Colloidal aggregation induced by long range attractions

The structure of colloidal clusters formed by long-range attractive interactions under diluted conditions is studied by means of Monte Carlo simulations. For a not-too-long attraction range, clusters show self-similar internal structure with lower density than that typical for diffusive aggregation. For long-range interactions, low kappa, nonfractal clusters are formed (dense at short scales but open at long ones). The dependence on the volume fraction shows that more-compact clusters are grown the higher the colloidal density for diffusive aggregation and attraction-driven aggregation in the fractal regime. The whole trend is explained in terms of the interpenetration among aggregates. In attraction-driven aggregations, the interpenetration of clusters competes with aggregation in the tips of the clusters, causing low-density clusters.     

Calorimetric unfolding of intramolecular triplexes: length dependence and incorporation of single AT --> TA substitutions in the duplex domain

DNA triplexes have been the subject of great interest due to their ability to interfere with gene expression. The inhibition of gene expression involves the design of stable triplexes under physiological conditions; therefore, it is important to have a clear understanding of the energetic contributions controlling their stability. We have used a combination of UV spectroscopy and differential scanning calorimetric (DSC) techniques to investigate the unfolding of intramolecular triplexes, d(A(n)C5T(n)C5T(n)), where n is 5-7, 9, and 11, and related triplexes with a single AT --> TA substitution in their duplex stem. Specifically, we obtain standard thermodynamic profiles for the unfolding of each triplex in buffer solutions containing 0.1 M or 1 M NaCl. The triplexes unfold in monophasic or biphasic transitions (triplex --> duplex --> coil) depending on the concentration of salt used and position of the substitution, and their transition temperatures are independent of strand concentration. The DSC curves of the unsubstituted triplexes yielded an unfolding heat of 13.9 kcal/mol for a TAT/TAT base-triplet stack and a heat capacity of 505 cal/ degrees C.mol. The incorporation of a single substitution destabilizes triplex formation (association of the third strand) to a larger extent in 0.1 M NaCl, and the magnitude of the effects also depends on the position of the substitution. The combined results show that a single AT --> TA substitution in a homopurine/homopyrimidine duplex does not allow triplex formation of the neighboring five TAT base triplets, indicating that the in vivo formation of triplexes, such as H-DNA, is exclusive to homopurine/homopyrimidine sequences.     

Nuclear magnetic resonance studies of sulfur inversion in bis(cyclopentadienyl)-molybdenum and -tungsten complexes with dithioethers

The metallocene thioether derivatives [Cp₂M(MeSCH₂CH₂SMe)][PF₆]₂ (1, M = MO; 2, M = W), [Cp₂Mo(SCH₂CH₂SMe)][PF₆] (3) and [Cp₂M(SCH₂CH₂S)] (4, M = Mo; 5, M = W) exhibit temperature-dependent fluxional behavior in solution, owing to the pyramidal sulfur inversion process. The activation energies for this process were determined from proton band-shape analysis in the cases of 1 (54.9 ± 2 kJ mol⁻¹), 2 (51.2 ± 4.6 kJ mol⁻¹) and 3 (30.0 ± 3.1 kJ mol⁻¹). Extended Hückel calculations on related model complexes suggest that local inversion at the sulfur atoms, rather that an inversion of the complete S---C---C---S chain, is responsible for the observed fluxional behaviour.     

Flow injection determination of bismuth in urine by successive retention of Bi(III) and tetrahydroborate(III) on an anion-exchange resin and hydride generation atomic absorption spectrometry

