu), γ⁴‐Glu(OtBu), and γ‐Aib) reveals that these amino acids adopted a helix favoring gauche conformations along the central C^γ? C^β bond. To study the behavior of γ⁴‐residues with functional side chains in peptide sequences, two short hybrid γ‐peptides P1 (Ac‐Aib‐γ⁴‐Asn‐Aib‐γ⁴‐Leu‐Aib‐γ⁴‐Leu‐CONH₂) and P2 (Ac‐Aib‐γ⁴‐Ser‐Aib‐γ⁴‐Val‐Aib‐γ⁴‐Val‐CONH₂) were designed, synthesized on solid phase, and their 12‐helical conformation in single crystals were studied. Remarkably, the γ⁴‐Asn residue in P1 facilitates the tetrameric helical aggregations through interhelical H bonding between the side‐chain amide groups. Furthermore, the hydroxyl side‐chain of γ⁴‐Ser in P2 is involved in the interhelical H bonding with the backbone amide group. In addition, the analysis of 87 γ⁴‐residues in peptide single‐crystals reveal that the γ⁴‐residues in 12‐helices are more ordered as compared with the 10/12‐ and 12/14‐helices.     

l‐Alanylglycyl‐l‐alanine monohydrate at 20 K

The X‐ray crystal structure of the title compound, C₈H₁₅N₃O₄·H₂O, at 20 K (space group P2₁) reveals that the mol­ecular conformation of the tripeptide is remarkably different from the water‐free form (space group P2₁2₁2₁) reported previously [Padiyar & Seshadri (1996), Acta Cryst. C 52 , 1693–1695].     

Poly[[μ2‐1,3‐bis(pyridin‐4‐yl)propane](μ3‐1,4‐phenylenediacetato)cadmium]

Solvothermal reaction between Cd(NO₃)₂, 1,4‐phenylenediacetate (1,4‐PDA) and 1,3‐bis(pyridin‐4‐yl)propane (bpp) afforded the title complex, [Cd(C₁₀H₈O₄)(C₁₃H₁₄N₂)]_n. Adjacent carboxylate‐bridged Cd^II ions are related by an inversion centre. The 1,4‐PDA ligands adopt a cis conformation and connect the Cd^II ions to form a one‐dimensional chain extending along the c axis. These chains are in turn linked into a two‐dimensional network through bpp bridges. The bpp ligands adopt an anti–gauche conformation. From a topological point of view, each bpp ligand and each pair of 1,4‐PDA ligands can be considered as linkers, while the dinuclear Cd^II unit can be regarded as a 6‐connecting node. Thus, the structure can be simplified to a two‐dimensional 6‐connected network.     

The Implications of (2S,4S)‐Hydroxyproline 4‐O‐Glycosylation for Prolyl Amide Isomerization

The conformations of peptides and proteins are often influenced by glycans O‐linked to serine (Ser) or threonine (Thr). (2S,4R)‐4‐Hydroxyproline (Hyp), together with L ‐proline (Pro), are interesting targets for O‐glycosylation because they have a unique influence on peptide and protein conformation. In previous work we found that glycosylation of Hyp does not affect the N‐terminal amide trans/cis ratios (K_trans/cis) or the rates of amide isomerization in model amides. The stereoisomer of Hyp—(2S,4S)‐4‐hydroxyproline (hyp)—is rarely found in nature, and has a different influence both on the conformation of the pyrrolidine ring and on K_trans/cis. Glycans attached to hyp would be expected to be projected from the opposite face of the prolyl side chain relative to Hyp; the impact this would have on K_trans/cis was unknown. Measurements of ³J coupling constants indicate that the glycan has little impact on the C^γ‐endo conformation produced by hyp. As a result, it was found that the D ‐galactose residue extending from a C^γ‐endo pucker affects both K_trans/cis and the rate of isomerization, which is not found to occur when it is projected from a C^γ‐exo pucker; this reflects the different environments delineated by the proline side chain. The enthalpic contributions to the stabilization of the trans amide isomer may be due to disruption of intramolecular interactions present in hyp; the change in enthalpy is balanced by a decrease in entropy incurred upon glycosylation. Because the different stereoisomers—Hyp and hyp—project the O‐linked carbohydrates in opposite spatial orientations, these glycosylated amino acids may be useful for understanding of how the projection of a glycan from the peptide or protein backbone exerts its influence.     

