A time-dependent quantum dynamics investigation of the guanine-cytosine system: a six-dimensional model

The dynamics of the guanine-cytosine base pair has been studied in the time-dependent quantum approach. A six-dimensional model involving the nonlinear three hydrogen bridges has been utilized. The modifications induced in the hydrogen transfer from a base to the other by the explicit inclusion of the out-of-plane hydrogen atom position in the three bridges have been evidenced and the consequences on stacking interaction and base pair opening are considered. The relevance of these aspects in biological properties has been suggested.     

Synthesis and characterization of facial and meridional tris-cyclometalated iridium(III) complexes

The synthesis, structures, electrochemistry, and photophysics of a series of facial (fac) and meridional (mer) tris-cyclometalated Ir(III) complexes are reported. The complexes have the general formula Ir(C'N)(3) [where C'N is a monoanionic cyclometalating ligand; 2-phenylpyridyl (ppy), 2-(p-tolyl)pyridyl (tpy), 2-(4,6-difluorophenyl)pyridyl (46dfppy), 1-phenylpyrazolyl (ppz), 1-(4,6-difluorophenyl)pyrazolyl (46dfppz), or 1-(4-trifluoromethylphenyl)pyrazolyl (tfmppz)]. Reaction of the dichloro-bridged dimers [(C'N(2)Ir(mu-Cl)(2)Ir(C'N)(2)] with 2 equiv of HC( wedge )N at 140-150 degrees C forms the corresponding meridional isomer, while higher reaction temperatures give predominantly the facial isomer. Both facial and meridional isomers can be obtained in good yield (>70%). The meridional isomer of Ir(tpy)(3) and facial and meridional isomers of Ir(ppz)(3) and Ir(tfmppz)(3) have been structurally characterized using X-ray crystallography. The facial isomers have near identical bond lengths (av Ir-C = 2.018 A, av Ir-N = 2.123 A) and angles. The three meridional isomers have the expected bond length alternations for the differing trans influences of phenyl and pyridyl/pyrazolyl ligands. Bonds that are trans to phenyl groups are longer (Ir-C av = 2.071 A, Ir-N av = 2.031 A) than when they are trans to heterocyclic groups. The Ir-C and Ir-N bonds with trans N and C, respectively, have bond lengths very similar to those observed for the corresponding facial isomers. DFT calculations of both the singlet (ground) and the triplet states of the compounds suggest that the HOMO levels are a mixture of Ir and ligand orbitals, while the LUMO is predominantly ligand-based. All of the complexes show reversible oxidation between 0.3 and 0.8 V, versus Fc/Fc(+). The meridional isomers are easier to oxidize by ca. 50-100 mV. The phenylpyridyl-based complexes have reduction potentials between -2.5 and -2.8 V, whereas the phenylpyrazolyl-based complexes exhibit no reduction up to the solvent limit of -3.0 V. All of the compounds have intense absorption bands in the UV region assigned into (1)(pi --> pi) transitions and weaker MLCT (metal-to-ligand charge transfer) transitions that extend to the visible region. The MLCT transitions of the pyrazolyl-based complexes are hypsochromically shifted relative to those of the pyridyl-based compounds. The phenylpyridyl-based Ir(III) tris-cyclometalates exhibit intense emission both at room temperature and at 77 K, whereas the phenylpyrazolyl-based derivatives emit strongly only at 77 K. The emission energies and lifetimes of the phenylpyridyl-based complexes (450-550 nm, 2-6 micros) and phenylpyrazolyl-based compounds (390-440 nm, 14-33 micros) are characteristic for a mixed ligand-centered/MLCT excited state. The meridional isomers for both pyridyl and pyrazolyl-based cyclometalates show markedly different spectroscopic properties than do the facial forms. Isolated samples of mer-Ir(C( wedge )N)(3) complexes can be thermally and photochemically converted to facial forms, indicating that the meridional isomers are kinetically favored products. The lower thermodynamic stabilities of the meridional isomers are likely related to structural features of these complexes; that is, the meridional configuration places strongly trans influencing phenyl groups opposite each other, whereas all three phenyl groups are opposite pyridyl or pyrazolyl groups in the facial complexes. The strong trans influence of the phenyl groups in the meridional isomers leads to the observation that they are easier to oxidize, exhibit broad, red-shifted emission, and have lower quantum efficiencies than their facial counterparts.     

