Helical Complexes Research Articles

Synthesis and Chirality Control of Bulk Crystals and Nanocrystals: From a Right-Handed Nonpolar Chain to a Left-Handed Polar Chain

Both bulk crystals and nanocrystals of two helical complexes, [Cu(μ2-L)(H2O)]n (1) and {[Cu(μ2-L)(H2O)]·2H2O}n (2) (H2L = thiazolidine-2,4-dicarboxylic acid), have been synthesized with the chiralities of right-handedness (1) and left-handedness (2), respectively. 4-Cyanopyridine and poly(vinylpyrrolidone) (PVP) have been applied to control the synthesis of complexes with different helicities in bulk-crystal and nanocrystal forms, respectively. 2 can be irreversibly transformed to 1 under heating. Associated with the conformation changing, the symmetry alters between nonpolar and polar space groups.

Production of Disulfide-stabilized Transmembrane Peptide Complexes for Structural Studies

Physical interactions among the lipid-embedded alpha-helical domains of membrane proteins play a crucial role in folding and assembly of membrane protein complexes and in dynamic processes such as transmembrane (TM) signaling and regulation of cell-surface protein levels. Understanding the structural features driving the association of particular sequences requires sophisticated biophysical and biochemical analyses of TM peptide complexes. However, the extreme hydrophobicity of TM domains makes them very difficult to manipulate using standard peptide chemistry techniques, and production of suitable study material often proves prohibitively challenging. Identifying conditions under which peptides can adopt stable helical conformations and form complexes spontaneously adds a further level of difficulty. Here we present a procedure for the production of homo- or hetero-dimeric TM peptide complexes from materials that are expressed in E. coli, thus allowing incorporation of stable isotope labels for nuclear magnetic resonance (NMR) or non-natural amino acids for other applications relatively inexpensively. The key innovation in this method is that TM complexes are produced and purified as covalently associated (disulfide-crosslinked) assemblies that can form stable, stoichiometric and homogeneous structures when reconstituted into detergent, lipid or other membrane-mimetic materials. We also present carefully optimized procedures for expression and purification that are equally applicable whether producing single TM domains or crosslinked complexes and provide advice for adapting these methods to new TM sequences.

Syntheses, Supramolecular Structures and Antibacterial Activities of Five Helical Transition Complexes with 5‐Chloro‐1‐phenyl‐1H‐pyrazole‐3,4‐dicarboxylic Acid

AbstractFive new transition metal complexes [Cu(HL)2(H2O)2] (1), [Cu(HL)2(phen)] (2), [Cu(HL)2(H2O)]2(4,4′‐bipy) (3), [Zn(HL)2(H2O)2]·(4,4′‐bipy) (4), [Ag(HL)(4,4′‐bipy)]n (5), (H2L=5‐chloro‐1‐phenyl‐1H‐pyrazole‐3,4‐dicarboxylic acid, phen=1,10‐phenanthroline; 4,4′‐bipy=4,4′‐bipyridine) have been synthesized and characterized. Complexes 1, 2 and 4 exhibit monomeric structures, 3 shows a dinuclear structure, 5 displays 1D chain structure, and all extend to 3D supramolecular network via rich hydrogen bonds. Complexes 1, 2, 3, 5 comprise single helical chains, while complex 4 generates quadruple‐stranded helical chains. Furthermore, the antibacterial activities of the titled complexes against bacterial species, three Gram positive bacteria (Staphylococcus aureus, Bacillus subtilis and Candida albicans) and two Gram negative bacteria (Escherichia coli and Pseudomonas aeruginosa) were studied and compared to the activities of free ligands by using the microdilution method.

