Transforming a Stable Amide into a Highly Reactive One: Capturing the Essence of Enzymatic Catalysis.

Aspartic proteinases, which include HIV-1 proteinase, function with two aspartate carboxy groups at the active site. This relationship has been modeled in a system possessing an otherwise unactivated amide positioned between two carboxy groups. The model amide is cleaved at an enzyme-like rate that renders the amide nonisolable at 35 °C and pH 4 owing to the joint presence of carboxy and carboxylate groups. A currently advanced theory attributing almost the entire catalytic power of enzymes to electrostatic reorganization is shown to be superfluous when suitable interatomic interactions are present. Our kinetic results are consistent with spatiotemporal concepts where embedding the amide group between two carboxylic moieties in proper geometries, at distances less than the diameter of water, leads to enzyme-like rate enhancements. Space and time are the essence of enzyme catalysis.

Imidazolium-based zwitterionic surfactants: characterization of normal and reverse micelles and stabilization of nanoparticles.

This paper presents the physicochemical properties of micellar aggregates formed from a series of zwitterionic surfactants of the type 3-(1-alkyl-3-imidazolio)propane-sulfonate (ImS3-n), with n = 10, 12, 14, and 16. The ImS3-n dipolar ionic surfactants represent a versatile class of dipolar ionic compounds, which form normal and reverse micelles. Furthermore, they are able to stabilize nanoparticles in water and in organic media. Aqueous solubility is too low at room temperature to allow characterization of micellar aggregates but increases with addition of salts, allowing determination of aggregation number and cmc. As expected, these parameters depend on the length of the alkyl chain, and cmc values follow Klevens equation. In the presence of NaClO4, all ImS3-n micelles become anionoid by incorporating ClO4(-) on the micellar interface. A special feature of these surfactants is the ability to form reverse micelles and solubilize copious amounts of saline solutions in chloroform. (1)H NMR and infrared spectroscopic evidence showed that the maximum water to surfactant molar ratio w0 achievable depends on the concentration and type of salt dissolved. Reverse micelles of the ImS3-n surfactants can be used to stabilize metallic nanoparticles, whose size may be tuned by the amount of water dissolved.

Exploring the charged nature of supramolecular micelles based on p-sulfonatocalix[6]arene and dodecyltrimethylammonium bromide.

The aggregation of supramolecular amphiphiles formed from hexamethylated p-sulfonatocalix[6]arene (SC6HM) and dodecyltrimethylammonium bromide (C12TAB) was studied by capillary electrophoresis experiments and by kinetic probes. The hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC) was used to investigate the micropolarity of the micellar aggregates and their ability to solubilize and stabilize labile organic compounds against hydrolysis. Further insights were obtained using a more sophisticated kinetic probe: the basic hydrolysis of p-nitrophenylvalerate (NPV). This probe provides information on the ionic composition of the micellar interface and on the potential of the aggregates to be used as nanoreactors. The results obtained revealed that the charge of the micellar aggregates can be tuned from anionic to cationic through the adjustment of the C12TAB : SC6HM molar ratio and confirmed that these micelles have good solubilization properties. On the other hand, the kinetics of the p-nitrophenylvalerate basic hydrolysis suggest that, in the concentration range comprised between the first and second CMCs, Br(-) anions do not take part in the micellar structure.

Final treatment of spent batteries by thermal plasma.

The growth in the use of wireless devices, notebooks and other electronic products has led to an ever increasing demand for batteries, leading to these products being commonly found in inappropriate locations, with adverse effects on the environment and human health. Due to political pressure and according to the environmental legislation which regulates the destination of spent batteries, in several countries the application of reverse logistics to hazardous waste is required. Thus, some processes have been developed with the aim of providing an appropriate destination for these products. In this context, a method for the treatment of spent batteries using thermal plasma technology is proposed herein. The efficiency of the method was tested through the determination of parameters, such as total organic carbon, moisture content and density, as well as analysis by atomic absorption spectrometry, scanning electron microscopy and X-ray fluorescence using samples before and after inertization. The value obtained for the density was 19.15%. The TOC results indicated 8.05% of C in the batteries prior to pyrolisis and according to the XRF analysis Fe, S, Mn and Zn were the most stable elements in the samples (highest peaks). The efficiency of the paste inertization was 97% for zinc and 99.74% for manganese. The results also showed that the most efficient reactor was that with the DC transferred arc plasma torch and quartzite sand positively influenced by the vitrification during the pyrolysis of the electrolyte paste obtain from batteries.