Selective steroid recognition by a partially bridged resorcin[4]arene cavitand

The partially bridged resorcin[4]arene cavitand featuring a cleft-shaped recognition site formed by two anti-quinoxaline bridges and four convergent HO-groups was prepared in three steps and characterised by X-ray crystallography; cavitand was found to be a selective receptor for steroidal substrates in CDCl3, with the best binding observed for steroids with a flat A-ring and two H-bonding sites on rings A and C/D.     

Modeling the geometric, electronic, and redox properties of iron(III)-containing amphiphiles with asymmetric [NN'O] headgroups

Two iron(III)-containing amphiphiles 1 and 2 have been synthesized with the [NN'O] ligands HL(tBu-ODA) (2-((octadecyl(pyridin-2-ylmethyl)amino)methyl)-4,6-di-tert-butylphenol) and HL(I-ODA) (2-((octadecyl(pyridin-2-ylmethyl)amino)methyl)-4,6-diiodophenol), respectively. Compound 1 is monometallic, whereas EXAFS data suggest that 2 is a mixture of mono- and bimetallic species. The archetypical [Fe(III)(L(NN'O))(2)](+) complexes 3-9 have been isolated and characterized in order to understand the geometric, electronic, and redox properties of the amphiphiles. Preference for a monometallic or bimetallic nuclearity is dependent on (i) the nature of the solvent used for synthesis and (ii) the type of the substituent in the phenol moiety. In methanol, the tert-butyl-, methoxy-, and chloro-substituted 3, 4, and 5 are monometallic species, whereas the bromo- and iodo-substituted 6 and 7 form bimetallic complexes taking advantage of stabilizing methoxo bridges generated by solvent deprotonation. In dichloromethane, the bromo- and iodo-substituted 8 and 9 are monometallic species; however, these species favor meridional coordination in opposition to the facial coordination observed for the tert-butyl- and methoxy-substituted compounds. Molecular structures for species 5, 7, 8, and 9 have been solved by X-ray diffraction. Furthermore, the electronic spectrum of the amphiphile 1 was expected to be similar to those of facial/cis archetypes with similar substituents, but close resemblance was observed with the profile for those meridional/cis species, suggesting a similar coordination mode. This trend is discussed based on DFT calculations, where preference for the meridional/cis coordination mode appears related to the presence of tertiary amine nitrogen on the ligand, as when a long alkyl chain is attached to the [NN'O] headgroup.     

Tris(porphyrinyl-oxinato)Ga Complexes as a Photosynthetic Antenna Miniature

Two oxinylporphyrins, 5-(8-hydroxy-5-quinolinyl)-10,15,20-( p -tolyl)- porphyrin ( 1 ) and 5,15-bis(8-hydroxy-5-quinolinyl)-10,20-bis( n -heptyl)-porphyrin ( 2 ), were prepared and coordinated with Ga(III) to afford tris(oxinato) complex 3 and poly(oxinato) complex 4 , respectively. The structure of 3 was analyzed by variable temperature NMR study with referring to tris(8-hydroxy-5-quinolinyl)Ga(III) complex 5 to be in a meridional form. Oxinato ligands of 3 and 5 were exchanged with one another, with keeping the meridional structure. UV-Vis and fluorescence spectra of tris(oxinato)complex 3 and poly(oxinato)complex 4 were compared with each monomeric compound 1 and 2 . The absorption spectra showed only a slight broadening of the Soret band, suggesting trivial electronic and excitonic interactions. The fluorescence intensity was increased significantly compared with each monomeric compound 1 and 2 . At the same time, efficient excitation energy transfer among three porphyrins has been observed.     

