Regulation of the rate of hydrolysis of phosphorus acid esters in organized systems based on amphiphilic pyrimidinophanes

The aggregation and catalytic activity of supramolecular systems based on new macrocyclic dimeric surfactants (pyrimidinophanes) was studied both in the absence and in the presence of polyethyleneimine (PEI). It was found that the critical micelle concentrations measured by tensiometry were independent of the structure of surfactants. The morphology of aggregates was responsible for various characters of the catalytic effects of pyrimidinophanes in the hydrolysis of phosphonic acid esters. A less hydrophobic pyrimidinophane exhibited a typical effect of cationic surfactants to accelerate the reaction both in the absence and in the presence of PEI. Unlike cationic micelles, a heminal-type pyrimidinophane exhibited an anomalous behavior: it had no effect on the rates of hydrolysis of the substrates and did not inhibit the hydrolysis. Upon the addition of lanthanum ions, the catalytic activity of dimeric surfactants increased. The overall catalytic effect due to the action of supramolecular systems based on pyrimidinophanes, PEI, and lanthanum ions can increase the rates of hydrolysis of the substrates by three orders of magnitude, as compared with that of alkaline hydrolysis.     

Nucleation of silica Stöber particles in the presence of methacryloxypropyltrimethoxysilane

The effect of 3-methacryloxypropyltrimethoxysilane (MPTMOS) on the nucleation of silica particles synthesized in a water?ethanol?ammonia?tetraethoxysilane (TEOS) mixture by the Stöber?Fink?Bohn method has been studied. It has been shown, using atomic force microscopy, that, as the content of MPTMOS in a TEOS + MPTMOS precursor mixture is increased from 0 to 12.5 mol %, the final silica particle size decreases from 470 to 10 nm, because the number of nucleation centers increases by several orders of magnitude. In contrast to TEOS, hydrolysis of MPTMOS yields a smaller amount of deprotonated orthosilicic acid monomers, the condensation of which is hindered by electrostatic repulsion. The polycondensation of electrically neutral products of MPTMOS hydrolysis gives rise to a larger number of nucleation centers in the reaction mixture.     

Direct measurement of molecular motion in freestanding polystyrene thin films

An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 nm. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with different dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at T(g,DSC) - 25 K (T(g,DSC) is the glass transition temperature of bulk polystyrene). At T(g,DSC) - 5 K, the mobility of these two subsets differs by 4 orders of magnitude. These data are interpreted as indicating the presence of a high-mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and equals 7 nm at T(g,DSC).     

Mimics of small ribozymes utilizing a supramolecular scaffold

For elucidating the mechanism of the general acid/base catalysis of the hydrolysis of RNA phosphodiester bonds, a number of cleaving agents having two cyclen moieties tethered to a 1,3,5-triazine core have been prepared and their ability to bind and cleave uridylyl-3',5'-uridine (UpU) studied over a wide pH range. Around neutral pH, the cleaving agents form a highly stable ternary complex with UpU and Zn(II) through coordination of the uracil N3 and the cyclen nitrogen atoms to the Zn(II) ions. Under conditions where the triazine core exists in the deprotonated neutral form, hydrolysis of UpU, but not of adenylyl-3',5'-adenosine (ApA), is accelerated by approximately two orders of magnitude in the presence of the cleaving agents, suggesting general base rather than metal ion catalysis. The probable mechanism of the observed catalysis and implications to understanding the general acid/base-catalyzed phosphodiester hydrolysis by ribozymes are discussed.     

Size-dependent ultrafast electronic energy relaxation and enhanced fluorescence of copper nanoparticles

The energy relaxation of the electrons in the conduction band of 12 and 30 nm diameter copper nanoparticles in colloidal solution was investigated using femtosecond time-resolved transient spectroscopy. Experimental results show that the hot electron energy relaxation is faster in 12 nm copper nanoparticles (0.37 ps) than that in 30 nm copper nanoparticles (0.51 ps), which is explained by the size-dependent electron-surface phonon coupling. Additional mechanisms involving trapping or energy transfer processes to the denser surface states (imperfection) in the smaller nanoparticles are needed to explain the relaxation rate in the 12 nm nanoparticles. The observed fluorescence quantum yield from these nanoparticles is found to be enhanced by roughly 5 orders of magnitude for the 30 nm nanoparticles and 4 orders of magnitude for the 12 nm nanoparticles (relative to bulk copper metal). The increase in the fluorescence quantum yield is attributed to the electromagnetic enhancement of the radiative recombination of the electrons in the s-p conduction band below the Fermi level with the holes in the d bands due to the strong surface plasmon oscillation in these nanoparticles.     