A single- and double-flash flash photolysis study of the sequential biphotonic photoprocesses of Cu(i) phenanthrolines. Comparison of the helicate complex, [Cu2(1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane)2]2+, and [Cu(2,9-dimethyl-1,10-phenanthroline)2]+ photoprocesses

Photochemical processes induced when two photons are sequentially absorbed by the helicate complex [Cu2(mphenpr)2](2+), where mphenpr = 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane, and [Cu(dmp)2](+) were investigated in CH2Cl2 containing solvents. A strong resemblance was observed between the fs-ns photophysics of [Cu2(mphenpr)2](2+) and Cu(I) phenanthroline complexes having a large steric hindrance. In the biphotonic regime, single-flash flash-photolyzed solutions were used for the determination of the product concentrations and quantum yields. The concentration of Cl(-) produced by the photoinduced decomposition of CH2Cl2 increases linearly with flash intensity as expected for a monophotonic process. In contrast, the concentration of a decomposed Cu(I) complex exhibits the quadratic dependence on flash intensity of a biphotonic process. Results of a sequential double-flash flash photolysis experiment are consistent with the decomposition of CH2Cl2 ahead of the flattened excited state formation and with the absorption of the second photon by the flattened MLCT excited state.

Heteroscorpionate aluminium complexes as chiral building blocks to engineer helical architectures

Treatment of heteroscorpionate ligand precursors pbptamH, pbpamH, sbpamH and (S)-mbpamH with 2 equivalents of AlR3 (R = Et, Me) yielded the corresponding binuclear organoaluminium complexes [Al2R4(μ-pbptam)] (R = Me 1, Et 2), [Al2R4(μ-pbpam)] (R = Me 3, Et 4), [Al2R4(μ-sbpam)] (R = Me 5, Et 6) and [Al2R4{μ-(S)-mbpam}] (R = Me 7, Et 8). These complexes have helical chirality due to the demands of the fixed pyrazole rings. The stereoisomerism and the self-assembly processes of these helicates have been studied in some detail in solution by NMR and in the solid state by X-ray diffraction. Mixtures of M- and P-handed enantiomers and mixtures of M- and P-handed diastereoisomers were obtained when achiral (1–4) and chiral (5–8) heteroscorpionate ligands were used as scaffolds, respectively. Re-crystallization from hexane allowed us to obtain M-homochiral architectures in the solid state for the helical complexes [Al2Et4(μ-sbpam)] (6) and [Al2Et4{μ-(S)-mbpam}] (8). The reaction of heteroscorpionate ligands with 3 equivalents of AlR3 (R = Me, Et) led to the corresponding trinuclear organoaluminium complexes [Al3R7(μ3-pbptam)] (R = Me 9, Et 10), [Al3R7(μ3-pbpam)] (R = Me 11, Et 12), [Al3R7(μ3-sbpam)] (R = Me 13, Et 14) and [Al3R7{μ3-(S)-mbpam}] (R = Me 15, Et 16). The extra AlR3 molecule contributes to the formation of a diastereomeric excess of the PS helicate for complexes 15 and 16. X-ray determination of some of the helical complexes allowed us to witness a versatile and efficient self-assembly process of the building blocks (heteroscorpionate aluminium complexes) directed by noncovalent intermolecular CH–π interactions. The structures of these complexes have been determined by spectroscopic methods and the X-ray crystal structures of 2, 6, 8, and 16 have also been established. Concentration-dependent 1H pulsed field-gradient spin echo (PFGSE) NMR experiments provided evidence for the self-assembly of the single molecular species of complex 2 in solution. The degree of aggregation was calculated for complex 2, with the average number of units constituting the aggregate (N) estimated to be a maximum of 4 molecules in solution before reaching the solid state.

Self-assembly of a helical zinc-europium complex: speciation in aqueous solution and luminescence

Two new tridentate(NNO)-bidentate(NN) compartmental ligands, HL5 and HL6, are synthesized from pyridine and benzimidazole synthons. They react in aqueous solution under physiological conditions with ZnII, LnIII, or a mixture thereof, to yield complexes of different stoichiometries, 1:3, 2:2, 2:3, 1:1:3, the speciation of which is established by UV-visible titrations and ESI mass spectrometry. Photophysical studies of the EuIII-containing solutions in Tris-HCl 0.1 M (pH = 7.4) show that lanthanide luminescence arises from a unique N6O3 coordination site with pseudo D3 symmetry. Relevant parameters such as crystal field splitting, lifetime, radiative lifetime, and intrinsic quantum yield perfectly match those reported for dinuclear 4f-4f helicates in which the EuIII ion has the same coordination environment.