Highly efficient, selective, and general method for the preparation of meridional homo- and heteroleptic tris-cyclometalated iridium complexes

A highly efficient and general method based on transmetalation with an organozinc reagent is developed for selective preparation of homo- and heteroleptic meridional tris-cyclometalated iridium complexes. The molecular structure of mer-Ir(1-piq)2(ppy) (2) has been determined by a single-crystal X-ray diffraction analysis. The emission properties of a series of meridional complexes are reported.     

Volume of a nanoscale water bridge

Water bridges formed through capillary condensation at nanoscale contacts first stretch and then break during contact rupture. Atomic force microscopy (AFM) pull-off experiments performed in air with hydrophilic tips and samples show that stretched nanoscopic water bridges are in mechanical equilibrium with the external pull-off force acting at the contact but not in thermodynamic equilibrium with the water vapor in air. The experimental findings are explained by a theoretical model that considers constant water volume and decrease of water meniscus curvature during meniscus stretching. The model predicts that the water bridge breakup distance will be roughly equal to the cubic root of the water bridge volume. A thermodynamic instability was noticed for large water bridges formed at the contact of a blunt AFM tip (curvature radius of 400 nm) with a flat sample. In this case, experiments showed rise and stabilization of the volume of the water at the contact in about 1 s. For sharp AFM tips (curvature radius below 50 nm), the experiments indicated that formation of stable water bridges occurs in a much shorter time (below 5 ms).     

Color-tunable and Highly Emissive Solid Materials Constructed from Tetraphenylethylene-o-carborane-based Building Blocks: Synthesis,Aggregation-induced emission,and Photophysics

A novel kind of expanded tetraphenylethylene (TPE)-carborane-TPE pentad has been synthesized by using two adjacent carborane moieties as central bridges and three TPE units in lateral positions. Its solid-state fluorescence quantum yield was substantially increased to 68.2% by expanding the number of bridges between carborane and TPE. Subsequently, the emission color shifted from blue to orange-yellow (126 nm). Mechanical insights into the electronic structure of the extended TPE-carborane-TPE pentads were obtained from density functional theory (DFT) calculations.     

Anomalous electrophoretic behavior of a very acidic protein: ribonuclease U2

Ribonuclease U2 is a low-molecular-weight acidic protein with three disulfide bridges. This protein displays an anomalous electrophoretic behavior on standard SDS-PAGE. The electrophoretic mobility of the nonreduced protein roughly corresponds to its molecular mass while the migration of the reduced protein would be in accordance with the expected molecular mass of the protein dimer. This study reveals that the protein does not bind SDS under the SDS-PAGE conditions, its electrophoretic mobility being only determined by its electrostatic charge and hydrodynamic properties. In addition, the nonreduced protein cannot be blotted to a membrane. Unfolding of the protein upon reduction of its disulfide bridges enables electrotransference to membranes due to a restricted diffusion along the electrophoresis gel.     

An unusual open cubane structure in a mu(1,1)-azido- and alkoxo-bridged tetranuclear copper(II) complex, [Cu4L2(mu(1,1)-N3)2].5H2O (H3L = N,N'-(2-Hydroxylpropane-1,3-diyl)bis-salicylideneimine)

A new end-on (EO) azido-bridged tetranuclear copper(II) complex [Cu(4)L(2)(mu(1,1)-N(3))(2)].5H(2)O derived from the ligand H(3)L (N,N'-(2-hydroxylpropane-1,3-diyl)bis-salicylideneimine) has been synthesized. Its X-ray structure shows an unusual Cu(4)O(2)N(2) open cubane core in which four copper(II) atoms are connected two by two through two mu(1,1)-azido species and three by three through two alkoxo bridges. The magnetic susceptibility data is dominated by strong antiferromagnetic interactions associated with the alkoxides and weak ferromagnetic interactions arising from the azides, in agreement with magneto-structural correlations found in the literature relative to such bridges in Cu(II) complexes.     