A novel three‐dimensional CdII coordination polymer: poly[[aquabis(2,2′‐bipyridine)(μ5‐pyrazol‐1‐ide‐3,5‐dicarboxylato)(μ4‐pyrazol‐1‐ide‐3,5‐dicarboxylato)(μ3‐pyrazole‐3,5‐dicarboxylato)tetracadmium(II)] dihydrate]

The title compound, {[Cd₄(C₅H₂N₂O₄)(C₅HN₂O₄)₂(C₁₀H₈N₂)₂(H₂O)]·2H₂O}_n, crystallized in the monoclinic space group P2₁/n and displays a three‐dimensional architecture. The asymmetric unit is composed of four crystallographically independent Cd^II centres, two triply deprotonated pyrazole‐3,5‐dicarboxylic acid molecules, one doubly deprotonated pyrazole‐3,5‐dicarboxylic acid molecule, two 2,2′‐bipyridine ligands, one coordinated water molecule and two interstitial water molecules. Interestingly, the Cd^II centers exhibit two different coordination numbers. Two Cd^II centres adopt a distorted octahedral arrangement and a third a trigonal–prismatic geometry, though they are all hexacoordinated. However, the fourth Cd^II center is heptacoordinated and displays a pentagonal–bipyramidal geometry. The three anionic ligands adopt μ₃‐, μ₄‐ and μ₅‐bridging modes, first linking Cd^II centers into a one‐dimensional wave‐like band, then into a wave‐like layer and finally into a three‐dimensional coordination framework, which is stabilized by hydrogen bonds.     

7‐Deaza‐2,8‐diaza­adenosine

In the title compound [systematic name: 4‐amino‐7‐(β‐d ‐ribofuranos­yl)‐7H‐pyrazolo[3,4‐d][1,2,3]triazine], C₉H₁₂N₆O₄, the torsion angle of the N‐glycosylic bond is high anti [χ = −83.2 (3)°]. The ribofuran­ose moiety adopts the C2′‐endo–C1′‐exo (²T₁) sugar conformation (S‐type sugar pucker), with P = 152.4° and τ_m = 35.0°. The conformation at the C4′—C5′ bond is +sc (gauche,gauche), with the torsion angle γ = 52.0 (3)°. The compound forms a three‐dimensional network that is stabilized by several hydrogen bonds (N—H⋯O, O—H⋯N and O—H⋯O).     

rac‐1‐[(2‐Methoxyethyl)sulfanyl]‐2‐[(2‐methoxyethyl)sulfinyl]benzene and its PdCl2 complex

As an extension of recent findings on the recovery of palladium with dithioether extractants, single crystals of the chelating vicinal thioether sulfoxide ligand rac‐1‐[(2‐methoxyethyl)sulfanyl]‐2‐[(2‐methoxyethyl)sulfinyl]benzene, C₁₂H₁₈O₃S₂, (I), and its square‐planar dichloridopalladium complex, rac‐dichlorido{1‐[(2‐methoxyethyl)sulfanyl]‐2‐[(2‐methoxyethyl)sulfinyl]benzene‐κ²S,S′}palladium(II), [PdCl₂(C₁₂H₁₈O₃S₂)], (II), have been synthesized and their structures analysed. The molecular structure of (II) is the first ever characterized involving a dihalogenide–Pd^II complex in which the palladium is bonded to both a thioether and a sulfoxide functional group. The structural and stereochemical characteristics of the ligand are compared with those of the analogous dithioether compound [Traeger et al. (2012). Eur. J. Inorg. Chem. pp. 2341–2352]. The sulfinyl O atom suppresses the electron‐pushing and mesomeric effect of the S—C...;C—S unit in ligand (I), resulting in bond lengths significantly different than in the dithioether reference compound. In contrast, in complex (II), those bond lengths are nearly the same as in the analogous dithioether complex. As observed previously, there is an interaction between the central Pd^II atom and the O atom that is situated above the plane.     

Poly[bis(μ‐benzene‐1,4‐dicarboxylato)bis[μ‐6‐(4‐pyridyl)‐5H‐imidazolo[4,5‐f][1,10]phenanthroline]dilead(II)]: an interpenetrating α‐Po net

