Interaction of Dipicolinatodioxovanadium(V) with Polyatomic Cations

6250

Langmuir 2005, 21, 6250-6258

Interaction of Dipicolinatodioxovanadium(V) with Polyatomic Cations and Surfaces in Reverse Micelles Jessica Stover, Christopher D. Rithner, Rae Anne Inafuku, Debbie C. Crans,* and Nancy E. Levinger* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872 Received March 28, 2005 In confined media such as reverse micelles, molecular probes frequently reside at and interact strongly with the interface. If the interface is charged, it is often difficult to separate effects arising from interactions with the charged species from the effect of the interfacial environment. With reverse micelles as a model system, the work reported here explores the interaction of the charged surfactant headgroups at a selfassembled interface with the dipicolinatodioxovanadium(V) coordination complex. The vanadium complex studied in these experiments serves as an excellent probe to investigate how charged metal complexes interact with lipid interfaces. For comparison, measurements were also carried out probing the interaction of the vanadium complex with a model cationic headgroup, tetramethylammonium bromide. The impact of the environment is gauged by changes in the 51V chemical shift, longitudinal relaxation times, and 1H NMR pulsed field gradient measurements. These measurements suggest that while interface component parts, as modeled by the dispersed systems, interact with the vanadium complex, the interfacial environment perturbs the complex substantially more strongly than the sum of the components alone. Coulomb attraction dominates the interaction in all systems probed and surprisingly orients the hydrophobic portion into the bulk water.

I. Introduction Understanding how charged molecules interact with lipid interfaces is important for biology, chemistry, physics, and other technical fields. Because most lipids are charged, interaction with a lipid surface depends at least in part on the Coulomb attraction and repulsion between charged species. Several models for characterization of lipid interfaces have been used, including simple systems such as reverse micelles and liposomes.1,2 To assess the impact of the intramicellar environment on molecules solubilized therein, we undertook a study to characterize the molecular interaction with the self-assembled interface. In such systems it can be difficult to separate the impact of the charged surfactant headgroup on the probe from the impact of confinement within reverse micelles. Here we describe a study designed to contrast the role of electrostatic interactions present in a system of ions dispersed in aqueous solution with the electrostatic interactions at a lipid interface.3,4 We utilize vanadium coordination complexes to explore the ligand-metal ion interactions which compete with that of the metal ion-lipid interface, and as such provide a force for destabilization of the complex.5 In this work, we examine interactions in both a reverse micellar system and an aqueous system of model ions that mimic the ionic headgroups of the surfactant molecules. Reverse micelles are self-assembled lipid structures that form in nonpolar media.1,6-9 At the appropriate concen* Corresponding authors: e-mail [email protected] or [email protected]. (1) Luisi, P. L.; Straub, B. E. Reverse Micelles: Biological and Technological Relevance of Amphiphilic Structures in Apolar Media; Plenum Press: New York, 1984. (2) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. Adv. Colloid Interface Sci. 1999, 82, 189. (3) Tsao, H. K.; Sheng, Y. J.; Lu, C. Y. D. J. Chem. Phys. 2000, 113, 10304. (4) Cuccovia, I. M.; Romsted, L. S.; Chaimovich, P. J. Colloid Interface Sci. 1999, 220, 96. (5) Fernandez, E.; Garcia-Rio, L.; Knorl, A.; Leis, J. R. Langmuir 2003, 19, 6611.

trations, surfactant molecules delineate polar solvent pools from nonpolar solvents. Frequently, the molar ratio of water to surfactant, w0 ) [water]/[surfactant], is used to characterize the size of particles in solution. For spherical reverse micelles, w0 is directly proportional to the micelle radius.7 Reverse micelles exist within biological membranes and it has been suggested that they are involved in vesicle transport across lipid interfaces.1 The research reported here utilizes reverse micelles as a model system to explore the interaction of amphiphilic vanadium coordination compounds that are dispersed in solution or self-assembled into reverse micelles. Vanadium coordination compounds have shown promise for their potential for treatment of diabetes in animal models and human beings.10-12 Yet little is understood about the interaction of these coordination compounds with interfaces or the mechanism of their transport across membranes to sites where they become biologically active. Although protein-based active transport systems are important in many systems,13 passive transport across membranes is also probably dependent on the chemical structure and charge of the complex.14,15 Some animal studies suggest that the bis(maltolato)oxovanadium(IV) decomposes in the stomach and the ligand separates from (6) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1988, 947, 209. (7) De, T.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (8) Zhang, J.; Bright, F. V. J. Phys. Chem. 1991, 95, 7900. (9) Levinger, N. E.; Riter, R. E. Solvation Dynamics at Liquid Interfaces. In Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications; Volkov, A. G., Ed.; Marcel Dekker: New York, 2001; p 399. (10) Thompson, K. H.; McNeill, J. H.; Orvig, C. Chem. Rev. 1999, 99, 2561. (11) Crans, D. C.; Smee, J.; Gaidamauskas, E.; Yang, L. Chem. Rev. 2004, 104, 849. (12) Sakurai, H.; Kojima, Y.; Yoshikawa, Y.; Kawabe, K.; Yasui, H. Coord. Chem. Rev. 2002, 226, 187. (13) Schrijvers, D. Clin. Pharmacokinet. 2003, 42, 779. (14) Yang, X.-G.; Yang, X.-D.; Yuan, L.; Wang, K.; Crans, D. C. Pharm. Res. 2004, 21, 1026. (15) Yang, X.; Wang, K.; Lu, J.; Crans, D. C. Coord. Chem. Rev. 2003, 237, 103.