Bismuth as BiCl₄⁻ and BH₄⁻ ware successively retained in a column (150 mm × 4 mm, length × i.d.) packed with Amberlite IRA-410 (strong anion-exchange resin). This was followed by passage of an injected slug of hydrochloric acid resulting in bismuthine generation (BiH₃). BiH₃ was stripped from the eluent solution by the addition of a nitrogen flow and the bulk phases were separated in a gas–liquid separator. Finally, bismutine was atomized in a quartz tube for the subsequent detection of bismuth by atomic absorption spectrometry. Different halide complexes of bismuth (namely, BiBr₄⁻, BiI₄⁻ and BiCl₄⁻) were tested for its pre-concentration, being the chloride complexes which produced the best results. Therefore, a concentration of 0.3 mol l⁻¹ of HCl was added to the samples and calibration solutions. A linear response was obtained between the detection limit (3σ) of 0.225 and 80 μg l⁻¹. The R.S.D.% (n = 10) for a solution containing 50 μg l⁻¹ of Bi was 0.85%. The tolerance of the system to interferences was evaluated by investigating the effect of the following ions: Cu²⁺, Co²⁺, Ni²⁺, Fe³⁺, Cd²⁺, Pb²⁺, Hg²⁺, Zn²⁺, and Mg²⁺. The most severe depression was caused by Hg²⁺, which at 60 mg l⁻¹ caused a 5% depression on the signal. For the other cations, concentrations between 1000 and 10,000 mg l⁻¹ could be tolerated. The system was applied to the determination of Bi in urine of patients under therapy with bismuth subcitrate. The recovery of spikes of 5 and 50 μg l⁻¹ of Bi added to the samples prior to digestion with HNO₃ and H₂O₂ was in satisfactory ranges from 95.0 to 101.0%. The concentrations of bismuth found in six selected samples using this procedure were in good agreement with those obtained by an alternative technique (ETAAS). Finally, the concentration of Bi determined in urine before and after 3 days of treatment were 1.94 ± 1.26 and 9.02 ± 5.82 μg l⁻¹, respectively.     

Rhodium(I) catalyzed four-component reaction of Fischer alkenyl carbene complexes and 1,1-diphenylallene

Alkyl substituted chromium Fischer carbene complexes react with 1,1-diphenylallene in the presence of rhodium(I) catalysts (10 mol%) to yield highly substituted dienyl indenone derivatives. In this process a catalytic chromium(0)-rhodium(I) exchange occurs, four new C-C bonds are created, and four-components (two allenes, the carbene ligand and one CO ligand) are joined in a chemo- and regioselective manner.     

A family of oxorhenium(v) complexes incorporating chelated monoanionic ONN reduced Schiff base and dianionic ONNO tetradentate ligands: synthesis, spectroscopic and electrochemical studies

The green colored complexes of the type Re(V)O(L(SB))Cl(2), 1, have been synthesised by reacting NBu(4)[ReOCl(4)] with HL(SB) in dry ethanol. Here, L(SB)(-) are the deprotonated forms of N-(2-hydroxybenzyl)-2-picolylamine (HL(SB)(1)); N-(2-hydroxybenzyl)-N',N'-dimethylethylenediamine (HL(SB)(2)) and N-(2-hydroxybenzyl)-N',N'-diethylethylenediamine (HL(SB)(3)). Similarly, NBu(4)[ReOCl(4)] reacted with N,N-bis(2-hydroxybenzyl)-2-picolylamine (H(2)L(1)); N,N-bis(2-hydroxybenzyl)-N',N'-dimethylethylenediamine (H(2)L(2)); N,N-bis(2-hydroxybenzyl)-N',N'-diethylethylenediamine (H(2)L(3)); [N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)]-2-aminoethanol (H(2)L(4)); [N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)]-2-methyl-2-amino-1-propanol (H(2)L(5)); N,N-bis(1-hydroxyethyl)-2-picolylamine (H(2)L(6)), to give the monochloro complexes Re(V)O(L)Cl, 2. The X-ray structures of the complexes are reported. The molecular structures observed in the solid state are preserved in solution ((1)H NMR). In acetonitrile solution the Re(V)O(L)Cl, 2, display a one-electron couple, Re(VI)O(L)Cl(+)-Re(V)O(L)Cl, near 1.0 V vs SCE. The electrogenerated hexavalent complexes [Re(VI)O(L)Cl]ClO(4), 3, are paramagnetic and display sextet EPR spectra in solution at room temperature (A(av) approximately 417 (G), g approximately 1.914).     

