Co‐Assembled Supramolecular Nanostructure of Platinum(II) Complex through Helical Ribbon to Helical Tubes with Helical Inversion

We demonstrated the morphology transformation of co‐assemblies based on terpyridine‐based ligands ( 1R and 1S ) possessing R‐ or S‐alanine analogues and their platinum(II) complex ( 2R‐Pt and 2S‐Pt ). The right‐handed helical ribbon of the co‐assembly formed with 0.5 equivalents of 2R‐Pt to 1R was converted into the left‐handed helical ribbon with 0.6 equivalents of 2R‐Pt . The left‐handed helical ribbon structure of the co‐assembly became a tubular structure in the presence of 0.8–1.0 equivalents of 2R‐Pt . The morphology transformation via helical inversion at the supramolecular level was due to an orientation change of the amide groups caused by non‐covalent Pt???Pt interactions between the terpyridine of 2R‐Pt and that of 2R‐Pt . This study provides insights into controlling the morphology of the transformation of helical ribbons into tubular structures through helicity inversion in co‐assembled supramolecular nanostructures based on platinum(II) complexes.     

Spontaneous Assembly of an Organic–Inorganic Nucleic Acid Z‐DNA Double‐Helix Structure

Herein, we report a hybrid polyoxometalate organic–inorganic compound, Na₂[(HGMP)₂Mo₅O₁₅]⋅7 H₂O ( 1 ; where GMP=guanosine monophosphate), which spontaneously assembles into a structure with dimensions that are strikingly similar to those of the naturally occurring left‐handed Z‐form of DNA. The helical parameters in the crystal structure of the new compound, such as rise per turn and helical twist per dimer, are nearly identical to this DNA conformation, allowing a close comparison of the two structures. Solution circular dichroism studies show that compound 1 also forms extended secondary structures in solution. Gel electrophoresis studies demonstrate the formation of non‐covalent adducts with natural plasmids. Thus we show a route by which simple hybrid inorganic–organic monomers, such as compound 1 , can spontaneously assemble into a double helix without the need for a covalently connected linear sequence of nucleic acid base pairs.     

Poly(p‐benzamide) having isopropyl‐substituted chiral tri(ethylene glycol) side Chain: Synthesis and helical conformation

Isopropyl‐substituted tri(ethylene glycol) is used as a chiral side chain of N‐substituted poly(p‐benzamide) in order to increase the difference of stability between the right‐ and left‐handed helical structures of the polymer. The target polymer is synthesized by the chain‐growth condensation polymerization of the corresponding monomer with an initiator using lithium 1,1,1,3,3,3‐hexamethyldisilazide as a base. A circular dichroism (CD) study of the polymer reveals that the CD signal is due to an excess of a thermodynamically controlled right‐handed helical structure of the polymer, and that the replacement of the methyl group with a bulkier isopropyl group at the side chain of poly(p‐benzamide) increases the abundance of right‐handed helical structure in chloroform. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1623–1628     

Folding pentapeptides into left and right handed alpha helices

Left or right handed alpha helicity can be induced in a pentapeptide (ANGYG) by appending left or right handed helical cycles as chiral templates. This sequence corresponds to a rare left handed helix found in the protein alanine racemase. Circular dichroism spectra reveal that pentapeptide ANGYG has no detectable structure in aq phosphate buffer, that it is an ambidextrous peptide in that it can be directed to fold into either a left handed or right handed alpha helix in water, with greater propensity for the uncommon left handed than the normal right handed conformation. A helix-inducing cyclic peptide at both ends of this peptide was more effective at inducing alpha helicity than a single cyclic peptide at one end. The alpha helical cyclic peptides provide novel tools for folding short peptides into thermodynamically unstable helices in water, and for studying factors that control chirality and helix induction.     

The Cooperative Effect of Both Molecular and Supramolecular Chirality on Cell Adhesion

Although helical nanofibrous structures have great influence on cell adhesion, the role played by chiral molecules in these structures on cells behavior has usually been ignored. The chirality of helical nanofibers is inverted by the odd–even effect of methylene units from homochiral l ‐phenylalanine derivative during assembly. An increase in cell adhesion on left‐handed nanofibers and weak influence of cell behaviors on right‐handed nanofibers are observed, even though both were derived from l ‐phenylalanine derivatives. Weak and negative influences on cell behavior was also observed for left‐ and right‐handed nanofibers derived from d ‐phenylalanine, respectively. The effect on cell adhesion of single chiral molecules and helical nanofibers may be mutually offset.     