Keto-enol/enolate equilibria in the isochroman-4-one system. Effect of a beta-oxygen substituent.

The enol of 1-tetralone was generated flash photolytically, and rates of its ketonization were measured in aqueous HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4(-), (CH2OH)3CNH3(+), and NH4(+) buffers. The enol of isochroman-4-one was also generated, by hydrolysis of its potassium salt and trimethylsilyl ether, and rates of its ketonization were measured in aqueous HClO4 and NaOH. Rates of enolization of the two ketones were measured as well. Combination of the enolization and ketonization data for isochroman-4-one gave the keto-enol equilibrium constant pK(E) = 5.26, the acidity constant of the enol ionizing as an oxygen acid p = 10.14, and the acidity constant of the ketone ionizing as a carbon acid p = 15.40. Comparison of these results with those for 1-tetralone shows that the beta-oxygen substituent in isochroman-4-one raises all three of these constants: K(E) by 2 orders of magnitude, by not quite 1 order of magnitude, and by nearly 3 orders of magnitude. The beta-oxygen substituent also retards the rate of hydronium-ion-catalyzed ketonization by more than 3 orders of magnitude. The origins of these substituent effects are discussed.     

Thorium nanochemistry: the solution structure of the Th(iv)-hydroxo pentamer

Tetravalent thorium exhibits a strong tendency towards hydrolysis and subsequent polymerization. Polymeric species play a crucial role in understanding thorium solution chemistry, since their presence causes apparent solubility several orders of magnitude higher than predicted by thermodynamic data bases. Although electrospray mass spectrometry (ESI MS) identifies Th(iv) dimers and pentamers unequivocally as dominant species close to the solubility limit, the molecular structure of Th(5)(OH)(y) polymers was hitherto unknown. In the present study, X-ray absorption fine structure (XAFS) spectroscopy, high energy X-ray scattering (HEXS) measurements, and quantum chemical calculations are combined to solve the pentamer structure. The most favourable structure is represented by two Th(iv) dimers linked by a central Th(iv) cation through hydroxide bridges.     

Excitation of vibrations in microtubules in living cells

Microtubules, which are thought to be the primary organizers of the cytoskeleton, are electrical polar structures with extraordinary elastic deformability at low stress and with energy supply from hydrolysis of guanosine triphosphate (GTP) to guanosine diphosphate (GDP). At least a part of the energy supplied from hydrolysis can excite vibrations. Energy is mainly lost by viscous damping of the surrounding cytosol. Viscous damping is diminished by a slip layer which is formed by an attracted ionic charge layer and by a thin surface layer of the microtubule. Relaxation time caused by viscous damping may be several orders of magnitude greater than period of vibrations at 10 MHz. Energy supplied to the microtubule is of the order of magnitude of 10(-14) W cm(-1) (per unit length of the microtubule).     

Aggregation and catalytic activity of 2-hydroxybenzylated polyethyleneimines in water-organic solutions

The aggregation of 2-hydroxybenzylated derivatives of polyethyleneimine in a water—DMF medium in the absence and in the presence of cationic surfactants was studied. The catalytic activity of modified polyethyleneimines in hydrolysis of phosphorus acid esters depends on the pH of the medium and increases with the formation of polymer-colloidal complexes. The polymer-colloidal complexes based on surfactants with two hydroxyethyl substituents in the head group have the maximum catalytic activity (increase in the reaction rate by up to three orders of magnitude).     

Interaction of hydrophobized filaments in aqueous electrolyte solutions

The apparatus developed earlier to measure dispersion forces has been applied in the measurement of the forces of attraction between hydrophobic (θ ≈ 100°) silica filaments immersed in water and the disjoining pressure before the samples are brought into contact, as a function of the distance separating the filaments, H (for gaps H=20 to 60 nm). The attraction forces and disjoining pressure exceeded by one to two orders of magnitude the dispersion forces, and decayed exponentially as the distance increased. The decay length was 12 to 13 nm — considerably larger than for the structural repulsive forces in thin interlayers of polar liquids adjoining lyophilic surfaces.

