A chiral luminescent liquid crystal with a tolane unit

We have synthesised a chiral liquid crystal with a tolane unit that exhibits intense fluorescence in both solution and the solid states. The liquid crystal can form the enantiotropic twist-grain boundary A (TGB_A*) and Blue II phases, and the helical pitch of the TGB_A* phase decreases with an increasing temperature.     

A new peptide motif in the formation of supramolecular double helices

The single crystal X-ray diffraction studies of a new tripeptide motif Boc-Tyr-Aib-Xaa-OMe (Xaa = Leu/Ile/Ala) reveal that the peptides adopt β-turn conformations which self-assemble to form a supramolecular double helical structure using various non-covalent interactions in the solid state and the peptides exhibit a type-III N(2) sorption isotherm.     

On the magnetic alignment of metal ions in a DNA-mimic double helix

We computed by spin-polarized DFT the structure and the electronic properties of an infinite periodic wire constituted of planar Cu-bridged hydroxypyridone chelator base pairs and of a similarly stacked finite dimer. The Cu centers undergo electronic hybridization with the bases. There is an unpaired spin per plane, and the majority-spins manifest ordering: The ferromagnetic and antiferromagnetic phases are energetically degenerate. The total magnetization of the ferromagnetic wire depends linearly on the number of planes in the stack. The combination of interplane spin coupling and intraplane metal-hydroxypyridone coupling makes this system very appealing for electronic and magnetic device exploitation.     

Interaction of water with DNA single and double helix in the B conformation

The interaction energy between water and B-DNA in the single and double helix is computed at a number of planar cross sections perpendicular to the helix long axis and for a few cylindrical surfaces enclosing the helix. In addition, Monte Carlo simulations are presented for a small cluster of water around regions of energy minima. On the base of these simulations the structure of water for B-DNA in solution, the quaternary structure of B-DNA, is proposed and discussed. The intermolecular interaction used in the Monte Carlo computation has been derived from ab initio computations of complexes between water and the DNA bases, diethylphosphate, a ribose derivative, and other model compounds.     

A triple helix of double helicates: three hierarchical levels of self-assembly in a single structure

The bis-bidentate bridging ligand L reacts with Ag(I) ions to form a conventional dinuclear [Ag(2)L(2)](2+) double helicate; individual double helicate units assemble via Ag···Ag interactions into infinite chains, three of which wrap around a central spine of anions to give a triple helical braid, which is therefore an infinite triple helix composed of molecular double helicate subunits.     

An efficient synthesis of a hexasaccharide—the repeating unit of the exopolysaccharide from Cryptococcus neoformans serovar A

A general method for the synthesis of 2-xylose or glucuronic acid branched (1→3)-linked mannose oligosaccharides has been developed. As a typical example, the synthesis of the methyl glycoside of β-d-GlcpA-(1→2)-α-d-Manp-(1→3)-[β-d-Xylp-(1→2)-]α-d-Manp-(1→3)-[β-d-Xylp-(1→2)-]α-d-Manp, the repeating unit of the exopolysaccharide from Cryptococcus neoformans serovar A, was achieved in a regio- and stereoselective manner.     

Incorporation of 3d-metal ions in the hexagonal channels of the Sr5(PO4)3OH apatite

Strontium phosphate apatites with compositions Sr₅(PO₄)₃Zn_0.15O_0.3(OH)_0.7, Sr₅(PO₄)₃Ni_0.2O_0.4(OH)_0.6, and Sr₅(PO₄)₃Co_0.2O_0.5(OH)_0.4 were synthesized by solid state reaction at 1400 °C in air. The samples were characterized by powder X-ray diffraction, EDX analysis, magnetic measurements and IR spectroscopy. The crystal structures were refined by the Rietveld method in the space group P6₃/m with lattice constants a = 9.7499(1), 9.7722(1), 9.7507(1) Å and c = 7.3066(1), 7.2962(1), 7.2988(1) Å, respectively. The 3d-metal atoms were found randomly distributed in the hexagonal channels formally substituting hydrogen in the initial hydroxyapatite. Zn and Ni atoms were twofold coordinated by oxygen atoms such that the linear O–M–O groups formed in the channel separated by the OH groups. Co atom was shifted from the channel center giving the O–Co–O fragment distorted from a linear geometry probably due to the additional coordination by the oxygen atoms of the phosphate groups.     

