Where Are Sodium Ions in AOT Reverse Micelles? Fluoride Anion Probes Nanoconfined Ions by 19F Nuclear Magnetic Resonance Spectroscopy.

Confining water to nanosized spaces creates a unique environment that can change water's structural and dynamic properties. When ions are present in these nanoscopic spaces, the limited number of water molecules and short screening length can dramatically affect how ions are distributed compared to the homogeneous distribution assumed in bulk aqueous solution. Here, we demonstrate that the chemical shift observed in 19F NMR spectroscopy of fluoride anion, F-, probes the location of sodium ions, Na+, confined in reverse micelles prepared from AOT (sodium dioctyl sulfosuccinate) surfactants. Our measurements show that the nanoconfined environment of reverse micelles can lead to extremely high apparent ion concentrations and ionic strength, beyond the limit in bulk aqueous solutions. Most notably, the 19F NMR chemical shift trends we observe for F- in the reverse micelles indicate that the AOT sodium counterions remain at or near the interior interface between surfactant and water, thus providing the first experimental support for this hypothesis.

Correlation of insulin-enhancing properties of vanadium-dipicolinate complexes in model membrane systems: phospholipid langmuir monolayers and AOT reverse micelles.

We explore the interactions of V(III) -, V(IV) -, and V(V) -2,6-pyridinedicarboxylic acid (dipic) complexes with model membrane systems and whether these interactions correlate with the blood-glucose-lowering effects of these compounds on STZ-induced diabetic rats. Two model systems, dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayers and AOT (sodium bis(2-ethylhexyl)sulfosuccinate) reverse micelles present controlled environments for the systematic study of these vanadium complexes interacting with self-assembled lipids. Results from the Langmuir monolayer studies show that vanadium complexes in all three oxidation states interact with the DPPC monolayer; the V(III) -phospholipid interactions result in a slight decrease in DPPC molecular area, whereas V(IV) and V(V) -phospholipid interactions appear to increase the DPPC molecular area, an observation consistent with penetration into the interface of this complex. Investigations also examined the interactions of V(III) - and V(IV) -dipic complexes with polar interfaces in AOT reverse micelles. Electron paramagnetic resonance spectroscopic studies of V(IV) complexes in reverse micelles indicate that the neutral and smaller 1:1 V(IV) -dipic complex penetrates the interface, whereas the larger 1:2 V(IV) complex does not. UV/Vis spectroscopy studies of the anionic V(III) -dipic complex show only minor interactions. These results are in contrast to behavior of the V(V) -dipic complex, [VO2 (dipic)](-) , which penetrates the AOT/isooctane reverse micellar interface. These model membrane studies indicate that V(III) -, V(IV) -, and V(V) -dipic complexes interact with and penetrate the lipid interfaces differently, an effect that agrees with the compounds' efficacy at lowering elevated blood glucose levels in diabetic rats.

Gel formulation containing mixed surfactant and lipids associating with carboplatin.

The interaction of amphiphilic molecules such as lipids and surfactants with the hydrophilic drug carboplatin was investigated to identify suitable self-assembling components for a potential gel-based delivery formulation. (1) H-NMR Studies in sodium bis(2-ethylhexyl) sulfosuccinate (aerosol-OT, AOT)-based reverse micelles show that carboplatin associates and at least partially penetrates the surfactant interface. Langmuir monolayers formed by dipalmitoyl(phosphatidyl)choline are penetrated by carboplatin. Carboplatin was found to also penetrate the more rigid monolayers containing cholesterol. A combined mixed surfactant gel formulation containing carboplatin and cholesterol for lymphatic tissue targeting was investigated for the intracavitary treatment of cancer. This formulation consists of a blend of the surfactants lecithin and AOT (1 : 3 ratio), an oil phase of isopropyl myristate, and an aqueous component. The phases of the system were defined within a pseudo-ternary phase diagram. At low oil content, this formulation produces a gel-like system over a wide range of H(2) O content. The carboplatin release from the formulation displays a prolonged discharge with a rate three to five times slower than that of the control. Rheological properties of the formulation exhibit pseudoplastic behavior. Microemulsion and Langmuir monolayer studies support the interactions between carboplatin and amphiphilic components used in this formulation. To target delivery of carboplatin, two formulations containing cholesterol were characterized. These two formulations with cholesterol showed that, although cholesterol does little to alter the phases in the pseudo-ternary system or to increase the initial release of the drug, it contributes significantly to the structure of the formulation under physiological temperature, as well as increases the rate of steady-state discharge of carboplatin.