Resolution and characterization of helicate dimer and trimer complexes of 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane with copper(i)

Complexation of copper(I) with the binucleating ligand, 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane, mphenpr, result in formation of helical dimers, [Cu2(mphenpr)2](2+). The resolution of the enantiomeric forms of the dimers has been carried out with Δ-[As(cat)3](-) as resolving agent and X-ray structures for two compounds, P-[Cu2(mphenpr)2](Δ-[As(cat)3])2 and P-[Cu2(mphenpr)2](Δ-[As(cat)3])2·4(CH3CN), are reported. The rate of racemization in poorly-coordinating solvents has been examined by (1)H NMR, and is slow. At saturating concentrations of [[Cu2(mphenpr)2](2+)] in acetonitrile, crystals of the helical trimeric complex [Cu3(mphenpr)3](ClO4)3 are obtained. The X-ray structure of the trimer is reported. This species has also been resolved. As with the helical dimer, racemization in poorly-coordinating solvents is slow, and circular dichroism and (1)H NMR spectra are reported. The absolute configuration of the resolved complex, P-[Cu3(mphenpr)3](Δ-[As(cat)3])3, has been determined by X-ray crystallography.

Silica mineralisation of DNA chiral packing: helicity control and formation mechanism of impeller-like DNA-silica helical architectures.

Understanding the formation of biologically relevant materials is essential for revealing the structural components of life and assists the development of new functional materials that mimic the state-of-the-art morphologies and structures observed in nature. In this study, we developed an in vitro mineral model system. This system, which was composed of a biomolecule template (DNA), a chiral inducing agent (Mg2+), a co-structure directing agent (N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride, TMAPS) and a silica source (tetraethoxysilane, TEOS) self-assembled into enantiomeric impeller-like helical DNA-silica complexes (IHDSCs). A right-handed excess impeller-like helical architecture with left-handed DNA chiral packing was produced at a higher reaction temperature (25 °C), whereas a left-handed architecture was synthesised at a lower temperature (0 °C). The pitch length of the IHDSCs was significantly decreased with increasing Mg2+ or TMAPS. The study of the DNA aggregates indicated that the addition of quaternary ammonium (N+) results in the formation of right-handed DNA packing, whereas the presence of Mg2+ results in left-handed DNA packing. Both the handedness and the pitch length of the DNA packing were determined through the competitive interaction between DNA-N+ and DNA-Mg2+. The macroscopic morphological evolution revealed that the impeller-like helical architectures were formed from DNA-silica platelets with parallel DNA packing to the breakage of blades with twisted chiral packing and subsequent growth along the bent blades with decreasing pitch length. The microscopic structural evolution revealed that the enantiomeric IHDSCs were transformed from right-handed DNA chiral packing, which can be reversed into left-handed chiral packing at the higher reaction temperature. Both of the morphological and structural transformations were explained in terms of kinetic charge matching and the competition between DNA-Mg2+/N+-silica wall.

A stable and strongly luminescent dinuclear Cu(i) helical complex prepared from 2-diphenylphosphino-6-methylpyridine

Treatment of 2-diphenylphosphino-6-methylpyridine (dpPyMe) with Cu(CH(3)CN)(4)BF(4) afforded the stable dinuclear Cu(I) complex [Cu(2)(μ-dpPyMe)(3)(CH(3)CN)](BF(4))(2). This compound is a weak emitter in solution, however a remarkably high emission quantum yield (46%) has been found in a rigid matrix at room temperature.

Synthesis and Characterization of a New Racemic Helically Chiral Ru(II) Complex and Its Catalytic Degradation of Eriochrome Blue Black B

A new helically chiral ruthenium complex of fused aromatic rings was prepared in a one stepsequence and fully characterized. The new complex was used as an heterogeneous catalyst for the degradation of Eriochrome Blue Black B (one of the azoic dyes) in aqueous medium in the presence of hydrogen peroxide. Using a fixed dose of H2O2, experiments revealed that the dye degradation was affected by the reaction temperature and the initial dye concentration. The energetic parameters were also determined, and the activation energy (E a) was found to be low (31 kJ mol−1), confirming that the prepared catalyst was very efficient for the degradation of anionic dyes using H2O2. The complex exhibited excellent catalytic activity and has the advantage of easy separation from the reaction medium. In addition, the ruthenium helical complex–H2O2 system could remain with good catalytic activity after five cycles.