Enantioselective Assembly of a Ruthenium(II) Polypyridyl Complex into a Double Helix

Evolution can increase the complexity of matter by self‐organization into helical architectures, the best example being the DNA double helix. One common aspect, apparently shared by most of these architectures, is the presence of covalent bonds within the helix backbone. Here, we report the unprecedented crystal structures of a metal complex that self‐organizes into a continuous double helical structure, assembled by non‐covalent building blocks. Built up solely by weak stacking interactions, this alternating tread stairs‐like double helical assembly mimics the DNA double helix structure. Starting from a racemic mixture in aqueous solution, the ruthenium(II) polypyridyl complex forms two polymorphic structures of a left‐handed double helical assembly of only the Λ‐enantiomer. The stacking of the helices is different in both polymorphs: a crossed woodpile structure versus a parallel columnar stacking.     

Enantioselective Synthesis of a Chiral Coordination Polymer with Circularly Polarized Visible Laser

Circular dichroism is known to be the feature of a chiral agent which has inspired scientist to study the interesting phenomena of circularly polarized light (CPL) modulated molecular chirality. Although several organic molecules or assemblies have been found to be CPL‐responsive, the influence of CPL on the assembly of chiral coordination compounds remains unknown. Herein, a chiral coordination polymer, which is constructed from achiral agents, was used to study the CPL‐induced enantioselective synthesis. By irradiation with either left‐handed or right‐handed CPL during the reaction and crystallization, enantiomeric excesses of the crystalline product were obtained. Left‐handed CPL resulted in crystals with a left‐handed helical structure, and right‐handed CPL led to crystals with a right‐handed helical structure. It is exciting that the absolute asymmetric synthesis of a chiral coordination polymer could be enantioselective when using CPL, and provides a strategy for the control of the chirality of chiral coordination polymers.     

Can Copper(II) Mediate Hoogsteen Base‐Pairing in a Left‐Handed DNA Duplex? A Pulse EPR Study

Pulse EPR spectroscopy is used to investigate possible structural features of the copper(II) ion coordinated to poly(dG‐dC)?poly(dG‐dC) in a frozen aqueous solution, and the structural changes of the polynucleotide induced by the presence of the metal ion. Two different copper species were identified and their geometry explained by a molecular model. According to this model, one species is exclusively coordinated to a single guanine with the N7 nitrogen atom forming a coordinative bond with the copper. In the other species, a guanine and a cytosine form a ternary complex together with the copper ion. A copper crosslink between the N7 of guanine and N3 of cytosine is proposed as the most probable coordination site. Moreover, no evidence was found for an interaction of either copper species with a phosphate group or equatorial water molecules. In addition, circular dichroism (CD) spectroscopy showed that the DNA of the Cu^II‐poly(dG‐dC)?poly(dG‐dC) adducts resembles the left‐handed Z‐form. These results suggest that metal‐mediated Hoogsteen base pairing, as previously proposed for a right‐handed DNA duplex, can also occur in a double‐stranded left‐handed DNA.     

Synthesis of Z‐Protected Aib‐ and Phe(2Me)‐Containing Pentapeptides and Their Crystal Structures

A series of pentapeptide derivatives containing α,α‐disubstituted α‐amino acids have been prepared by a combination of the ‘azirine/oxazolone method’ and segment condensations. X‐Ray crystal‐structure determinations of the molecular structures confirmed the presence of helical conformations stabilized by β‐turns of type III or III′. Pentapeptides containing (R)‐Phe(2Me) form a right‐handed helix, whereas those containing (S)‐Phe(2Me) adopt a left‐handed helical structure.     

Structural Basis for the Identification of an i‐Motif Tetraplex Core with a Parallel‐Duplex Junction as a Structural Motif in CCG Triplet Repeats

CCG triplet repeats can fold into tetraplex structures, which are associated with the expansion of (CCG)_n trinucleotide sequences in certain neurological diseases. These structures are stabilized by intertwining i‐motifs. However, the structural basis for tetraplex i‐motif formation in CCG triplet repeats remains largely unknown. We report the first crystal structure of a CCG‐repeat sequence, which shows that two dT(CCG)₃A strands can associate to form a tetraplex structure with an i‐motif core containing four C:C⁺ pairs flanked by two G:G homopurine base pairs as a structural motif. The tetraplex core is attached to a short parallel‐stranded duplex. Each hairpin itself contains a central CCG loop in which the nucleotides are flipped out and stabilized by stacking interactions. The helical twists between adjacent cytosine residues of this structure in the i‐motif core have an average value of 30°, which is greater than those previously reported for i‐motif structures.     

Non‐Covalent Interactions Enable the Length‐Controlled Generation of Discrete Tubes Capable of Guest Exchange

Supramolecular chemistry in confined spaces constructed from macrocyclic molecules has attracted much attention because it can utilize the specific binding properties of macrocyclic cavities. Herein we report the preparation of length‐controlled discrete tubular structures by homo‐/co‐assembly of rim‐differentiated and peralkylamino‐substituted pillar[5]arenes via hydrogen bonds and salt bridges. By dimerization and trimerization, the expanded tubes show a fivefold helical structure and stepwise binding, respectively. We found that the exchange speed of guest molecules in the tubes could be controlled by varying the tube length.     