Antidiabetic vanadium compound and membrane interfaces: interface-facilitated metal complex hydrolysis.

The interactions of metabolites of the antidiabetic vanadium-containing drug bis(maltolato)oxovanadium(IV) (BMOV) with lipid interface model systems were investigated and the results were used to describe a potentially novel mechanism by which these compounds initiate membrane-receptor-mediated signal transduction. Specifically, spectroscopic studies probed the BMOV oxidation and hydrolysis product interaction with interfaces created from cetyltrimethylammonium bromide (CTAB) which mimics the positively charged head group on phosphatidylcholine. (1)H and (51)V NMR spectroscopies were used to determine the location of the dioxobis(maltolato)oxovanadate(V) and the maltol ligand in micelles and reverse micelles by measuring changes in the chemical shift, signal linewidth, and species distribution. Both micelles and reverse micelles interacted similarly with the complex and the ligand, suggesting that interaction is strong as anticipated by Coulombic attraction between the positively charged lipid head group and the negatively charged complex and deprotonated ligand. The nature of the model system was confirmed using dynamic light scattering studies and conductivity measurements. Interactions of the complex/ligand above and below the critical micelle concentration of micelle formation were different, with much stronger interactions when CTAB was in the form of a micelle. Both the complex and the ligand penetrated the lipid interface and were located near the charged head group. These studies demonstrate that a lipid-like interface affects the stability of the complex and raise the possibility that ligand exchange at the interface may be important for the mode of action for these systems. Combined, these studies support recently reported in vivo observations of BMOV penetration into 3T3-L1 adipocyte membranes and increased translocation of a glucose transporter to the plasma membrane.

Layered structure of room-temperature ionic liquids in microemulsions by multinuclear NMR spectroscopic studies.

Microemulsions form in mixtures of polar, nonpolar, and amphiphilic molecules. Typical microemulsions employ water as the polar phase. However, microemulsions can form with a polar phase other than water, which hold promise to diversify the range of properties, and hence utility, of microemulsions. Here microemulsions formed by using a room-temperature ionic liquid (RTIL) as the polar phase were created and characterized by using multinuclear NMR spectroscopy. (1)H, (11)B, and (19)F NMR spectroscopy was applied to explore differences between microemulsions formed by using 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]) as the polar phase with a cationic surfactant, benzylhexadecyldimethylammonium chloride (BHDC), and a nonionic surfactant, Triton X-100 (TX-100). NMR spectroscopy showed distinct differences in the behavior of the RTIL as the charge of the surfactant head group varies in the different microemulsion environments. Minor changes in the chemical shifts were observed for [bmim](+) and [BF(4)](-) in the presence of TX-100 suggesting that the surfactant and the ionic liquid are separated in the microemulsion. The large changes in spectroscopic parameters observed are consistent with microstructure formation with layering of [bmim](+) and [BF(4)](-) and migration of Cl(-) within the BHDC microemulsions. Comparisons with NMR results for related ionic compounds in organic and aqueous environments as well as literature studies assisted the development of a simple organizational model for these microstructures.

Effect of micellar and reverse micellar interface on solute location: 2,6-pyridinedicarboxylate in CTAB micelles and CTAB and AOT reverse micelles.

