Neat Aqueous Solutions Research Articles

Test‐Strip‐Based Fluorometric Detection of Fluoride in Aqueous Media with a BODIPY‐Linked Hydrogen‐Bonding Receptor

The measurement of biologically relevant anions, such as fluoride, is an important task in analytical chemistry, in particular, for dental health and osteoporosis. Although a large number of fluoride probes are known, the applicability under relevant conditions is limited to a few examples. To improve this situation, BODIPY-amidothiourea dyes with varying hydrogen-bond donating strengths were developed, the most H-acidic of which (1 c) could detect F(-) from an inorganic source (NaF) in 50 % aqueous solution (DMSO/water 1:1, v/v) with 0.01 ppm sensitivity through selective fluorescence quenching by a photoinduced electron-transfer (PET) process. Use of the probe and a reference dye with a test-strip assay and a portable and rapidly recording lateral-flow fluorescence reader made determination of F(-) in neat aqueous solutions, such as spiked water samples and toothpaste extracts, possible in a self-referenced manner, achieving a detection limit of 0.2 ppm.

Ground and excited state prototropism of 2-(4′-pyridyl)benzimidazole in micelles

Abstract The effect of micelles on ground and excited state prototropism of 2-(4′-pyridyl)benzimidazole (4PBI) has been studied by fluorescence spectroscopy. It has been observed that in sodium dodecyl sulfate (SDS) micelle, 4PBI is preferentially protonated at the pyridyl nitrogen atom, in contrast to the situation in neat aqueous solutions, in which imidazole and pyridyl nitrogen atoms have equal propensity for protonation. In SDS, the protonation of the benzimidazole nitrogen occurs only in the excited state. Such effects are not observed in cationic cetyl trimethyl ammonium bromide (CTAB) and neutral Triton X-100 (TX-100). Thus, 4PBI shows very good sensitivity for negatively charged interfaces between aqueous and apolar compartments in microheterogeneous media.

(2,2′-Bipyridyl)-3-3′-diol in Nafion: Stabilization of Unusual Ground and Excited States

(2,2'-Bipyridyl)-3-3'-diol (BP(OH)(2)) undergoes excited state intramolecular double proton transfer (ESIDPT). The photophysics of this molecule has been investigated, in the highly acidic water channels inside nafion membrane. The dianionic enolate form of the dye, which is formed only in alkaline conditions in neat aqueous solutions, is found to be present in its excited state in native nafion membrane at low water content. This surprising phenomenon has been explained in the light of loss of protons from the fluorophore to the medium, due to increased electrostatic interactions with the pendant sulfonate groups of nafion, at the lower water content. This observation fortifies the model of microscopic interactions that we have proposed in recent past. Further, the ground state of the diketo form, observed only in neat aqueous solutions so far, is found to be present in Na(+)-exchanged membranes at lower levels of hydration. The steady-state spectral features of the dye is different in (CH(3))(4)N(+)-exchanged membranes than that of Na(+)-exchanged membranes, unlike in our earlier studies with coumarin 102 and 2-(2'-pyridyl)benzimidazole, here we observed formation of an unusual ground state in Na(+)-exchanged membranes, while no such feature was observed in (CH(3))(4)N(+)-exchanged membranes.

Effect of Urea on the Ion-Association Capillary Electrophoresis Separation of the Anionic Metal Complexes Using Hydrophobic Tetraalkylammonium as Ion-Association Agent

The effect of urea as an electrophoretic buffer solution modifier on the ion-association (IA) capillary electrophoresis (CE) separation of four anionic metal complexes of Al(III), Co(III), Cr(III), and Fe(III) with 2,2'-dihydroxyazobenezene-5,5'-disulfonate (DHABS) using a hydrophobic ion-association agent, tetrapentylammonium bromide, was studied. The mutual separation of the four anionic metal-DHABS complexes was not achieved without the addition of urea in the electrophoretic buffer solution. However, the addition of 1.5 M urea in the electrophoretic buffer solution brought about a complete separation of the four metal complexes. The ion-association constants between all metal-DHABS complexes and tetrapentylammonium in an aqueous urea solution were smaller than those in a neat aqueous solution. This indicates the hydrophobic interaction contributing to the ion-association between analytes and ion-association agent during IA-CE separation processes can be controlled with the addition of urea to the electrophoretic buffer solution. Another advantage of adding urea was a substantial enhancement of separation efficiency with a reduction of the half-bandwidth of the peaks. Also, a reduction of the electrophoretic mobility of the electroosmotic flow when urea was added was much less than when organic solvents were used.