The result for gaps H<20 nm (the decay length equal to about 3 nm) is in qualitative agreement with the works of Israelachvili, Pashley and Claesson.

The attractive forces decayed still more slowly as the distance increased after the samples' contact position was reached. The presence of hysteresis of forces allows a conclusion to be drawn on the formation of a vapour-gas bubble when the samples are brought into contact. This confirms the Schchukin-Yushchenko statement.     

Supramolecular systems based on aminomethylated calix[4]resorcinarene and a cationic surfactant: Catalysts of the hydrolysis of esters of phosphorus acids

Kinetics of alkaline hydrolysis of 4-nitrophenyl esters of tetracoordinated phosphorus acids in micellar solutions of aminomethylated calix[4]resorcinarene containing sulfonatoethylene groups on the lower rim of the macrocycle, 4-aza-1-hexadecyl-azoniabicyclo[2.2.2]octane bromide and their mixtures was investigated spectrophotometrically. It is established that the catalytic effect of aggregates depends on the concentration of calixarene and surfactants, pH, presence of lanthanum salt and reaches more than two orders of magnitude. The parameters of the catalyzed reactions and their dependence on the composition are determined.     

Catalytic Hydrolysis of Phosphate Diester with Metal Complexes of Macrocyclic Tetraamine in Comicellar Solution

Four novel pyridine or benzene ring‐containing pendant macrocyclic dioxotetraamines 2,6‐dixo‐l,4,7,10‐tetraazacyclododecane ligands have been synthesized. Their metal complexes have been investigated as catalysts for the hydrolysis of bis(p‐nitrophenyl) phosphate (BNPP) in aqueous comicellar solution. The results indicate mat the hydrophobic interaction between substrate and metal complex, the nature of transition metal ion, and the micellar microenvironment are important factors for the hydrolysis of BNPP. Large rate enhancement (up to over two‐three orders of magnitude) employing 5 has been observed.     

The effects of magnetite particles encapsulated in dihexadecyl phosphate (DHP) vesicles on the luminescence characteristics of molecules

Luminescence of organic molecules is observed in DHP vesicles in the absence and presence of magnetite encapsulated in the vesicles. Room temperature phosphorescence calibration curves for carbazole were constructed at 297/440 nm and at 330/440 nm in the absence and presence of magnetite. The linear dynamic range was found to be over 2.5 orders of magnitude. At 297/440 nm, the limits LOD's were found to be 30 ng/ml and 9 ng/ml in the absence and presence of magnetite, respectively. At 330/440 nm, the LOD's were found to be 30 ng/ml both in the presence and absence of magnetite. The effect of the internal magnetic field on the luminescence properties of various molecules is discussed. The precision of the method ranged from ∼ 1 % to ∼ 40%.     

Interfacial reactions in confinement: kinetics and temperature dependence of the surface hydrolysis of polystyrene-block-poly(tert-butyl acrylate) thin films

The effect of confinement on the kinetics of the surface hydrolysis of polystyrene-block-poly(tert-butyl acrylate) (PS(n)-b-PtBA(m)) thin films on oxidized silicon substrates in 3 M aqueous hydrochloric acid was systematically investigated. As shown by X-ray photoelectron spectroscopy (XPS) and contact angle measurements, a skin layer of acid-sensitive PtBA is present on the surface of PS(n)-b-PtBA(m) films, consistent with the lower surface tension of PtBA compared to that of PS. The thickness of the skin layer was determined by angle-dependent XPS as approximately 8 nm for PS(690)-b-PtBA(1210). Tapping mode atomic force microscopy showed an increasing surface coverage of swollen poly(acrylic acid)-rich globules with increasing hydrolysis time. Using ex situ Fourier transform infrared spectroscopy, the reaction kinetics was determined quantitatively as a function of temperature, polymer film thickness, thermal pretreatment of the films, and block copolymer composition. The initial stages of the hydrolysis can be described as a pseudo-first-order reaction under all conditions investigated. The corresponding rate constants were found to be 2 orders of magnitude lower than those reported for the hydrolysis of tert-butyl acetate in solution and depended linearly on the fraction of PtBA exposed at the surface. However, the polymer film thickness, thermal pretreatment of the films, block copolymer composition, and local composition did not affect the rate constants. The negative value of the activation entropy (DeltaS(298)++ = -103 J/mol K), determined according to the Arrhenius equation and transition state theory, indicates that the tightness of the transition state is more pronounced in the PS(n)-b-PtBA(m) film compared to reactions in solution. Thus, the spatial constraints due to the incorporation of the reactive ester groups in thin polymer films are responsible for the observed reduced reactivity.     