A novel triazidoruthenium(III) building block for the construction of polynuclear compounds

Reaction of [Ru(VI)(N)(sap)Cl] with excess NaN(3) affords a novel paramagnetic triazidoruthenium(III) complex [Ru(III)(sap)(N(3))(3)](2-), which is isolated as a PPh(4)(+) salt (1). Reaction of 1 with Ni(2+) and Co(2+) ions produce two isostructural hexanuclear [Ru(4)M(2)] compounds, [Ru(IV)(4)M(II)(2)(μ(3)-OMe)(2)(μ-OMe)(2)(μ-N)(2)(μ-N(3))(2)(μ-O(phenoxy))(2)(sap)(4) (MeOH)(4)] (M = Ni 2 or Co 3). The molecular structures of 1-3 have been determined by X-ray crystallography. 1 is a mononuclear ruthenium(III) compound where three azide ligands are bonded to ruthenium in a meridional fashion, while compounds 2 and 3 are isostructural hexanuclear compounds containing a defective face-sharing dicubane-like core with two missing vertexes. Variable-temperature dc magnetic susceptibility studies have been carried out for 2 and 3. These data indicate that there are four diamagnetic Ru(IV) ions in 2 and 3 and there is ferromagnetic interaction between the two Ni(2+) in 2 and Co(2+) in 3 via the methoxy bridges.     

Wetting Kinetics for a Single Detergent Granule

A technique for investigating the rate of wetting of a detergent granule has been developed. The evolution with respect to time of the meridional profile of the granule/water meniscus system is recorded using a high speed video camera. Glass spheres of different hydrophilicity were used to calibrate the system. A real detergent granule (LAS) was shown to give wetting behavior intermediate to those shown by the calibrants. In all cases the characteristic time for the wetting process was of the order 10 milliseconds. The protocol developed seems likely to be applicable to powder systems having medium particle size. It might thus be useful for the study of wetting rates in powdered food systems such as soup powders and building materials such as sand.     

Weak exchange interaction supported by a biologically relevant long chemical bridge in a Cu-peptide model compound

The copper complex of the dipeptide L-alanyl-L-phenylalanine, catena-(L-alaninate-L-phenylalaninate-copper(II) monohydrate), identified as Cu(II)Ala-Phe, provides a convenient system to study a weak exchange interaction between unpaired spins transmitted through a biologically relevant long chemical bridge (18.34 A). In this complex, the copper ions are arranged in two symmetry-related anisotropic layers parallel to the ab plane at 13.17 A, separated by a double layer of water molecules. The equatorial-equatorial bridge considered as the most relevant path for exchange interactions between copper ions in neighbor layers contains 11 diamagnetic atoms (including three hydrogens), with two covalent amidate bridges plus three weak and moderate H bonds that go across the water layer. This interaction was studied using electron paramagnetic resonance in single-crystal samples, at 9.5 and 34.5 GHz. The measured magnitude of the interlayer interaction, |J3|/kB = 1.7(2) x 10(-3) K, is discussed in terms of values obtained for similar paths in other model compounds and in proteins. These results in model systems provide information that may be important in understanding biological functions at the molecular level.     

New Ru complexes containing the N-tridentate bpea and phosphine ligands: consequences of meridional vs facial geometry

The synthesis and isolation of the complex cis,fac-[RuIICl2(bpea)(PPh3)][3; bpea = N,N-bis(2-pyridylmethyl)ethylamine] and three geometrical isomers of the complex [RuIICl(bpea)(dppe)](BF4) [4; dppe = (1,2-diphenylphosphino)ethane], trans,fac (4a), cis,fac (4b), and mer(down) (4c), have been described (see Chart 1 for a drawing of their structures). These complexes have been characterized through analytical, spectroscopic (IR, UV/vis, and 1D and 2D NMR), and electrochemical (cyclic voltammetry) techniques. In addition, complexes 3, 4a, and 4b have been further characterized in the solid state through monocrystal X-ray diffraction analysis. The molecular and electronic structures of isomers 4a, 4b, 4c, and 4d (the mer(up) isomer) have also been studied by means of density functional theory (DFT) calculations. Furthermore, their low-energy electronic transitions have been simulated using time-dependent DFT approaches, which have allowed unraveling of their metal-to-ligand charge-transfer nature. Complexes 3 and 4a-c are capable of catalyzing H-transfer types of reactions between alcohols and aromatic ketones such as acetophenone and 2,2-dimethylpropiophenone (DP). A strong influence of the facial versus meridional geometry in the bpea ligand coordination mode is observed for these catalytic reactions, with the meridional isomer being much more active than the facial one. The meridional isomer is even capable of carrying out the H-transfer reaction of bulky substrates such as DP at room temperature.     