Noncovalent interactions in supramolecular complexes: a study on corannulene and the double concave buckycatcher

Stimulated by the recent observation of pi-pi interactions between C60 and corannulene subunits in a molecular tweezer arrangement (J Am Chem Soc 2007, 129, 3842), a density functional theory study was performed to analyze the electronic structure and properties of various noncovalent corannulene complexes. The theoretical approach is first applied to corannulene complexes with a series of benchmark molecules (CH4, NH3, and H2O) using several new-generation density functionals. The performance of nine density functionals, illustrated by computing binding energies of the corannulene complexes, demonstrates that Zhao and Truhlar's MPWB1K and M05-2X functionals provide energies similar to that obtained at the SCS-MP2 level. In contrast, most of the other popular density functionals fail to describe this noncovalent interaction or yield purely repulsive interactions. Further investigations with the M05-2X functional show that the binding energy of C60 with corannulene subunits in the relaxed molecular receptor clip geometry is -20.67 kcal/mol. The results of this calculation further support the experimental interpretation of pure pi-pi interactions between a convex fullerene and the concave surfaces of two corannulene subunits.     

A five-membered Ru5 ring in a hexagonal La14 cage: the La14Cl20Ru5 structure

The title compound features a five-membered Ru(5) ring embedded in a La(14) hexagonal wheel-like cage, an incommensurate combination of the two building units. A formal electron partition of (La(3+))(14)(Cl(-))(20)(Ru(5))(22-) results in a (Ru(5))(22-) ring isoelectronic to (Cd(5))(2-). However, computational studies show that there is significant electron back-donation from the Ru(5) ring to the La(14) wheel. This interaction strongly stabilizes the Ru(5) ring. The resistivity and magnetic susceptibility of the compound have also been investigated.     

Iso-energy contour maps for the interaction of water with DNA double helix in the B conformation

The interaction energy between water with B-DNA double helix is computed for few cylindrical surfaces (enclosing the helix) using analytical pair potentials. The iso-energy contour maps indicate a strong attraction for water extending to three water layers surrounding DNA and very stable bridging structure of water molecules connecting two successive phosphate groups along a single helix in the innermost layer.     

Oligomerization state of photosynthetic core complexes is correlated with the dimerization affinity of a transmembrane helix

In the Rhodobacter (Rba.) species of photosynthetic purple bacteria, a single transmembrane α-helix, PufX, is found within the core complex, an essential photosynthetic macromolecular assembly that performs the absorption and the initial processing of light energy. Despite its structural simplicity, many unresolved questions surround PufX, the most important of which is its location within the photosynthetic core complex. One proposed placement of PufX is at the center of a core complex dimer, where two PufX helices associate in the membrane and form a homodimer. Inability for PufX of certain Rba. species to form a homodimer is thought to lead to monomeric core complexes. In the present study, we employ a combination of computational and experimental techniques to test the hypothesized homodimerization of PufX. We carry out a systematic investigation to measure the dimerization affinity of PufX from four Rba. species, Rba. blasticus , Rba. capsulatus , Rba. sphaeroides , and Rba. veldkampii , using a molecular dynamics-based free-energy method, as well as experimental TOXCAT assays. We found that the four PufX helices have substantially different dimerization affinities. Both computational and experimental techniques demonstrate that species with dimeric core complexes have PufX that can potentially form a homodimer, whereas the one species with monomeric core complexes has a PufX with little to no dimerization propensity. Our analysis of the helix-helix interface revealed a number of positions that may be important for PufX dimerization and the formation of a hydrogen-bond network between these GxxxG-containing helices. Our results suggest that the different oligomerization states of core complexes in various Rba. species can be attributed, among other factors, to the different propensity of its PufX helix to homodimerize.     