Control of the Diels-Alder-Addition Regioselectivity by Remote Olefins. Syntheses and Cycloadditions of 2,3,5-Trimethylidenebicyclo[2.2.1]heptane and 2,3,5,6,7-Pentamethylidenebicyclo[2.2.2]octane

The syntheses of 2,3,5-trimethylidenebicyclo[2.2.1]heptane ( 1 ) and 2,3,5,6,7-pentamethylidenebicyclo[2.2.2]-octane ( 2 ) are reported. The Diels-Alder additions of the diene moieties of these polyenes can be regioselective, probably because of a possible transannular interaction between the homoconjugated methylidene and s-cis-buta-diene groups.     

[M(bpy)3] (M = Fe,Ru, Os): New Crystalline Materials from the reductive electrocrystallization of [M(bpy)3](PF6)2

Reductive electrocrystallization at a constant current density (11.0–11.5 μA/cm²) of millimolar solutions of [M(bpy)₃](PF₆)₂, where M = Fe, Ru, or Os, and bpy = 2,2′-bipyridine in acetonitrile containing 0.1M Bu₄NPF₆ results in the formation of dark crystals on the Pt cathode. The crystals grow as long, thin, and shiny needles having a hexagonal cross section of 0.1–0.5 mm in diameter. Combustion microanalyses results are consistent with the composition for [Fe(bpy)₃], [Ru(bpy)₃], and [Os(bpy)₃]. In addition, the chromophores are conserved, as confirmed by recording both the electronic and the ¹H-NMR spectra after reoxidation of the electrocrystals in humid air. The spectra are identical to those for authentic samples of [Fe(bpy)₃]²⁺, [Ru(bpy)₃]²⁺, and [Os(bpy)₃]²⁺. A ratio of 2.0 ± 0.1 e^?/molecule is observed upon completion of the controlled potential electrolysis of a solution of [M(bpy)₃]²⁺, which results in the precipitation of a dark solid and the almost complete fading of the color of the original solution. Unexpectedly, the crystals do not exhibit an ESR signal. These data indicate the formation of novel materials, crystalline [Fe(bpy)₃], [Ru(bpy)₃], and [Os(bpy)₃].     

Regularity of solutions and interfaces of a generalized porous medium equation inR N

Summary We consider the Cauchy problem for the generalized porous medium equation u_t=(u) where u=u(x, t), xRⁿ and t>0, and the initial datum u(x, 0) is assumed to be nonnegative, integrable mid to nave compact support. The nonlinearity (u) is a C¹ function defined for uO which grows like a power of u. Our assumptions generalize the porous medium case, (u)=u^m, m>1, and also include the equation of the Marshak waves. This problem has finite speed of propagation. We estimate the rate of growth of the support of the solution with precise estimates for t 0 and t. Our main result deals with the regularity of the solutions. We show that after a certain time t₀ the pressure, defined by v=(u), with (u)=(u)/u and (0)=0, is a Lipschitz-continuous function of x and t and the interface is a Lipschitz-continuous surface in R^N+1; the solution u is Hölder continuous for all times t> 0.Both authors partially supported by CAICYT, Project 2805-83. The second author also supported by USA-Spain Joint Research Grant CCB-8402023.     

Harmonic maps from compact Kähler manifolds to exceptional hyperbolic spaces

It has been conjectured that a lattice in a noncompact group of real rank one, other than SU(1,n), cannot be isomorphic to the fundamental group of a compact Kähler manifold; moreover, it is known to be true for SO(1,n). In this note it is shown that this conjecture also holds for the case of uniform lattices in F_4(?20), the group of isometries of the Cayley hyperbolic plane. The result is a consequence of a classification theorem for harmonic maps between Kähler and Cayley hyperbolic manifolds.