Questioning the paradigm of metal complex promoted phosphodiester hydrolysis: [Mo7O24](6-) polyoxometalate cluster as an unlikely catalyst for the hydrolysis of a DNA model substrate

The first example of a phosphodiester bond cleavage promoted by a highly negatively charged polyoxometalate cluster has been discovered: the hydrolysis of the phosphodiester bond in a DNA model substrate bis(p-nitrophenyl)phosphate (BNPP) is promoted by the heptamolybdate anion [Mo7O24](6-) with rates which represent an acceleration of nearly four orders of magnitude compared to the uncatalyzed cleavage.     

Analysis of carbohydrates in wood and pulps employing enzymatic hydrolysis and subsequent capillary zone electrophoresis

An efficient method for determining the carbohydrate composition of extractive-free delignified wood and pulp is described here. The polysaccharides in the sample are first hydrolyzed using a mixture of commercially available preparations of cellulase and hemicellulase. The reducing saccharides in the hydrolysate thus obtained are subsequently derivatized with 4-aminobenzoic acid ethyl ester and thereafter quantitated by capillary zone electrophoresis (CZE) in an alkaline borate buffer with monitoring of the absorption at 306 nm. All reducing sugars (i.e., neutral monosaccharides and uronic acids) which occur as structural elements in the polysaccharides of wood and pulp can be quantitated in a single such analytical run, which can also determine the contents of 4-deoxy-beta-L-threo-hex-4-enopyranosyluronic acid (HexA) residues present in pulps obtained from alkaline processes. CZE analyses were performed using linear regression of standard curves over a concentration range spanning approximately three orders of magnitude. Carbohydrate constituents constituting approximately 0.1% of the dry mass of the sample could be quantitated. The overall precision of this analytical procedure--involving enzymatic hydrolysis, derivatization and CZE--was good (RSD=2.2-7.5%), especially considering the heterogeneity of the wood and pulp samples. The total yield of carbohydrates (93-97%) obtained employing the procedure developed here was consistently higher than that obtained upon applying the traditional procedure for carbohydrate analysis (85-93%) (involving acid hydrolysis and gas chromatographic analysis) to the same pulps. The trisaccharide HexA-xylobiose was the only HexA-containing saccharide detected using the conditions for enzymatic hydrolysis developed here (i.e., 30 h incubation at pH 4 and 40 degrees C); whereas mixtures of HexA-xylobiose and HexA-xylotriose were obtained when the incubation was performed at pH 5 or 6.     

Surface-enhanced Raman scattering in the ultraviolet spectral region: UV-SERS on rhodium and ruthenium electrodes

We report the first observation of surface-enhanced Raman scattering (SERS) excited with ultraviolet (UV) light from transition metal electrodes. Adsorbed pyridine and SCN- on rough rhodium (Rh) and ruthenium (Ru) electrodes, respectively, have been studied using 325 nm laser excitation. In contrast, the best enhancers in the visible and near infrared, silver and gold, do not produce UV-SERS. The experimental data of UV-SERS are in agreement with our preliminary theoretical calculation based on the electromagnetic enhancement mechanism. The enhancement factor is about 2 orders of magnitude for the Rh and Ru electrodes when they are excited at 325 nm.     