Early events associated with the excited state proton transfer in 2-(2′-pyridyl)benzimidazole

2-(2(')-Pyridyl)benzimidazole (2PBI) undergoes excited state proton transfer (ESPT) in acidic solutions, leading to a tautomer emission at 460 nm. This photoprocess has been studied using ultrafast fluorescence spectroscopic techniques in acidic neat aqueous solutions, in viscous mixtures of glycerol with water, as well as in sucrose solutions. The tautomer is found to be stabilized in the more viscous medium, leading to a greater relative quantum yield as well as lifetime. The long rise time in tautomer emission is not affected by viscosity though. Rather, it appears to have the same value as the long component of the decay of the cationic excited state (C(*)). In addition to the subnanosecond lifetime reported earlier, C(*) is found to exhibit a decay time of 2 ps. This is assigned to its protonation to form the nonfluorescent dication in its excited state (D(*)) considering the ground and excited state pK(a) values reported earlier. An additional rising component of 100 ps is observed in the region of C(*) emission. This is likely to arise from a structural change or charge redistribution in C(*) immediately after its creation and before the phototautomerization.

An enigmatic peptide ligation reaction: protease-catalyzed oligomerization of a native protein segment in neat aqueous solution.

We report an enigmatic peptide ligation reaction catalyzed by Glu-specific Staphylococcus aureus V8 protease that occurs in neat aqueous solution around neutral pH utilizing a totally unprotected peptide substrate containing free alpha-carboxyl and alpha-amino groups. V8 protease catalyzed a chain of ligation steps between pH 6 and 8 at 4 degrees C, producing a gamut of covalent oligomers (dimer through octamer or higher) of a native protein segment TAAAKFE (S39) derived from ribonuclease A (RNAse A). Size-exclusion chromatography suggested the absence of strong interaction between the reacting peptides. The circular dichroism spectra of monomer through pentamer showed length-dependent enhancement of secondary structure in the oligomers, suggesting that protease-catalyzed ligation of a monomer to an oligomer resulted in a product that was more structured than its precursor. The relative conformational stability of the oligomers was reflected in their ability to resist proteolysis, indicating that the oligomerization reaction was facilitated as a consequence of the "conformational trapping" of the product. The ligation reaction proceeded in two phases-slow formation and accumulation of the dimer followed by a fast phase of oligomerization, implying that the conformational trap encountered in the oligomerization reaction was a two-step process. The Gly substitution at any position of the TAAAKFE sequence was deleterious, suggesting that the first step of the conformational trap, namely the dimerization reaction, that proceeded very slowly even with the parent peptide, was quite sensitive to amino acid sequence. In contrast, the oligomerization reaction of an Ala analog, AAAAKFE, occurred in much the same way as S39, albeit with faster rate, suggesting that Ala substitution stabilized the overall conformational trapping process. The results suggest the viability of the product-directed "conformational trap" as a mechanism to achieve peptide ligation of totally unprotected peptide fragments in neat aqueous solution. Further, the study projects the presence of considerable innate synthetic potential in V8 protease, baring rich possibilities of protein engineering of this enzyme to generate a "V8 peptide ligase."

A colorimetric molecular probe for Cu(ii) ions based on the redox properties of Ru(ii) phthalocyanines

The one-electron oxidation of a Ru(II) phthalocyanine induces a dramatic colour change and provides a useful tool for the selective and highly sensitive colorimetric detection of copper(II) ions in neat aqueous solutions.

Formation of α-ω(4,7,10,13-pentaoxa-16-azacyclooctadecane) hexadecane micelles in aqueous solution – effect of HCl addition

The structure of aggregates of a new bolaform surfactant [α-ω (4,7,10,13-pentaoxa-16-azacyclooctadecane) hexadecane] in neat aqueous solution has been investigated by small-angle neutron scattering. Preliminary data analysis unequivocally indicates that globular micelles are formed. This finding has been confirmed by the analysis of data obtained upon addition of hydrochloric acid. The observed significant reduction of micelle aggregation number has been attributed to the peculiar complexing ability of the two aza-crown ether units in the surfactant molecule; this is in line with information inferred from the complexation of ions into the non alkylated azacrown ethers. As a consequence of hydrogen ion uptake into the aza-crown ether unit, the partial ionization of surfactant causes a decrease in the micelle dimension that follows the increased repulsion among head groups.

