Kinetics and mechanism of the reaction of hydroxopentaaquarhodium(III) ion with <Emphasis Type="SmallCaps">l</Emphasis>-Arginine in aqueous solution

The kinetics of the aqua ligand substitution from hydroxopentaaquarhodium(III) ion, [Rh(H₂O)₅(OH)]²⁺, by l-Arginine has been studied spectrophotometrically as a function of Arginine concentration, and temperature, at pH 4.3. The reaction proceeds via a rapid outer sphere association complex formation step followed by two consecutive steps. The first of these involves ligand-assisted anation, while the second involves chelation as the second aqua ligand is displaced. The association equilibrium constant for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The activation parameters for both steps have been evaluated using Eyring’s equation. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants are also consistent with an associative mode of activation. The product of the reaction has been characterized by conductivity measurement and IR spectroscopic analysis.     

Kinetic and mechanistic studies on the reaction of thiosemicarbazide with [(H<Subscript>2</Subscript>O)(tap)<Subscript>2</Subscript>RuORu(tap)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)]<Superscript>2+</Superscript> {tap = 2-(m-tolylazo)pyridine}

The interaction of thiosemicarbazide with the title complex has been studied spectrophotometrically in aqueous medium as a function of [complex], [thiosemicarbazide], pH and temperature at constant ionic strength. At pH 7.4, the reaction shows two distinct paths; both of which are [thiosemicarbazide] dependent. A parallel reaction scheme fits well with the experimental findings. An associative interchange mechanism is proposed for both the paths; the activation parameters calculated from Eyring plots are ΔH₁^≠ = 14.2 ± 0.8 kJ mol⁻¹, ΔS₁^≠ = −241 ± 2 JK⁻¹ mol⁻¹, ΔH₂^≠ = 30.8 ± 1.4 kJ mol⁻¹ and ΔS₂^≠ = −236 ± 4 JK⁻¹ mol⁻¹. From the temperature dependence of the outer sphere association complex equilibrium constants, the thermodynamic parameters calculated are ΔH₁^° = 34.25 ± 1.9 kJ mol⁻¹, ΔS₁^° = 146 ± 6 J K⁻¹ mol⁻¹ and ΔH₂^° = 9.4 ± 1.1 kJ mol⁻¹, ΔS₂^° = 71 ± 3 JK⁻¹ mol⁻¹, which gives a negative ΔG° at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex.     

Rapid exploration of phase behavior in surfactant systems using flow in microchannels

The paper describes a study for the determination of the phase behavior of a self-assembling dilute aqueous cetyl trimethylammonium bromide (CTAB) and dodecyl benzene sulfonic acid (HDBS) system using flow in microchannels. The diffusional length scales of approximately 10-100 microm and volumes on the order of a few tens of nanoliters allow fast composition and temperature homogeneity compared to "bulk" experiments, where characteristic volumes and length scales are on the order of milliliters and centimeters, respectively. Fluorescence emission of a polarity-sensitive fluorophore was used with the surfactants for phase characterization. To demonstrate the validity of the new approach, the critical micelle concentrations (cmc) for CTAB and HDBS were first shown to agree with the cmc obtained in the literature under bulk conditions. Subsequently, the microstructures of dilute (less than 0.8 wt % total surfactant) aqueous mixtures of CTAB and HDBS were examined. The range of desired concentrations and accurate flow dilutions of the samples were achieved by imposing controlled pressure gradients across the channel network. Marked changes in slopes of fluorescence emission intensity versus composition were used to demarcate phase boundaries. A series of microstructures ranging from mixed micelles (M), vesicles (V), and giant vesicles (GV) was observed in the ternary CTAB/HDBS/water system. Experimental data from the microfluidic method was found to be consistent with the results obtained from bulk phase experiments using fluorescence, turbidity, dynamic light scattering, and cryogenic transmission electron microscopy.     

Fluorometric and isothermal titration calorimetric studies on binding interaction of a telechelic polymer with sodium alkyl sulfates of varying chain length

Steady-state fluorescence measurements and isothermal titration calorimetric experiments have been performed to study the interaction between a telechelic polymer, pyrene-end-capped poly(ethylene oxide) (PYPY), and sodium alkyl sulfate surfactants having decyl, dodecyl, and tetradecyl hydrocarbon tails. Fluorometric results suggest polymer-surfactant interaction in the very low range of polymer concentrations. The relative variation in the excimer to monomer pyrene emission intensities with varying surfactant concentration reveals that initial addition of surfactant favors intramolecular preassociation until the surfactant molecules start binding with the ethylene oxide (EO) chain. With the growing number of surfactant aggregates along the EO chain, the association becomes hindered due to the polyelectrolyte effect. The results from microcalorimetric titrations in the low concentration range of PYPY solution (approximately 10(-6) M) with alkyl sulfates suggest two kinds of surfactant-polymer interactions, one with the polymer hydrophobic end groups and the other with the ethylene oxide backbone. The overall polymer-surfactant interaction starts at a much lower surfactant concentration for the hydrophobically modified polymers compared to that in the case of unsubstituted poly(ethylene oxide) homopolymer. From the experiments critical aggregation concentration values and the second critical concentration where free micelles start forming have been determined. An endeavor has been made to unveil the mechanism underlying the corresponding associations of the surfactants with the polymer.     