Small-Molecule Inhibition of c-MYC:MAX Leucine Zipper Formation Is Revealed by Ion Mobility Mass Spectrometry

The leucine zipper interaction between MAX and c-MYC has been studied using mass spectrometry and drift time ion mobility mass spectrometry (DT IM-MS) in addition to circular dichroism spectroscopy. Peptides comprising the leucine zipper sequence with (c-MYC-Zip residues 402-434) and without a postulated small-molecule binding region (c-MYC-ZipΔDT residues 406-434) have been synthesized, along with the corresponding MAX leucine zipper (MAX-Zip residues 74-102). c-MYC-Zip:MAX-Zip complexes are observed both in the absence and in the presence of the reported small-molecule inhibitor 10058-F4 for both forms of c-MYC-Zip. DT IM-MS, in combination with molecular dynamics (MD), shows that the c-MYC-Zip:MAX-Zip complex [M+5H](5+) exists in two conformations, one extended with a collision cross section (CCS) of 1164 ± 9.3 Å(2) and one compact with a CCS of 982 ± 6.6 Å(2); similar values are observed for the two forms of c-MYC-ZipΔDT:MAX-Zip. Candidate geometries for the complexes have been evaluated with MD simulations. The helical leucine zipper structure previously determined from NMR measurements (Lavigne, P.; et al. J. Mol. Biol. 1998, 281, 165), altered to include the DT region and subjected to a gas-phase minimization, yields a CCS of 1247 Å(2), which agrees with the extended conformation we observe experimentally. More extensive MD simulations provide compact complexes which are found to be highly disordered, with CCSs that correspond to the compact form from experiment. In the presence of the ligand, the leucine zipper conformation is completely inhibited and only the more disordered species is observed, providing a novel method to study the effect of interactions of disordered systems and subsequent inhibition of the formation of an ordered helical complex.

Syntheses, structures, and fluorescent properties of two helical complexes based on a long rigid N-heterocyclic ligand

A long N-heterocyclic ligand, 2,6-bis(3-(pyrid-3-yl)-1,2,4-triazolyl)pyridine (H2bptp), and Zn(II)/Pb(II) yield {[Zn(bptp)(H2O)] · 2H2O·CH3CN} n (1) and {[Pb(bptp)] · H2O} n (2). Single-crystal X-ray diffraction analysis reveals that 1 and 2 possess 2-D networks containing alternating left- and right-handed helical motifs. Topologically, 1 features a (4,4) topology, while 2 exhibits a (6,3) topology. The bptp2− in 1 and 2 adopts syn-anti and syn-syn conformations, respectively. The results indicate that the long rigid N-heterocyclic ligand can adopt different conformations to coordinate with metals, beneficial to construction of helical structures with diverse topologies. The difference between the photoluminescence properties of the two complexes reveals that metal ions and coordination environment have significant influences on photoluminescence behavior.

Helical Assembly Based on Quinoline-Based Molecular Clips

Molecule clips, di(2-quinoline-carboxaldehyde)-2,2′-bibenzoyl-hydrazone(H2L), with potential atropisomeric chirality was used as helicand for construction of dinuclear helicates. Reaction of the ligand with Co(NO3)2 and Cu(BF4)2 afforded dinuclear double helical complexes with the metal centers exhibiting the same absolute configurations. Thus the atropisomeric chiral bridging mode is main factor for the ligand to translate chirality of metal centers from one center to another.