Zwitterionic-surfactant-stabilized palladium nanoparticles as catalysts in the hydrogen transfer reductive amination of benzaldehydes.

Palladium nanoparticles (NPs) stabilized by a zwitterionic surfactant are revealed here to be good catalysts for the reductive amination of benzaldehydes using formate salts as hydrogen donors in aqueous isopropanol. In terms of environmental impact and economy, metallic NPs offer several advantages over homogeneous and traditional heterogeneous catalysts. NPs usually display greater activity due to the increased metal surface area and sometimes exhibit enhanced selectivity. Thus, it is possible to use very low loadings of expensive metal. The methodology eliminates the use of a hydrogen gas atmosphere or toxic or expensive reagents. A range of aromatic aldehydes were converted to benzylamines when reacted with primary and secondary amines in the presence of the Pd NPs, which also displayed good activity when supported on alumina. In every case, the Pd NPs could be easily recovered and reused up to three more times, and at the end of the process, the product was metal-free.

Simultaneous nondestructive analysis of palladium, rhodium, platinum, and gold nanoparticles using energy dispersive X-ray fluorescence.

A selective method is proposed for the determination of palladium, gold, and sulfur in catalytic systems, by direct liquid analysis using energy dispersive X-ray fluorescence (EDXRF), under an atmosphere of helium or air. This method allows a nondestructive analysis of palladium, rhodium, platinum, and gold nanoparticulate catalysts stabilized by imidazolium propane sulfonate based zwitterionic surfactants, allowing the samples to be reused for catalytic studies. The signals from palladium, rhodium, platinum, and gold samples in the presence of imidazolium propane sulfonate-based zwitterionic surfactants obtained using EDXRF before (Pd(2+), Rh(2+), Pt(2+), and Au(3+)) and after (Pd(0), Rh(0), Pt(0), and Au(0)) formation of nanoparticles are essentially identical. The results show that the EDXRF method is nondestructive and allows detection and quantification of the main components of platinum, gold, rhodium, and palladium NPs, including the surfactant concentration, with detection and quantification limits in the range of 0.4-3 mg L(-1). The matrices used in such samples present no problems, even allowing the detection and quantification of interfering elements.

Surface charge of zwitterionic sulfobetaine micelles with 2-naphthol as a fluorescent probe.

2-Naphthol (2NOH) was used as a fluorescent probe in order to examine and quantify the changes in hydrogen ion concentration in micelles formed by the zwitterionic 3-(tetradecyldimethylammonium)-propanesulfonate (SB3-14) surfactant or by anionic sodium dodecyl sulfate (SDS). In the presence of SDS, 2NOH is incorporated into the anionic micelle and the neutral form of the probe becomes the dominant species. The results are consistent with a microenvironment probably with a higher acidity and/or lower polarity in the micellar surface. The addition of SB3-14 generates a plateau at pH 3 to 9 with a fluorescent component of low intensity, which indicates the partial formation of 2NO(-)*, promoted by proton transfer to water. Theoretical results provided information on the structural parameters, emission wavelength, and changes in ΔpK(a) values due to the solvent, which are consistent with a solubilization site similar to aqueous ethanol. Zwitterionic surfactants concentrate anions such as trifluoroacetate in zwitterionic micelles, and as a result, the micellar surface charge becomes negative and promotes hydrogen ion incorporation into the micellar surface. Effects observed on the proton transfer between 2NOH* and anions in zwitterionic micellar solutions are complex and, besides the well-known anion incorporation, include changes in the surface potential and acidity of the surface. Zwitterionic micelles are able to emulate the mostly zwitterionic nature of biological membranes, and in the complex nature of zwitterionic micelles, we found reasons for the selection of zwitterionic headgroups in surfactants in natural systems as major components of biological interfaces.

The chameleon-like nature of zwitterionic micelles: effect of cation binding.