The asymmetric unit of the title compound, [Pb₂(C₈H₄O₄)₂(C₁₈H₁₁N₅)₂]_n, contains two Pb^II atoms, two benzene‐1,4‐dicarboxylate (1,4‐bdc) dianions and two 6‐(4‐pyridyl)‐5H‐imidazolo[4,5‐f][1,10]phenanthroline (L) ligands. Each Pb^II atom is eight‐coordinated by three N atoms from two different L ligands and five carboxylate O atoms from three different 1,4‐bdc dianions. The two 1,4‐bdc dianions (1,4‐bdc₁ and 1,4‐bdc₂) show different coordination modes. Each 1,4‐bdc₁ coordinates to two Pb^II atoms in a chelating bis‐bidentate mode. Each carboxylate group of the 1,4‐bdc₂ anion connects two Pb^II atoms in a chelating–bridging tridentate mode to form a dinuclear unit. Neighbouring dinuclear units are connected together by the aromatic backbone of the 1,4‐bdc dianions and the L ligands into a three‐dimensional six‐connected α‐polonium framework. The most striking feature is that two identical three‐dimensional single α‐polonium nets are interlocked with each other, thus leading directly to the formation of a twofold interpenetrated three‐dimensional α‐polonium architecture. The framework is held together in part by strong N—H...O hydrogen bonds between the imidazole NH groups of the L ligands and the carboxylate O atoms of 1,4‐bdc dianions within different α‐polonium nets.     

A new optically pure half‐sandwich Cp–Ru diphosphine complex with a chiral metal centre, (S)‐Ru(η5‐C5H5)(EPHOS)Cl {EPHOS is (+)‐(1R,2S)‐2‐[(diphenylphosphino)methylamino]‐1‐phenylpropyl diphenylphosphinite}

The crystal structure of the title compound, chloro(η⁵‐cyclopenta­dienyl){(1R,2S)‐2‐[(di­phenyl­phosphino)­methyl­amino]‐1‐phenyl­propyl di­phenyl­phosphinite‐κ²P,P′}ruthenium(II), [Ru(C₅H₅)Cl(C₃₄H₃₃NOP₂)], is reported. The pseudo‐octa­hedral complex is chiral and the configuration at the Ru atom is S. The seven‐membered metallacycle adopts a boat‐like conformation.     

(E)‐2‐[2‐(1‐Naphthyl)­vinyl]‐3‐tosyl‐2,3‐di­hydro‐1,3‐benzo­thia­zole

The title compound, C₂₆H₂₁NO₂S₂, which consists of a benzo­thia­zole skeleton with α‐naphthyl­vinyl and tosyl groups at positions 2 and 3, respectively, was prepared by palladium–copper‐catalyzed heteroannulation. The E configuration of the mol­ecule about the vinyl C=C bond is established by the benzothiazole–naphthyl C—C—C—C torsion angle of 177.5 (4)°. The five‐membered heterocyclic ring adopts an envelope conformation with the Csp³ atom 0.380 (6) Å from the C₂NS plane. The two S—C [1.751 (4) and 1.838 (4) Å] and two N—C [1.426 (5) and 1.482 (5) Å] bond lengths in the thia­zole ring differ significantly.     

N‐Ethylpyridinium bis(2‐thioxo‐1,3‐dithiole‐4,5‐dithiolato)nickelate

The title compound, (C₇H₁₀N)[Ni(C₃S₅)₂] or (Etpy)[Ni(dmit)₂] (where Etpy is the N‐ethyl­pyridinium cation, C₇H₁₀N⁺, and dmit is the 2‐thio­xo‐1,3‐di­thiole‐4,5‐di­thiol­ate dianion, C₃S₅²⁻), crystallizes in the P space group with two mol­ecules in the asymmetric unit. The [Ni(dmit)₂]⁻ monoanion has a planar D_2h conformation, with the central Ni atom and the four coordinated S atoms forming an NiS₄ square plane. The six‐membered ring of the Etpy cation also shows good planarity, as expected. There are two main types of disorder in the two Etpy cations. Several short intermolecular interactions are present, such as S⋯S, Ni⋯S and Ni⋯Ni, which help to form the enhanced three‐dimensional structure of the crystal.     

N‐(tert‐Butoxycarbonyl)‐α‐aminoisobutyryl‐α‐aminoisobutyric acid methyl ester: two polymorphic forms in the space group P21/n

The title compound (systematic name: methyl 2‐{2‐[(tert‐butoxycarbonyl)amino]‐2‐methylpropanamido}‐2‐methylpropanoate), C₁₄H₂₆N₂O₅, (I), crystallizes in the monoclinic space group P2₁/n in two polymorphic forms, each with one molecule in the asymmetric unit. The molecular conformation is essentially the same in both polymorphs, with the α‐aminoisobutyric acid (Aib) residues adopting ϕ and ψ values characteristic of α‐helical and mixed 3₁₀‐ and α‐helical conformations. The helical handedness of the C‐terminal residue (Aib2) is opposite to that of the N‐terminal residue (Aib1). In contrast to (I), the closely related peptide Boc‐Aib‐Aib‐OBn (Boc is tert‐butoxycarbonyl and Bn is benzyl) adopts an α_L‐P_II backbone conformation (or the mirror image conformation). Compound (I) forms hydrogen‐bonded parallel β‐sheet‐like tapes, with the carbonyl groups of Aib1 and Aib2 acting as hydrogen‐bond acceptors. This seems to represent an unusual packing for a protected dipeptide containing at least one α,α‐disubstituted residue.     