Antiferromagnetic three-dimensional order induced by carboxylate bridges in a two-dimensional network of [Cu3(dcp)2(H2O)4] trimers

A new Cu(II) complex, [Cu(3)(dcp)(2)(H(2)O)(4)](n), with the ligand 3,5-pyrazoledicarboxylic acid monohydrate (H(3)dcp) has been prepared by hydrothermal synthesis, and it crystallizes in the monoclinic space group P2(1)/c with a = 11.633(2) A, b = 9.6005(14) A, c = 6.9230(17) A, beta = 106.01(2) degrees, and Z = 2. In the solid state structure of [Cu(3)(dcp)(2)(H(2)O)(4)](n), trinuclear [Cu(3)(dcp)(2)(H(2)O)(4)] repeating units in which two dcp(3-) ligands chelate the three Cu(II) ions with the central Cu(II) ion, Cu(1) (on an inversion center), link to form infinite 2D sheets via syn-anti equatorial-equatorial carboxylate bridges between Cu(2) atoms in adjacent trimers. These layers are further linked by syn-anti axial-equatorial carboxylate bridging between Cu(1) atoms in adjacent sheets resulting in the formation of a crystallographic 3D network. A detailed analysis of the magnetic properties of [Cu(3)(dcp)(2)(H(2)O)(4)](n) reveals that the dcp(3-) ligand acts to link Cu(II) centers in three different ways with coupling constants orders of magnitude apart in value. In the high temperature region above 50 K, the dominant interaction is strongly antiferromagnetic (J/k(B) = -32 K) within the trimer units mediated by the pyrazolate bridges. Below 20 K, the trimer motif can be modeled as an S = 1/2 unit. These units are coupled to their neighbors by a ferromagnetic interaction mediated by the syn-anti equatorial-equatorial carboxylate bridge. This interaction has been estimated at J(2D)/k(B) = +2.8 K on the basis of a 2D square lattice Heisenberg model. Finally, below 3.2 K a weak antiferromagnetic coupling (J(3D)/k(B) = -0.1 K) which is mediated by the syn-anti axial-equatorial carboxylate bridges between the 2D layers becomes relevant to describe the magnetic (T, H) phase diagram of this material.     

Heat exchange in apparatus with a blade agitator

Three-dimensional model of a convective heat exchange in the apparatus with two blade agitators equipped with an outside heating device was formulated using the equations of laminar motion of viscous fluid and of energy. The Arrhenius-Frenkel-Eyring equation was used to account for the temperature dependence on the fluid viscosity. A numerical computations of the fields of velocity, pressure, and temperature were performed with algorithm Simple. The results were presented as isotherms in the meridional, and in the horizontal sections of apparatus.     

Synthesis of the First

A "true" allenophane with several rings and allenic bridges was synthesized. The key step in the preparation of 1 was the cyclization to a 28-membered macrocycle (52 % yield) starting from a linear molecule, which already contains three of the four allene units.     

Synthesis, structure and magnetism of a new ferromagnetic hexanuclear nickel cluster with a dicubane-like core

A new ferromagnetic nickel(II) hexameric structure based on the versatile N,O3-ligand 2,2',2"-nitrilotribenzoic acid has been prepared and characterised by X-ray crystallography and magnetic measurements; the compound represents a rare example of a nickel cluster with a dicubane-like core having only oxygen bridges.     

Spin transition charted in a fluorophore-tagged thermochromic dinuclear iron(II) complex

The first crystal structures of a dinuclear iron(II) complex with three N1,N2-1,2,4-triazole bridges in the high-spin and low-spin states are reported. Its sharp spin transition, which was probed using X-ray, calorimetric, magnetic, and (57)Fe Mossbauer analyses, is also delineated in the crystalline state by variable-temperature fluorimetry for the first time.