Synthesis and characterisation of dinuclear Co(II), Ni(II) and Cu(II) unsaturated helical complexes from a novel dipyridyl-bispyrazole ligand

Reported here is the synthesis of a novel bis-bidentate ligand 4,4′-methylenebis(1-(2-pyridyl)-3,5-dimethylpyrazole), L, along with the synthesis and structural characterisation of three unsaturated dinuclear helical complexes that it forms with Co(II), 1, Ni(II), 2, and Cu(II), 3. The structure of 1, [LCoCl(H2O)]2[CoCl4]·2MeCN, is comprised of two Co(II) ions with octahedral geometry around which two ligand molecules wrap to generate a compact unsaturated double helicate, with coordination about each Co(II) completed by a water molecule and chloride ion. Packing of the complex is such that helicate species align head-to-tail, with the metal bound water molecule and chloride ligand on adjacent helicates hydrogen-bonding to each other in a reciprocal fashion to form a 1D polymer. Complex 2, [LNiCl]2·xH2O, possesses a similar molecular structure to that found in 1, however in this instance the coordination about each octahedral Ni(II) ion is comprised from two ligand molecules and two chloride ions to yield a charge neutral unsaturated double helical species. Packing of adjacent helical species in 2 creates large void spaces throughout the crystal lattice within which disordered solvent molecules reside. The structure of 3, [LCu2Cl4], is a single stranded dinuclear helical species whereby one ligand molecule wraps about two highly distorted square planar Cu(II) centres, with two chloride ions completing the coordination sphere to yield a neutral species.

Perspectives on the history of research on starch Part IV: On the visualization of granule architecture

AbstractStarch has been used over several millennia for a number of different applications. However, research on understanding this substance only spans about three centuries starting with Leeuwenhoek who observed it in 1716. This story of discovery of the molecular structure and architectural makeup of starch is chronicled in a series of six essays of which this is the fourth with a focus on visualizing the granule, its development and architecture. Since the starch granule was visualized initially, researchers focused on the content and composition of the granule and also tried to understand what they saw by way of layers, polarization, and staining properties using a microscope. Two schools of thought prevailed in the 1800s with regards to granule development. Confusion prevailed with respect to terminologies and composition through this period. The first model of the crystallized granule was proposed in the late 19th century and it was not until mid‐20th century that the blocklet and crystalline arrangement of starch was understood. However, the crystallinity and helical complex between AM and iodine continued to obscure the source of starch crystallinity during the second half of the 20th century. magnified imagePlease read the Editorial for more details.

Preparation of starch-stabilized silver nanoparticles from amylose–sodium palmitate inclusion complexes

Starch-stabilized silver nanoparticles (AgNP) were prepared from amylose–sodium palmitate helical inclusion complexes by first converting sodium palmitate within the amylose helix to silver palmitate by an ion-exchange reaction with silver nitrate, and then reducing the complexed silver palmitate salt with NaBH4. This process yielded stable aqueous solutions that could be dried and then re-dispersed in water for end-use applications. Reaction products were characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), UV–VIS spectroscopy, X-ray diffraction, TEM, SEM and light microscopy. Addition of acid to reduce the pH of aqueous starch-AgNP solutions produced an increase in viscosity, and nearly quantitative precipitation of starch-AgNP was observed at low pH. Smaller AgNP and higher conversions of silver nitrate to water-soluble starch-AgNP were obtained in this process, as compared with a process carried out under similar conditions using a commercial soluble starch as a stabilizer.

Nanospheres with Tunable Size and Chirality from Helical Polymer–Metal Complexes

A new family of nanospheres is made by complexation of divalent metals (i.e., Ca(2+), Ba(2+)) and poly(phenylacetylene) polymers bearing α-methoxyphenylacetic acid (MPA) pendants with high content of the cis isomer responsible for their helical structures. The resulting helical polymer-metal complex (HPMC) nanospheres present two interesting properties: (a) their diameter can be tuned to different sizes, to growth or to shrink, by changing the metal ion or the polymer/metal ion ratio, and (b) the helicity on the surface and the interior of the particle can be tuned to any of the two helical senses (M or P) by selection of the metal ion. The role of the solvent, the metal ion, and the helicity of the polymer in the aggregation are discussed. The ability of these nanospheres to encapsulate is demonstrated with examples.