Effect of nanocavity confinement on the rotational relaxation dynamics: 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine in micelles

In continuation of our recent study on the steady state photophysics of a biologically active beta-carboline derivative, 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ), in the present article we have investigated the effect of nanocavity confinement on the excited state dynamics and rotational relaxation of the probe using picosecond time resolved fluorescence and fluorescence anisotropy techniques. The polarity dependent intramolecular charge transfer process is responsible for the remarkable sensitivity of this biological fluorophore in micellar environments. The fluorescence anisotropy decay of AODIQ incorporated inside the micelle is biexponential. The rotational motion of the probe was interpreted on the basis of a two step model consisting of a fast restricted rotation of the probe and a slow lateral diffusion of the probe in the micelle; both coupled to the overall rotation of the micelle. Experimental results reveal that micellar environment causes significant retardation of both the wobbling as well as the translational motion of the probe.     

Triangulating Smooth Submanifolds with Light Scaffolding

We propose an algorithm to sample and mesh a k-submanifold M{\mathcal{M}} of positive reach embedded in \mathbbR^d{\mathbb{R}^{d}} . The algorithm first constructs a crude sample of M{\mathcal{M}} . It then refines the sample according to a prescribed parameter e{\varepsilon} , and builds a mesh that approximates M{\mathcal{M}} . Differently from most algorithms that have been developed for meshing surfaces of \mathbbR ³{\mathbb{R} ^3} , the refinement phase does not rely on a subdivision of \mathbbR ^d{\mathbb{R} ^d} (such as a grid or a triangulation of the sample points) since the size of such scaffoldings depends exponentially on the ambient dimension d. Instead, we only compute local stars consisting of k-dimensional simplices around each sample point. By refining the sample, we can ensure that all stars become coherent leading to a k-dimensional triangulated manifold [^(M)]{\hat{\mathcal{M}}} . The algorithm uses only simple numerical operations. We show that the size of the sample is O(e^-k){O(\varepsilon ^{-k})} and that [^(M)]{\hat{\mathcal{M}}} is a good triangulation of M{\mathcal{M}} . More specifically, we show that M{\mathcal{M}} and [^(M)]{\hat{\mathcal{M}}} are isotopic, that their Hausdorff distance is O(e²){O(\varepsilon ^{2})} and that the maximum angle between their tangent bundles is O(e){O(\varepsilon )} . The asymptotic complexity of the algorithm is T(e) = O(e^-k2-k){T(\varepsilon) = O(\varepsilon ^{-k^2-k})} (for fixed M, d{\mathcal{M}, d} and k).     

Amyloid-like fibril-forming supramolecular β-sheets from a β-turn forming tripeptide containing non-coded amino acids: the crystallographic signature

The crystal structure of a terminally protected tripeptide Boc-Leu-Aib-β-Ala-OMe 1 containing non-coded amino acids reveals that it adopts a β-turn structure, which self-assembles to form a supramolecular β-sheet via non-covalent interactions. The SEM image of peptide 1 exhibits amyloid-like fibrillar morphology in the solid state.     

Photophysical study of 3-acetyl-4-oxo-6,7-dihydro-12H-indolo[2,3-a]quinolizine in biomimetic reverse micellar nanocavities: a spectroscopic approach

Photophysical properties of 3-acetyl-4-oxo-6,7-dihydro-12H-indolo[2,3-a]quinolizine (AODIQ), a bioactive molecule, has been investigated in well-characterized, monodispersed biomimicking nanocavities formed by sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane using steady-state and picosecond time resolved fluorescence and fluorescence anisotropy. The emission behavior of AODIQ is very much dependent upon the water/surfactant mole ratio (W), i.e., on the water pool size of the reverse micellar core. AODIQ exhibits a sharp decrease in fluorescence anisotropy with increasing W, implying that the overall motional restriction experienced by the molecule is decreased with increased hydration. Some of the depth-dependent relevant fluorescence parameters, namely, fluorescence maxima and fluorescence anisotropy (r), have been monitored for exploiting the distribution and microenvironment around the probe in the reverse micelles. Fluorescence spectral position and fluorescence quenching studies suggest that the probe does not penetrate into the reverse micellar core; rather it binds at the interfacial region. Quantitaive estimates of the micropolarity and microviscosity at the binding sites of the probe molecule have been determined as a function of W.