Addition of salts, especially perchlorates, to zwitterionic micelles of SB3-14, C(14)H(29)NMe(2)(+)(CH(2))(3)SO(3)(-), induces anionic character and uptake of H(3)O(+) by SB3-14 micelles. Thus, the addition of alkali metal perchlorates accelerates the acid hydrolysis of 2-(p-heptoxyphenyl)-1,3-dioxolane, HPD, in the presence of SB3-14 micelles, which depends on the local proton concentration at the micelle surface. The addition of metal chlorides to solutions of such perchlorate-modified SB3-14 micelles decreases both the negative zeta potential of the micelles and the observed rate constant for acid hydrolysis of HPD. The effect of the monovalent cations Li(+), Na(+), and K(+) is smaller than that of the divalent cations Be(2+), Mg(2+), and Ca(2+), and much smaller than that of the trivalent cations Al(3+), La(3+), and Er(3+). The major factor responsible for this cation valence dependence of these effects is shown to be electrostatic in nature, reflecting the strong dependence of the micellar surface potential on the cation valence. The fact that the salt effects are not identical after correction for the electrostatic effects indicates that additional secondary nonelectrostatic effects may contribute as well.

Polymers containing hydroxamate groups: nanoreactors for hydrolysis of phosphoryl esters.

A polyhydroxamicalkanoate (PHA) polymer containing the functional groups hydroxamic acid and carboxylic acid with the ability to accelerate dephosphorylation reactions is proposed. The methodology used to prepare this polymer favored the position of the two functional groups next to each other, which allows for the cooperativity between these groups. This cooperative effect has an important role when one wants to mimic enzymes. The catalytic effect promoted by the polymer was evaluated on the cleavage of the bis(2,4-dinitrophenyl) phosphate (BDNPP) and diethyl 2,4-dinitrophenyl phosphate (DEDNPP) esters. Indeed, PHA was very efficient and promiscuous because it increased the rate of both reactions by a factor of up to 10(6)-fold. Isothermal titration calorimetry (ITC) experiments showed that the presence of PHA aids the formation of cetyltrimethylammonium bromide (CTABr) micelles. Thus, the effect of the cationic surfactant CTABr on the dephosphorylation of BDNPP by PHA was also investigated, and it was observed that, when CTABr is added to PHA, the reaction is ca. 15-fold faster compared to the reaction when only PHA is present.

Mechanism of intramolecular catalysis in the hydrolysis of alkyl monoesters of 1,8-naphthalic acid.

Hydrolysis of alkyl 1,8-naphthalic acid monoesters 1a-d is subject to highly efficient intramolecular nucleophilic catalysis by the neighboring COOH group. The reactivity for the COOH reaction depends on the leaving group pK(a), with values of ß(LG) of -0.50, consistent with a mechanism involving rate determining breakdown of tetrahedral addition intermediates. The release of the steric strain of the peri-substitiuents in the highly reactive alkyl 1,8-naphthalic acid monoesters is fundamental to understand the observed special reactivity in this intramolecular reaction. DFT calculations show how the proton transfers involved in the cleavage of the neutral ester can be catalyzed by solvent water, thus facilitating the departure of poor alkoxide leaving groups.

Ionic quenching of naphthalene fluorescence in sodium dodecyl sulfate micelles.

Micellar effects on luminescense of organic compounds or probes are well established, and here we show that quenching is highly favored in aqueous sodium dodecyl sulfate (SDS) micelles, which concentrate a naphthalene probe and cations of lanthanides, transition metals, and noble metals. Interactions have been studied by steady state and time-resolved fluorescence in examining the fluorescence suppression of naphthalene by metal ions in anionic SDS micelles. The quenching is collisional and correlated with the unit charge and the reduction potential of the metal ion. The rate constants, calculated in terms of local metal ion concentrations, are close to the diffusion control limit in the interior of SDS micelles, where the microscopic viscosity decreases the transfer rate, following the Stokes-Einstein relation.

The role of hydrophobicity in supramolecular polymer/surfactant catalysts: An understandable model for enzymatic catalysis.