The interface-solute interactions, including solute location, surfactant charge, and geometry of solute interactions were studied in CTAB micelles and reverse micelles and were found to be similar as measured using (1)H NMR spectroscopy and a pH-sensitive probe. (1)H NMR spectra were recorded in the presence and absence of 2,6-pyridinedicarboxylate probe at CTAB concentrations above and below the critical micelle concentration showing interaction between dipic-probe and the micellar self-assembled structure. Downfield chemical shifts are observed for the CTAB surfactant signals upon aggregation and micelle formation. The effect of micelle formation on CTAB chemical shifts was quantitated, and simple ion pairing was ruled out. No significant change in CTAB surfactant signals are observed in the presence of monoanionic probe, whereas significant shifts are observed in the presence of the dianionic probe. The (1)H NMR spectra of the dipic-probe are diagnostic of the protonation state and isomeric form of the dipic-probe. The (1)H NMR chemical shifts in micelles are sensitive to the location of the dipic-probe, and the downfield chemical shift suggests location of part of the molecule in the Stern layer near the charged interface. Other parts of the probe show an upfield chemical shifts consistent with a deeper penetration of the dipic-probe into the surfactant layer. Probe location was confirmed using the 2D ROESY. Spectra recorded of the dipic-probe at various pH values demonstrate that both CTAB micellar and CTAB/pentanol/cyclohexane reverse micellar interfaces are different than those reported in aqueous solution and in AOT/isooctane reverse micelles (Crans et al. J. Org. Chem. 2008, 73, 9633-9640) and suggest interface penetration by dipic(2-). Together these observations and comparisons provide guidelines for future interpretation of chemical shift changes in both micelles and reverse micelles and point to headgroup charge as being a key factor determining the direction of chemical shift change and the depth of solute penetration.

Is vanadate reduced by thiols under biological conditions? Changing the redox potential of V(V)/V(IV) by complexation in aqueous solution.

Although dogma states that vanadate is readily reduced by glutathione, cysteine, and other thiols, there are several examples documenting that vanadium(V)-sulfur complexes can form and be observed. This conundrum has impacted life scientists for more than two decades. Investigation of this problem requires an understanding of both the complexes that form from vanadium(IV) and (V) and a representative thiol in aqueous solution. The reactions of vanadate and hydrated vanadyl cation with 2-mercaptoethanol have been investigated using multinuclear NMR, electron paramagnetic resonance (EPR), and UV-vis spectroscopy. Vanadate forms a stable complex of 2:2 stoichiometry with 2-mercaptoethanol at neutral and alkaline pH. In contrast, vanadate can oxidize 2-mercaptoethanol; this process is favored at low pH and high solute concentrations. The complex that forms between aqueous vanadium(IV) and 2-mercaptoethanol has a 1:2 stoichiometry and can be observed at high pH and high 2-mercaptoethanol concentration. The solution structures have been deduced based on coordination induced chemical shifts and speciation diagrams prepared. This work demonstrates that both vanadium(IV) and (V)-thiol complexes form and that redox chemistry also takes place. Whether reduction of vanadate takes place is governed by a combination of parameters: pH, solute- and vanadate-concentrations and the presence of other complexing ligands. On the basis of these results it is now possible to understand the distribution of vanadium in oxidation states (IV) and (V) in the presence of glutathione, cysteine, and other thiols and begin to evaluate the forms of the vanadium compounds that exert a particular biological effect including the insulin-enhancing agents, antiamoebic agents, and interactions with vanadium binding proteins.

Complexation of bisphosphonates with ytterbium(III): application of phosphate and ATP detection assay based on Yb(3+)-pyrocatechol violet.

The coordination chemistry of bisphosphonates with Yb(3+) was investigated to evaluate the potential of the UV-vis based detection method using the Yb(3+)-pyrocatechol complexation reaction as a sensor for bisphosphonates. The complexation chemistry of Yb(3+) with phosphate and ATP analogs was previously described (E. Gaidamauskas, K. Saejueng, A.A. Holder, S. Bharuah, B.A. Kashemirov, D.C. Crans, C.E. McKenna, J. Biol. Inorg. Chem. 13 (2008) 1291-1299), and we here studied the complexation chemistry of bisphosphonates in this system. The spectrophotometric assay yields direct evidence for formation of a 4:3 metal to ligand complex at neutral pH. Direct evidence for Yb(3+):methylenebis(phosphonate) complexes with 1:1 and 1:2 stoichiometry was also obtained by potentiometry at acidic and basic pH. Direct evidence for complex formation was obtained using (1)H NMR spectroscopy although the stoichiometry was not accessed at neutral pH. Our results suggest that the spectroscopic observation of the YbPV complex can be used to conveniently measure concentrations of bisphosphonates down to 2-3 microM.