Supramolecular ssDNA Templated Porphyrin and Metalloporphyrin Nanoassemblies with Tunable Helicity

Free‐base and nickel porphyrin–diaminopurine conjugates were formed by hydrogen‐bond directed assembly on single‐stranded oligothymidine templates of different lengths into helical multiporphyrin nanoassemblies with highly modular structural and chiroptical properties. Large red‐shifts of the Soret band in the UV/Vis spectroscopy confirmed strong electronic coupling among assembled porphyrin–diaminopurine units. Slow annealing rates yielded preferentially right‐handed nanostructures, whereas fast annealing yielded left‐handed nanostructures. Time‐dependent DFT simulations of UV/Vis and CD spectra for model porphyrin clusters templated on the canonical B‐DNA and its enantiomeric form, were employed to confirm the origin of observed chiroptical properties and to assign the helicity of porphyrin nanoassemblies. Molar CD and CD anisotropy g factors of dialyzed templated porphyrin nanoassemblies showed very high chiroptical anisotropy. The DNA‐templated porphyrin nanoassemblies displayed high thermal and pH stability. The structure and handedness of all assemblies was preserved at temperatures up to +85 °C and pH between 3 and 12. High‐resolution transition electron microscopy confirmed formation of DNA‐templated nickel(II) porphyrin nanoassemblies and their self‐assembly into helical fibrils with micrometer lengths.     

On the Helical Motif of the Complexes Created by Association of Helix‐Forming Schizophyllan (SPG) and Helix‐Forming Polythiophene Derivatives

π‐Conjugated polymers can finely tune their electrical and optical properties in response to their conformational changes. We believe that a deeper understanding of their higher‐order structures will stimulate further development of their applications. We had revealed that one helix‐forming natural polysaccharide (SPG) and one polythiophene derivative (PT‐1) formed a stable one‐dimensional complex and in the polythiophene main chain a helical conformation was induced through the dynamic conformational changes. The objective of our present research is to obtain a better mechanistic understanding on the interaction between SPG and polythiophenes. Here we have used particular left‐ and right‐handed helix‐forming polythiophene derivatives (D ‐ and L ‐POWTs, respectively) and studied their influence on the helical motif of the complexes. We observed that SPG interacts with both D ‐ and L ‐POWTs through their dynamic conformational changes and both D ‐ and L ‐POWTs form the right‐handed co‐helical complexes with SPG according to the inherent helical motif of SPG. In addition, it was confirmed that 1) the complexes do not coagulate in aqueous solution, and 2) the exchange in the helical motif can occur only when the polymers experience the denature–renature process. We believe, therefore, that the mechanism of the helical induction of the SPG/POWT complexes is very unique, being different from conventional equilibrium reactions.     

A Novel DNA Helical Wire Containing HgII‐Mediated T:T and T:G Pairs

Numerous applications of metal‐mediated base pairs (metallo‐base‐pairs) to nucleic acid based nanodevices and genetic code expansion have been extensively studied. Many of these metallo‐base‐pairs are formed in DNA and RNA duplexes containing Watson–Crick base pairs. Recently, a crystal structure of a metal–DNA nanowire with an uninterrupted one‐dimensional silver array was reported. We now report the crystal structure of a novel DNA helical wire containing Hg^II‐mediated T:T and T:G base pairs and water‐mediated C:C base pairs. The Hg‐DNA wire does not contain any Watson–Crick base pairs. Crystals of the Hg‐DNA wire, which is the first DNA wire structure driven by Hg^II ions, were obtained by mixing the short oligonucleotide d(TTTGC) and Hg^II ions. This study demonstrates the potential of metallo‐DNA to form various structural components that can be used for functional nanodevices.     

Polymorphism of hydrogen bonding in the short double helixes of oligonucleotides: Semiempirical quantum chemical study

The origin of heterogeneity of nucleotide steps geometry in short double helixes is studied theoretically. By using the semiempirical MNDO/PM3 technique, the stability of “propeller‐like” and “step‐like” forms of base H‐pairing is examined in the structure of oligonucleotide duplexes of different types. The influence of end effects on the process of nucleotides packing, as well as the dependence of duplex curvature on the nature of bonded oligonucleotides, are examined. It is concluded that the structural polymorphism of base pairs most likely determines the unique packing of complementary pairs and their flexibility in DNA structure. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Trismaleimide Dendrimers: Helix‐to‐Superhelix Supramolecular Transition Accompanied by White‐Light Emission