Photophysics of aggregated 9-methylthiacarbocyanine bound to polyanions

The photophysical properties of 3,3 ′-diethyl-9-methylthiacarbocyanine (DTC) were studied in the presence of polystyrene sulfonate (PSS), polyacrylic acid (PAA) and polymethacrylic acid (PMA). The absorption spectra reflect a monomer/dimer equilibrium in neat aqueous solution and a shift towards bound H-aggregates, bound dimers and bound monomers on increasing the ratio of polyanion residue to dye concentrations ( r). These equilibria also determine the photodeactivation modes of DTC. The fluorescence intensity is reduced, when dimers and aggregates are present and strongly enhanced for low dye loading ( r=10 4). In contrast, the quantum yield of intersystem crossing is enhanced for bound dimers ( r=10 3).

Photodecarboxylation of phenylglyoxylic acid: influence of para-substituents on the triplet state properties

The photochemistry of several para-substituted derivatives of phenylglyoxylic acid (R–PA, R = OCH3, CH3, F, Cl, Br and CN) was studied in polar solvents. The triplet state was detected by laser flash photolysis and phosphorescence in acetonitrile, acetone or acetic acid at room temperature and by phosphorescence in polar glasses at –196 °C. In fluid media the phosphorescence intensity and the triplet lifetime are both reduced on addition of propan-2-ol or water. The rate constant for H-atom abstraction by the triplet of the R–PA from propan-2-ol, 2 × 104–8 × 107 dm3 mol–1 s–1 in acetonitrile, increases with the electron-accepting power of the substituent. The intermediate is the corresponding α-carboxy-α-hydroxybenzyl radical. The main transient in neat aqueous solution is the triplet state of the corresponding benzaldehyde as secondary intermediate. Based on results of triplet quenching by water in mixtures with acetonitrile and of time-resolved conductivity measurements, heterolytic α-splitting of the triplet anion involving the benzoyl anion is suggested. Photodecarboxylation occurs in substantial yield for R = CH3, F, Cl, Br and CN in acetonitrile and Φ(–CO2) is enhanced in the presence of water, Φ(–CO2) = 0.4–0.8 at 5–20% H2O. The values are larger than in neat aqueous solution, where a decreasing dependence vs. pH was generally found. A reaction scheme is presented accounting for the observed intermediates and the dependence of Φ(–CO2) on the water content and the pH.

Effect of Organic Cosolvents on Enantio-Enrichments via Cyclodextrin-Based Precipitations: An Examination of Production Efficiency

Crystallization or precipitation of cyclodextrins in the presence of racemates has proven to be a useful method to enrich enantiomers. The technique is commonly performed in neat aqueous solutions which limits its utility in resolving many hydrophobic compounds with poor water solubility. In this work we demonstrate that it is possible to enhance the enantiomeric enrichment of (±)-3-dimethyl-aminopropiophenone using organic cosolvents via β-cyclodextrin precipitation. In addition, an equation that allows the calculation of the production efficiency of an enantiomeric enrichment is proposed and utilized. Water plus various organic solvents, including methanol, ethanol, isopropanol, 1-propanol, tert-butanol, and acetonitrile, are studied as cosolvents. Several factors are investigated in terms of their effect on the enantiomeric enrichment including a) molar ratio of β-cyclodextrin to analyte, b) solubility of the analyte, c) solubility of β-cyclodextrin, d) solution pH, and e) amount of analyte co...

D2O–H2O isotope effect on nuclear magnetic relaxation of alkali halide nuclei and preferential solvation in mixed solvents

The effect of substitution of H2O by D2O on the nuclear magnetic relaxation of the ionic nuclei 23Na, 87Rb and 81Br, which all relax by quadrupole interaction, has been investigated in several NaBr and RbBr solutions. The solvents were neat water and binary mixtures of water with methanol, formamide and dimethylsulphoxide. In the neat aqueous solutions it was found experimentally that the isotope effect is similar to that on the viscosity and that even at high salt concentrations, where all water is “hydration water”, this effect is still relatively strong. In the mixed solvents the composition dependence of the isotope effect on the relaxation rates has been studied in the water rich region, showing e.g. in methanol + water mixtures a quite different behaviour for cationic and anionic nuclei. A new method is presented, which, with certain assumptions, allows us to calculate the local mole fraction of water in the inner solvation sphere of the ions from the measured D2O–H2O isotope effect as a function of the macroscopic solvent composition. The analysis yielded the result that Na+ and Rb+ in methanol + water and to a smaller extent also Rb+ in formamide + water are preferentially hydrated. In the same way it was found that Br– is weakly preferentially solvated by methanol and Na+ by dimethylsulphoxide, whereas Br– is not selectively solvated in formamide + water. The results are discussed and compared with those obtained by other methods.