Enzymes are highly significant catalysts, essential to biological systems, and a source of inspiration for the design of artificial enzymes. Although many models have been developed describing enzymatic catalysis, a deeper understanding of these biocatalysts remains a major challenge. Herein we detail the formation, characterization, performance, and catalytic mechanisms of a series of bio-inspired supramolecular polymer/surfactant complexes acting as artificial enzymes. The supramolecular complexes were characterized and exhibited exceptional catalytic efficiency for the dephosphorylation of an activated phosphate diester, the reaction rate being highly responsive to: (a) pH, (b) surfactant concentration, and (c) the length of the hydrophobic chain of the surfactant. Under optimal conditions (at pH > 8 for the more hydrophobic systems and at pre-micellar concentrations), enzyme-like rate enhancements of up to 6.0 × 109-fold over the rate of the spontaneous hydrolysis reaction in water were verified. The catalytic performance is a consequence of synergy between the hydrophobicity of the aggregates and the catalytic functionalities of the polymer and the catalytic mechanism is modulated by the nature of the hydrophobic pockets of these catalysts, changing from a general base mechanism to a nucleophilic mechanism as the hydrophobicity increases. Taken as a whole, the present results provide fundamental insights, through an understandable model, which are highly relevant to the design of novel bioinspired enzyme surrogates with multifunctional potentialities for future practical applications.

Anion-specific binding to n-hexadecyl phosphorylcholine micelles.

Hexadecyl phosphorylcholine (HPC) micelles incorporate anions rather than cations in the interfacial region, giving an anionoid micelle with a negative zeta potential. Hydronium ion incorporation in the micellar pseudophase parallels the increase in the negative zeta potential, and salts increase the rate of A1 hydrolysis of 2-(p-heptyloxyphenyl)-1,3-dioxolane in micellized HPC and inhibit the reaction of OH(-) with naphthoic anhydride. The kinetic effects are larger with NaClO(4) than with NaCl. The increased micellar negative charge with added salts increases the repulsion between headgroups and decreases the aggregation number. These observations are relevant to understanding the behaviors of biological phosphorylcholine amphiphiles.

Synthesis of a new zwitterionic surfactant containing an imidazolium ring. Evaluating the chameleon-like behavior of zwitterionic micelles.

Synthesis of a new zwitterionic surfactant containing the imidazolium ring 3-(1-tetradecyl-3-imidazolio)propanesulfonate (ImS3-14) is described. The solubility of ImS3-14 is very low but increases on addition of a salt which helps to stabilize the micellized surfactant. Fluorescence quenching and electrophoretic evidence for ImS3-14 shows that the micellar aggregation number is only slightly sensitive to added salts, as is the critical micelle concentration, but NaClO(4) markedly increases zeta potentials of ImS3-14 in a similar way as in N-tetradecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-14) micelles. The rate of specific hydrogen ion catalyzed hydrolysis of 2-(p-heptoxyphenyl)-1,3-dioxolane and equilibrium protonation of 1-hydroxy-2-naphthoate ion in zwitterionic micelles of ImS3-14 and SB3-14 are increased markedly by NaClO(4) which induces anionoid character and uptake of H(3)O(+), but NaCl is much less effective in this respect. Comparison of ImS3-14 with SB3-14 is based on experimental evidence, and computational calculations indicate similarities and differences in structures of both compounds.

The mechanism of dephosphorylation of bis(2,4-dinitrophenyl) phosphate in mixed micelles of cationic surfactants and lauryl hydroxamic acid.

Mixed micelles of cetyltrimethylammonium bromide (CTABr) or dodecyltrimethylammonium bromide (DTABr) and the alpha-nucleophile, lauryl hydroxamic acid (LHA) accelerate dephosphorylation of bis(2,4-dinitrophenyl)phosphate (BDNPP) over the pH range 4-10. With a 0.1 mole fraction of LHA in DTABr or CTABr, dephosphorylation of BDNPP is approximately 10(4)-fold faster than its spontaneous hydrolysis, and monoanionic LHA(-) is the reactive species. The results are consistent with a mechanism involving concurrent nucleophilic attack by hydroxamate ion (i) on the aromatic carbon, giving an intermediate that decomposes to undecylamine and 2,4-dinitrophenol, and (ii) at phosphorus, giving an unstable intermediate that undergoes a Lossen rearrangement yielding a series of derivatives including N,N-dialkylurea, undecylamine, undecyl isocyanate, and carbamyl hydroxamate.

Suicide nucleophilic attack: reactions of benzohydroxamate anion with bis(2,4-dinitrophenyl) phosphate.