Reported here are unprecedented fluorescent superhelices composed of primary, supramolecular polymers of the opposite helical twist. A new class of functional dendrimers was synthesized by amino‐ene click reactions, and they demonstrate an alternating OFF/ON fluorescence with generation growth. A peripherally alkyl‐modified dendrimer displays helix‐sense‐selective supramolecular polymerization, which predominantly forms right‐handed (or left‐handed) helical supramolecular polymers in the solution containing chiral solvents. With increasing the concentration, these primary helical supramolecular polymers spontaneously twist around themselves in the opposite direction to form superhelical structures. Atomic force microscopy and circular dichroism measurements were used to directly observe the helix‐to‐superhelix transition occurring with a reversal in the helical direction. Exceptional white‐light emission was observed during superhelix formation.     

New Types of Planar Chiral [2.2]Paracyclophanes and Construction of One‐Handed Double Helices

New types of planar chiral (R_p)‐ and (S_p)‐4,7,12,15‐tetrasubstituted [2.2]paracyclophanes were synthesized from racemic 4,12‐dihydroxy[2.2]paracyclophane as the starting compound. Regioselective dibromination and transformation afforded a series of planar chiral (R_p)‐ and (S_p)‐4,7,12,15‐tetrasubstituted [2.2]paracyclophanes, which can be used as chiral building blocks. In this study, left‐ and right‐handed double helical structures were constructed via chemoselective Sonogashira–Hagihara coupling. The double helical compounds were excellent circularly polarized luminescence (CPL) emitters with large molar extinction coefficients, good photoluminescence quantum efficiencies, and large CPL dissymmetry factors.     

De Novo Left‐Handed Synthetic Peptidomimetic Foldamers

The development of peptidomimetic helical foldamers with a wide repertoire of functions is of significant interest. Herein, we report the X‐ray crystal structures of a series of homogeneous l ‐sulfono‐γ‐AA foldamers and elucidate their folding conformation at the atomic level. Single‐crystal X‐ray crystallography revealed that this class of oligomers fold into unprecedented dragon‐boat‐shaped and unexpected left‐handed helices, which are stabilized by the 14‐hydrogen‐bonding pattern present in all sequences. These l ‐sulfono‐γ‐AApeptides have a helical pitch of 5.1 Å and exactly four side chains per turn, and the side chains lie perfectly on top of each other along the helical axis. 2D NMR spectroscopy, computational simulations, and CD studies support the folding conformation in solution. Our results provide a structural basis at the atomic level for the design of novel biomimetics with a precise arrangement of functional groups in three dimensions.     

甲壳素类液晶高分子的研究Ⅶ.N,O-苄基壳聚糖的胆甾螺旋行为

N ,O-苄基壳聚糖在浓溶液中形成胆甾液晶相 .用圆偏光二向色性谱 (CD)研究了这一聚合物的螺旋行为 ,主要包括螺距和螺旋方向 .浓度越高 ,螺距P越大 ,意味着胆甾相的扭转力随浓度增加而减弱 .CD谱图上观测到两类吸收 ,即在 5 70nm附近较宽但较强的吸收和 330nm附近较尖但较弱的吸收 .前者归属于胆甾相层片的超分子螺旋构象 ,而后者可以归属于分子链的螺旋构象 .改变浓度或溶剂性质时这两个层次的构象都会发生符号的变化 .提高浓度 (固定二氧六环为溶剂 )时两种螺旋结构先后发生反转 .以氯仿为溶剂 (固定浓度为 6 5 % )时两种螺旋结构均为左旋 (正Cotton效应 ) ,但二氧六环和四氢呋喃为溶剂时均变为右旋 (负Cotton效应 ) .溶剂的影响可能与溶剂和高分子间形成氢键的能力有关     

Ambidextrous α,γ‐Hybrid Peptide Foldamers

Molecular chirality is ubiquitous in nature. The natural biopolymers, proteins and DNA, preferred a right‐handed helical bias due to the inherent stereochemistry of the monomer building blocks. Here, we are reporting a rare co‐existence of left‐ and right‐handed helical conformations and helix‐terminating property at the C‐terminus within a single molecule of α,γ‐hybrid peptide foldamers composed of achiral Aib (α‐aminoisobutyric acid) and 3,3‐dimethyl‐substituted γ‐amino acid (Adb; 4‐amino‐3,3‐dimethylbutanoic acid). At the molecular level, the left‐ and right‐handed helical screw sense of α,γ‐hybrid peptides are representing a macroscopic tendril perversion. The pronounced helix‐terminating behaviour of C‐terminal Adb residues was further explored to design helix–Schellman loop mimetics and to study their conformations in solution and single crystals. The stereochemical constraints of dialkyl substitutions on γ‐amino acids showed a marked impact on the folding behaviour of α,γ‐hybrid peptides.