Ordered blue luminescent ultrathin films by the effective coassembly of tris(8-hydroxyquinolate-5-sulfonate)aluminum and polyanions with layered double hydroxides

This article reports a novel method to assemble a small anion with exfoliated Mg-Al-layered double hydroxide (LDH) nanosheets into ordered ultrathin films (UTFs) by employing the layer-by-layer assembly technique. The premixing solution of tris(8-hydroxyquinolate-5-sulfonate)aluminum(III) (AQS(3-)) with three kinds of polyanions-poly(acrylic acid), ((C(3)H(4)O(2))(n), PAA), poly(styrene 4-sulfonate) ([CH(2)CH(C(6)H(4))SO(3)](m), PSS), and poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylene vinylene] (C(12)H(13)O(5)S)(n), PPV)-has been used as building blocks to assemble alternatively with LDH nanosheets. The UV-vis absorption and fluorescence spectroscopy of (AQS-polyanion/LDH)(n) UTFs presents stepwise growth upon increasing deposited cycles in comparison with the (AQS/LDH)(n) film under the same experimental process. (AQS-PPV/LDH)(n) UTF displays complex fluorescence originating from AQS and PPV. The (AQS/LDH)(n) and (AQS-polyanion/LDH)(n) UTFs exhibit higher blue-polarized photoemission character with a luminescence anisotropy (r) of ca. 0.12-0.20 and a longer fluorescence lifetime than that of the Na(3)AQS film with r = 0.04. X-ray diffraction, scanning electron microscopy, and atomic force microscopy demonstrated that the UTFs were orderly periodically layered structures with a thickness of ca. 3.0 nm per bilayer. Therefore, this work gives a feasible method for immobilizing small anions into the gallery of LDHs.     

Toward a designed, functioning genetic system with expanded-size base pairs: solution structure of the eight-base xDNA double helix

We describe the NMR-derived solution structure of the double-helical form of a designed eight-base genetic pairing system, termed xDNA. The benzo-homologous xDNA design contains base pairs that are wider than natural DNA pairs by ca. 2.4 A (the width of a benzene ring). The eight component bases of this xDNA helix are A, C, G, T, xA, xT, xC, and xG. The structure was solved in aqueous buffer using 1D and 2D NMR methods combined with restrained molecular dynamics. The data show that the decamer duplex is right-handed and antiparallel, and hydrogen-bonded in a way analogous to that of Watson-Crick DNA. The sugar-phosphate backbone adopts a regular conformation similar to that of B-form DNA, with small dihedral adjustments due to the larger circumference of the helix. The grooves are much wider and more shallow than those of B-form DNA, and the helix turn is slower, with ca. 12 base pairs per 360 degrees turn. There is an extensive intra- and interstrand base stacking surface area, providing an explanation for the greater stability of xDNA relative to natural DNA. There is also evidence for greater motion in this structure compared to a previous two-base-expanded helix; possible chemical and structural reasons for this are discussed. The results confirm paired self-assembly of the designed xDNA system. This suggests the possibility that other genetic system structures besides the natural one might be functional in encoding information and transferring it to new complementary strands.     

KBa7Mg2B14O28F5, a new borate with an unusual heptaborate group and double perovskite unit

A new borate, potassium barium magnesium borate fluoride, KBa₇Mg₂B₁₄O₂₈F₅, with a nominal 7:1 composition of BaB₂O₄ to KMg₂F₅, has been found during the growth of BaMgBO₃F crystals with a KF flux. It crystallized in the space group C2/c and is composed of isolated heptaborate [B₇O₁₄]⁷⁻ groups and double perovskite [Mg₂O₆F₅]¹³⁻ units.     

Novel dinucleoside phosphotriester unit conjugated with an intercalative moiety in a stereospecific manner enhances thermal stability of an alternate-stranded triple helix

A novel phosphoramidite synthon of a dinucleoside phosphotriester unit bearing an intercalative moiety at its internucleotide linkage in a stereospecific manner was prepared and successfully incorporated into the middle portion of α-β chimeric oligoDNA. One of the resulting stereoisomeric chimera DNAs strongly enhances the thermal stability of an alternate-stranded triplex formed between the chimera and a double-stranded DNA.