Effects of lipids on enzymatic hydrolysis and physical properties of starch

This study aimed to understand effects of lipids, including corn oil (CO), soy lecithin (SL), palmitic acid (PA), stearic acid (SA), oleic acid (OA), and linoleic acid (LA), on the enzymatic hydrolysis and physical properties of normal corn (NCS), tapioca (TPS), waxy corn (WCS), and high-amylose corn (HA7) starch, and to elucidate mechanisms of interactions between the starches and lipids. After cooking with the lipids (10%, w/w, dsb), NCS, TPS, and HA7 showed significant decreases in enzymatic hydrolysis, and their DSC thermograms displayed amylose–lipid-complex dissociation peaks except with the CO. 13C NMR spectra of amylodextrin with CO showed downfield changes in the chemical shifts of carbons 1 and 4 of the anhydroglucose unit, indicating helical complex formation. Generally, free fatty acids (FFAs) reduced, but SL increased the peak viscosities of starches. FFAs and SL decreased, but CO increased the gel strength of NCS. These lipids displayed little impacts on the enzymatic hydrolysis and physical properties of WCS because it lacked amylose.

Dinuclear triple-stranded complexes of ReV with bis(benzene-o-dithiolato) ligands

The reaction of K2ReCl6 with 1,2-bis(2,3-dimercaptobenzamido)ethane (H4-1), and 1,2-bis(2,3-dimercaptobenzamido)benzene (H4-2) in the presence of Na2CO3 in methanol affords dinuclear complexes of ReV. Experimental evidence supports the presence of self-assembled complexes with two {Re(S2C6H3)3} units connected in a triple-stranded fashion. Density Functional Theory (DFT) studies on geometry and electronic properties were conducted employing the hybrid B3LYP and PBE1PBE functionals. The helical (ΔΔ and ΛΛ) and meso-helical (ΔΛ) isomers were considered. For the helicate complexes included in this study, differences in the stability of the isomers were observed originating in different steric and strain interactions between the three ligand strands. The geometries at the minimum exhibit a distorted trigonal-prismatic coordination environment at the metal centers. Natural bond orbitals (NBO) analysis indicates the presence of Re–S bonds which are strongly polarized toward the non-metal. Time-Dependent DFT (TD-DFT) calculations were performed for a further understanding of the optical spectra. The calculations show the occupied 5d orbitals of the rhenium lying beneath occupied sulfur-based MOs. The general features of the electronic spectra in the visible region are reasonably reproduced by the calculations. The analysis of molecular orbitals also allows the assignment of the origin for all experimentally detected absorption bands. In the high-energy region of the spectrum the absorptions are attributed to ligand-to-metal–ligand charge transfer (LMLCT), in which sulfur-based orbitals and unoccupied orbitals at the rhenium atom and the benzene-o-dithiolato groups are involved. Also in the blue region, shoulders originating from LMLCT are observed.

Contrasting enantioselective DNA preference: chiral helical macrocyclic lanthanide complex binding to DNA

There is great interest in design and synthesis of small molecules which selectively target specific genes to inhibit biological functions in which particular DNA structures participate. Among these studies, chiral recognition has been received much attention because more evidences have shown that conversions of the chirality and diverse conformations of DNA are involved in a series of important life events. Here, we report that a pair of chiral helical macrocyclic lanthanide (III) complexes, (M)-Yb[LSSSSSS]3+ and (P)-Yb[LRRRRRR]3+, can enantioselectively bind to B-form DNA and show remarkably contrasting effects on GC-rich and AT-rich DNA. Neither of them can influence non-B-form DNA, nor quadruplex DNA stability. Our results clearly show that P-enantiomer stabilizes both poly(dG-dC)2 and poly(dA-dT)2 while M-enantiomer stabilizes poly(dA-dT)2, however, destabilizes poly(dG-dC)2. To our knowledge, this is the best example of chiral metal compounds with such contrasting preference on GC- and AT-DNA. Ligand selectively stabilizing or destabilizing DNA can interfere with protein–DNA interactions and potentially affect many crucial biological processes, such as DNA replication, transcription and repair. As such, bearing these unique capabilities, the chiral compounds reported here may shed light on the design of novel enantiomers targeting specific DNA with both sequence and conformation preference.