The reaction between the benzohydroxamate anion (BHO(-)) and bis(2,4-dinitrophenyl)phosphate (BDNPP) has been examined kinetically, and the products were characterized by mass and NMR spectroscopy. The nucleophilic attack of BHO(-) follows two reaction paths: (i) at phosphorus, giving an unstable intermediate that undergoes a Lossen rearrangement to phenyl isocyanate, aniline, diphenylurea, and O-phenylcarbamyl benzohydroxamate; and (ii) on the aromatic carbon, giving an intermediate that was detected but slowly decomposes to aniline and 2,4-dinitrophenol. Thus, the benzohydroxamate anion can be considered a self-destructive molecular scissor since it reacts and loses its nucleophilic ability.

2-All-yloxy-5-nitro-benzoic acid.

The mol-ecule of the title compound, C(10)H(9)NO(5), is approximately planar, with the mean planes of the nitro, carboxyl and all-yloxy groups rotated by 8.1 (3), 7.9 (3) and 4.52 (18)°, respectively, from the plane of the benzene ring. Bond lengths in the aromatic ring are influenced by both electronic effects and strain induced by ortho-substitution. In the crystal structure, centrosymmetrically related mol-ecules are paired into dimers through strong O-H⋯O hydrogen bonds.

The chameleon-like nature of zwitterionic micelles: the intrinsic relationship of anion and cation binding in sulfobetaine micelles.

The rate of specific hydrogen ion-catalyzed hydrolysis of 2-( p-heptoxyphenyl)-1,3-dioxolane and acid-base equilibrium of 4-carboxy-1-n-dodecylpyridinium in zwitterionic micelles of SB3-14, C14H29NMe2+(CH2)3SO3(-) are controlled by NaClO4, which induces anionic character and uptake of H3O+ in the micelles. Other salts, e.g., NaF, NaCl, NaBr, NaNO3, NaI, NaBF4, have similar, but smaller, effects on the uptake of H3O+. Salt effects upon zeta potentials of SB3-14 micelles, estimated by capillary electrophoresis, are anion specific, and the anion order is similar to that of the rates of acid hydrolysis and of acid-base equilibria. Fluorescence quenching shows that the micellar aggregation number is not very sensitive to added salts, consistent with electrophoretic evidence. These specific anion effects follow the Hofmeister series and are related to anion hydration free energies.

Fluorescence of Zn(II) 8-hydroxyquinoline complex in the presence of aqueous micellar media: The special cetyltrimethylammonium bromide effect.

Mixtures of Zn(II) and 8-hydroxyquinoline (8QOH) in a 1:2 proportion, in aqueous solutions, result in fast complexation, followed by precipitation. Addition of 0.05 M sodium dodecyl sulfate (SDS), N-dodecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-12), N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-16) or Triton X-100 results in considerable retardation of precipitation. In the presence of SDS, SB3-12, SB3-16 and Triton X-100 the 8QOH chelates are only kinetically stable in solution and after 24 h, the precipitation is almost quantitative. Conversely, upon addition of the cationic surfactant hexadecyltrimethylammonium bromide (CTABr), the absorbance of the complex remains constant even after at least six months. The interaction of the ligand 8QOH (and of the (8QO)(2)Zn(II) complex) with the cationic surfactant was studied by ultraviolet and NMR spectroscopy and 8QOH has a pK(a)=9.05 in the presence of the cationic surfactant and the ligand intercalates in the micelle, being preferentially located near the headgroup of the micelle. Although the solubilization site of the (8QO)(2)Zn(II) complex is similar to that of 8QOH, the interaction of the aromatic moiety with the CTA(+) headgroup is much stronger, due to the increased electron density in the aromatic ring of the ligand. As a consequence of this interaction, sphere to rod transition and an increase in microscopic and macroscopic viscosity are observed.

Distribution of hexavalent Cr species across the clay mineral surface-water interface.

The adsorption isotherms of Cr(VI) on kaolinite, montmorillonite, and alumina were adequately treated with Langmuir model showing behavior characteristic of single-layer adsorption. The efficiency of the adsorbents in removing Cr(VI) from water follows the order alumina > kaolinite > montmorillonite > silica. Speciation studies indicate that hydrogen chromate ions were the major adsorbed species and simultaneous adsorption of dichromate ion occurred at concentrations greater than approximately 10(-3) mol L(-1). It is most probable that the mechanism of adsorption of the hydrogen chromate ion at the surface of alumina is predominantly electrostatic adsorption, with outer sphere complex formation.