Deprotonation of beta-cyclodextrin in alkaline solutions.

Variable pH (13)C NMR and (1)H NMR spectroscopic studies of the beta-cyclodextrin (beta-CD) in alkaline aqueous solutions revealed that beta-CD does not deprotonate at pH<12.0. Further increase in solution pH results in the deprotonation of OH-groups adjacent to C-2 and C-3 carbon atoms of beta-CD glucopyranose units, whereas the deprotonation of OH-groups adjacent to C-6 carbon atoms is expressed less markedly. The pK(a) values for beta-CD OH-groups adjacent to C-2 and C-3 carbon atoms are rather close, pK(a1,2) being 13.5+/-0.2 (22.5 degrees C).

Metal complexation chemistry used for phosphate and nucleotide determination: an investigation of the Yb3+-pyrocatechol violet sensor.

Metal complexation reactions can be used effectively as sensors to measure concentrations of phosphate and phosphate analogs. Recently, a method was described for the detection of phosphate or ATP in aqueous solution based on the displacement by these ligands of pyrocatechol violet (PV) from a putative 2:1 (Yb3+)2PV complex. We have not been able to reproduce this stoichiometry and report this work in order to correct the coordination chemistry upon which sensor applications are based. In our work, colorimetric and spectrophotometric detection of phosphate was confirmed qualitatively (blue PV+Yb3+; yellow+Pi); however, the sequence of visual changes on the titration of PV with 2 equiv. of Yb3+ and back titration with ATP as described previously could not be reproduced. In contrast to the linear response to Pi that was reported previously, the absorbance response at 443 or 623 nm was found to be sigmoidal using the recommended 2:1 Yb3+:PV solution (100 microM:50 microM, pH 7, HEPES). Furthermore, both continuous variation titration and molar ratio analysis (Job plot) experiments are consistent with 1:1, not 2:1, YbPV complex stoichiometry at pH 7 in HEPES buffer, indicating that the deviation from linearity is produced by excess Yb3+. Indeed, using a 1:1 Yb3+:PV ratio produces a linear response in DeltaAbs at 443 or 623 nm on back titration with analyte (phosphate or ATP). In addition, speciation analysis of the Yb-ATP system demonstrates that a 1:1 complex containing Yb3+ and ATP predominates in solution at microM metal ion and ATP concentrations. Paramagnetic 1H NMR spectroscopy directly establishes the formation of Yb3+-solute complexes in dilute aqueous solution. The 1:1 YbPV complex can be used for the colorimetric measurement of phosphate and ATP concentrations from approximately 2 microM.

Impairment of ascorbic acid's anti-oxidant properties in confined media: inter and intramolecular reactions with air and vanadate at acidic pH.

The anti-oxidant properties of L-ascorbic acid were investigated in the confined medium produced by a sodium bis(2-ethylhexyl)sulfosuccinate (aerosol-OT, AOT) self-assembled reverse micelle. Using 1H-1H NOESY (proton-proton 2D nuclear overhauser enhancement correlation spectroscopy) NMR spectroscopy, the location of ascorbic acid was investigated and found to be at the AOT-interface in contrast to earlier studies where the ascorbate was assumed to be in the water pool in these microemulsions. The reaction of ascorbic acid with oxygen was investigated using EPR spectroscopy. A delocalized monoanionic ascorbate radical was observed in microemulsions prepared from pH 5.6 stock solutions. This is in contrast to studies carried out in aqueous media where no radical formation was observed. The oxidation of ascorbic acid by aqueous V(V) was investigated in reverse micelles. Modest changes in the kinetic parameters were observed for this system compared to that in water. Details of these reactions were examined and can be summarized as the microemulsion solvating and stabilizing reactive intermediates via rate inhibition or enhancement. The inhibition of the oxidation is due to solvation stabilization of ascorbic acid in microemulsion media. Since ascorbate is a valuable marker of oxidative stress, our results suggest that compartmentization can modify the stabilization of the ascorbate radical and the changes in properties could be important in biological systems.