7‐Amino‐1‐(2‐deoxy‐β‐erythro‐pentofuranosyl)‐1H‐1,2,3‐triazolo[4,5‐d]pyrimidine: an 8‐azaadenine nucleoside with the nucleobase in a syn conformation

The title compound, C₉H₁₂N₆O₃, shows a syn‐glycosylic bond orientation [χ = 64.17 (16)°]. The 2′‐deoxyfuranosyl moiety exhibits an unusual C1′‐exo–O4′‐endo (₁T⁰; S‐type) sugar pucker, with P = 111.5 (1)° and τ_m = 40.3 (1)°. The conformation at the exocyclic C4′—C5′ bond is +sc (gauche), with γ = 64.4 (1)°. The two‐dimensional hydrogen‐bonded network is built from intermolecular N—H...O and O—H...N hydrogen bonds. An intramolecular bifurcated hydrogen bond, with an amino N—H group as hydrogen‐bond donor and the ring and hydroxymethyl O atoms of the sugar moiety as acceptors, constrains the overall conformation of the nucleoside.     

A commonly used spin label: S‐(2,2,5,5‐tetramethyl‐1‐oxyl‐Δ3‐pyrrolin‐3‐ylmethyl) methanethiosulfonate

The title compound, C₁₀H₁₈NO₃S₂, which finds application as a spin label, has triclinic (P) symmetry at 100 (2) K with two independent molecules in the asymmetric unit. Both molecules are very similar with respect to bond lengths and angles, but molecule 2 shows disordering of its side chain. The pyrroline rings differ slightly with respect to the position of the NO group, which in both cases are sterically shielded by the surrounding methyl groups. The crystal structure of the title compound represents the first example of a 2,2,5,5‐tetramethyl‐1‐oxyl‐Δ³‐pyrroline derivative with a side chain at the double bond which is linked to it through an sp³‐hybridized C atom. In the solid state, the side chain adopts a conformation with the methyl group above/below the pyrroline ring and a H atom directed towards a C atom of the double bond. The disordered side chain of molecule 2 represents a second conformation with low potential energy. Both molecules exhibit planar chirality, but in the solid state both pairs of stereoisomers are present. These four stereoisomers are stacked one behind the other in four different columns, denoted A, A′, B and B′, the angle between the vectors of the N—O bonds in columns A and B being 80.38 (8)°.     

7‐De­aza‐2′‐deoxy‐7‐propynyl­guanosine

The title compound, C₁₄H₁₆N₄O₄, adopts the anti conformation at the gly­cosylic bond [χ−117.1 (5)°]. The sugar pucker of the 2′‐deoxy­ribo­furan­osyl moiety is C2′‐endo–C3′‐exo, ²T₃ (S‐type). The orientation of the exocyclic C4′—C5′ bond is +sc (gauche). The propynyl group is linear and coplanar with the nucleobase moiety. The structure of the compound is stabilized by several hydrogen bonds (N—H⋯O and O—H⋯O), leading to the formation of a multi‐layered network. The nucleobases, as well as the propynyl groups, are stacked. This stacking might cause the extraordinary stability of DNA duplexes containing this compound.     

Poly[di‐μ‐aqua‐μ‐1,2‐bis(pyridin‐4‐yl)ethene‐κ2N:N′‐tetrakis(4‐iodobenzoato‐κ2O,O′)dicadmium]

Colourless crystals of the title compound, [Cd₂(C₇H₄IO₂)₄(C₁₂H₁₀N₂)(H₂O)₂]_n, were obtained by the self‐assembly of Cd(NO₃)₂·4H₂O, 1,2‐bis(pyridin‐4‐yl)ethene (bpe) and 4‐iodobenzoic acid (4‐IBA). Each Cd^II atom is seven‐coordinated in a pentagonal–bipyramidal coordination environment by four carboxylate O atoms from two different 4‐IBA ligands, two O atoms from two water molecules and one N atom from a bpe ligand. The Cd^II centres are bridged by the aqua molecules and bpe ligands, which lie across centres of inversion, to give a two‐dimensional net. Topologically, taking the Cd^II atoms as nodes and the μ‐aqua and μ‐bpe ligands as linkers, the two‐dimensional structure can be simplified as a (6,3) network.