Systems based on nonionic amphiphilic compounds: aggregation and catalytic properties

The micellization properties, solubilization capability, and catalytic effect of conventional nonionic surfactants and amphiphilic compounds of oligomeric (Tyloxapol) and polymeric (Synperonic F-68, Pluronic F-127) structure were compared. The systems studied demonstrate a marked catalytic effect toward basic hydrolysis of p-nitrophenyl laurate, which exceeds the effect of aqueous alkali solutions by two orders of magnitude. Correlations between the solubilization capacity of aggregates and their catalytic effect were observed. The maximum efficiency was found for the Tyloxapol solution. The synergetic enhancement of the catalytic effect was observed for the mixed Tyloxapol-cetyltrimethylammonium bromide systems in the presence of small amounts of cationic surfactant     

Assessment of acyl groups and reaction conditions in the competition between perhydrolysis and hydrolysis of acyl resorufins for developing an indicator reaction for fluorometric analysis of hydrogen peroxide

Perhydrolysis of acetyl resorufin (AR) was reported previously to work as a fluorometric indicator reaction for glucose determination using only glucose oxidase. However, hydrolysis of AR in blank solution rendered the working concentration range of this method less than two orders of magnitude. To exclude or at least significantly reduce this interference, acyl groups and reaction conditions in the competition between perhydrolysis and hydrolysis of various acyl resorufins were assessed. Fluorometric evaluation of reactions in the presence or absence of H202 in phosphate buffer (pH 7.5, 100 mm)-CH3CN at 25 degrees C demonstrated that in tert-butylacetyl, isobutyryl, cyclohexanecarbonyl and pivaloyl resorufins (TBAR, IBR, CHR and PVR, respectively) among 10 acyl resorufins examined here, the competitive situation was shifted in a much more favorable way to perhydrolysis than in AR, although fluorometric responses due to their H2O2-dependent deacylation were suppressed in comparison with AR. Examination of the effects of pH, components and concentrations of buffers as well as reaction temperature established reaction conditions that not only allowed perhydrolysis of each of these four compounds to prevail over hydrolysis more effectively, but also improved the H2O2-based fluorometric responses. Thus, perhydrolysis of TBAR, IBR, CHR and PVR in phosphate buffer (pH 8.0, 20 mM)-CH3CN at 25 degrees C worked effectively as fluorometric indicator reactions for H2O2 analysis, affording a calibration curve over a concentration range of three orders of magnitude. Taking sensitivity, reproducibility and the response for blank solution into consideration, PVR seemed to be the best choice as a fluorochromogen for H2O2 determination under these conditions. For H2O2 analysis at lower pH, perhydrolysis of IBR in phosphate buffer (pH 7.5, 20 mm)-CH3CN was shown to effectively function as an indicator reaction. Applicability of the fluorometric methods with PVR and IBR to blood glucose determination was also discussed, comparing with Trinder's method with phenol, 4-aminoantipyrine and peroxidase (POD).     

Inverted microcontact printing on polystyrene-block-poly(tert-butyl acrylate) films: a versatile approach to fabricate structured biointerfaces across the length scales

The combination of the recently introduced soft lithographic technique of inverted microcontact printing (i-muCP) and spin-coated films of polystyrene- block-poly( tert-butyl acrylate) (PS 690- b-P tBA 1210) as a reactive platform is shown to yield a versatile approach for the facile fabrication of topographically structured and chemically patterned biointerfaces with characteristic spacings and distances that cross many orders of magnitude. The shortcomings of conventional muCP in printing of small features with large spacings, due to the collapse of small or high aspect ratio stamp structures, are circumvented in i-muCP by printing reactants using a featureless elastomeric stamp onto a topographically structured reactive polymer film. Prior to molecular transfer, the substrate-supported PS 690- b-P tBA 1210 films were structured by imprint lithography resulting in lateral and vertical feature sizes between >50 microm-150 nm and >1.0 microm-18 nm, respectively. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and water contact angle measurements provided evidence for the absence of surface chemical transformations during the imprinting step. Following the previously established hydrolysis and activation protocol with trifluoroacetic acid and N-hydroxysuccinimide, amino end-functionalized poly(ethylene glycol) (PEG-NH 2), as well as bovine serum albumin and fibronectin as model proteins, were successfully transferred by i-muCP and coupled covalently. As shown, i-muCP yields increased PEG coverages and thus improved performance in suppressing nonspecific adsorption of proteins by exploiting the high local concentrations in the micro- and nanocontacts during molecular transfer. The i-muCP strategy provides access to versatile biointerface platforms patterned across the length scales, as shown for guided cancer cell adhesion, which opens the pathway for systematic cell-surface interaction studies.