Evidence of two-step deprotonation of D-mannitol in aqueous solution.

Deprotonation of D-mannitol was studied in aqueous basic solutions by means of potentiometry and (13)C NMR spectroscopy. Two-step dissociation in the pH range from 12 to 13.8 was shown, and successive dissociation constants K(a1) and K(a2) were determined. In a solution with ionic strength I = 1.0 M (NaOH + NaNO(3)) pK(a1) = 13.1 +/- 0.1 and pK(a2) = 13.8 +/- 0.2. With increasing ionic strength from 0.75 to 3.0 M, both pK(a1) and pK(a2) values decrease. Deprotonation-induced chemical shifts in pH-variable (13)C NMR spectra show that the OH-groups next to internal carbon atoms C-3 and C-4 dissociate to a greater extent compared to OH-groups next to external carbon atoms C-1 and C-6.

Inhibition of yeast growth by molybdenum-hydroxylamido complexes correlates with their presence in media at differing pH values.

The effects of Mo-hydroxylamido complexes on cell growth were determined in Saccharomyces cerevisiae to investigate the biological effects of four different Mo complexes as a function of pH. Studies with yeast, an eukaryotic cell, are particularly suited to examine growth at different pH values because this organism grows well from pH 3 to 6.5. Studies can therefore be performed both in the presence of intact complexes and when the complexes have hydrolyzed to ligand and free metal ion. One of the complexes we examined was structurally characterized by X-ray crystallography. Yeast growth was inhibited in media solutions containing added Mo-dialkylhydroxylamido complexes at pH 3-7. When combining the yeast growth studies with a systematic study of the Mo-hydroxylamido complexes' stability as a function of pH and an examination of their speciation in yeast media, the effects of intact complexes can be distinguished from that of ligand and metal. This is possible because different effects are observed with complex present than when ligand or metal alone is present. At pH 3, the growth inhibition is attributed to the forms of molybdate ion that exist in solution because most of the complexes have hydrolyzed to oxomolybdate and ligand. The monoalkylhydroxylamine ligand inhibited yeast growth at pH 5, 6 and 7, while the dialkylhydroxylamine ligands had little effect on yeast growth. Growth inhibition of the Mo-dialkylhydroxylamido complexes is observed when a complex exists in the media. A complex that is inert to ligand exchange is not effective even at pH 3 where other Mo-hydroxylamido complexes show growth inhibition as molybdate. These results show that the formation of some Mo complexes can protect yeast from the growth inhibition observed when either the ligand or Mo salt alone are present.

Cu(II) complex formation with xylitol in alkaline solutions.

The formation of four Cu(II)-xylitol complexes was observed in aqueous alkaline solutions (11.0< or =pH< or =14.0, I=1.0, 20 degrees C) by means of direct current polarography and VIS spectrophotometry. Mononuclear hydroxy complexes, CuXyl(OH)- (log beta=17.7 +/- 0.5), CuXyl(OH)2(2-) (log beta=20.2 +/- 0.3) and CuXyl2(OH)2(4-) (log beta=22.4 +/- 0.3), are formed at high ligand-to-metal ratios (L:M> or =10), whereas dinuclear complex Cu2Xyl (log beta=29.2 +/- 0.3) is the predominant species at low ligand-to-metal ratio (L:M=0.5). Diffusion coefficients and molar absorptivities of the complex species were determined. pH variable 13C NMR suggested that pKa values of xylitol are rather similar and equal to 13.8 +/- 0.2, 13.9 +/- 0.1 and 13.9 +/- 0.2 for OH-groups adjacent to (C-1,C-5), (C-3) and (C-2,C-4) carbon atoms, respectively.