10.1021/la0508137 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/03/2005

Dipicolinatodioxovanadium(V) in Reverse Micelles

Figure 1. Structure of [VO2dipic]- complex.

the metal ion.10,16 However, similar modes of action may not be in effect for more acid stable compounds. One system that has shown promise as an antidiabetes compound is the dioxovanadium(V) 2,6-pyridinecarboxylate17 ([VO2dipic]-) coordination complex, whose structure is shown in Figure 1.18-20 Vanadate forms a complex with dipicolinic acid to yield the five-coordinate anionic coordination complex ([VO2dipic]-).21 The [VO2dipic]- complex has both hydrophilic and hydrophobic character and thus has the potential of binding to both lipids and water. Given the spectroscopic properties of [VO2dipic]- and its amphiphilic faces,18,22,23 it serves as an ideal probe of interactions with lipid interfaces. The complex displays a Gaussian stability profile with a maximum stability at pH 3-4; it hydrolyzes to form vanadate and dianionic dipicolinate above pH 6.18,23 The complex is highly sensitive to temperature, ionic strength, and other characteristics of its environment. In the presence of free ligand, the complex undergoes pHdependent ligand exchange, which is slowest at pH 3.55.18,23 The charge, high water solubility, and NMR chemical shifts of [VO2dipic]- make it a useful molecular probe for investigating molecular and dynamic properties of the system. The goal of the experiments described here was to probe the interaction of [VO2dipic]- with dispersed ions and model lipid interfaces. We explored how polyatomic ions with charge and structure similar to the headgroups found in the reverse micelles impact the [VO2dipic]- complex. We utilized tetramethylammonium bromide (TMAB) in various aqueous solutions to model the headgroup of cetyltrimethylammonium bromide (CTAB). These studies provide comparison of electrostatic interactions between the vanadium compound and a quaternary ammoniumtype cation in confined and dispersed environments. Results from these experiments impact our interpretation of data reporting on anion interactions at cationic interfaces. Using NMR chemical shifts, lifetimes, and pulsed field gradient studies, we show that [VO2dipic]- interacts with TMAB (that is, the TMA+ ion) in aqueous solution. We find that the TMA+ ion interacts more strongly with [VO2dipic]- than smaller cations, such as K+, but that size is not the only property affecting the nature of the interaction. Using these data, we compare and contrast results for [VO2dipic]- in CTAB reverse micelles. Together these results suggest that [VO2dipic]- in CTAB reverse micelles is located near the lipid interface, but the (16) Dikanov, S. A.; Liboiron, B. D.; Thompson, K. H.; Vera, E.; Yuen, V. G.; McNeill, J. H.; Orvig, C. J. Am. Chem. Soc. 1999, 121, 11004. (17) Also known as (dipicolinato)dioxovanadate(V) or [VO2dipic]-. (18) Crans, D. C. J. Inorg. Biochem. 2000, 80, 123. (19) Jakusch, T.; Jin, W.; Yang, L.; Crans, D. C.; Kiss, T. J. Inorg. Biochem. 2003, 95, 1. (20) Buglyo, P.; Crans, D. C.; Nagy, E. M.; Lindo, R. L.; Yang, L.; Smee, J. J.; Chi, L.-H.; Godzala, M. E., III; Willsky, G. R. Inorg. Chem., in press. (21) Nuber, B.; Weiss, J.; Wieghardt, K. Z. Naturforsch. B 1978, 33, 265. (22) Wieghardt, K. Inorg. Chem. 1978, 17, 57. (23) Crans, D. C.; Yang, L. Q.; Jakusch, T.; Kiss, T. Inorg. Chem 2000, 39, 4409.

Langmuir, Vol. 21, No. 14, 2005 6251

interaction is different in the reverse micelle as compared to the interaction with simple monatomic and polyatomic cations. In addition, we explored differences in the interaction as we modify the interface by changing the size of the reverse micelles, the membrane structure, and fluidity by changing the organic solvent outside the reverse micelle.7,24 Our results show that interactions between [VO2dipic]- and the lipid interface differ substantially from the interaction with ions dispersed in aqueous solution. II. Experimental Methods Materials. All chemicals except CTAB were used as received. Tetramethylammonium bromide (TMAB) and ammonium bromide (NH4Br) were purchased from Alfa Aesar. Potassium chloride (KCl), amyl alcohol (n-pentanol), and methanol were purchased from Fisher. Deuterium oxide was purchased from Cambridge Isotope Laboratories. Cyclohexane, cetyltrimethylammonium bromide (CTAB), and other chemicals used were purchased from Aldrich Chemical Co. To remove water, CTAB was recrystallized three times from 100% ethanol. Water was distilled and passed through an ion-exchange column (Millipore, 18MΩ‚cm).Oxovanadium(V)2,6-pyridinecarboxylate([VO2dipic]-) was prepared as reported previously.18,22,23 The ammonium salt of [VO2dipic]- was used for most of these studies, but experiments with the tetramethylammonium salt were also used for comparison studies. {The tetramethylammonium salt of [VO2dipic]-, TMA[VO2dipic], was prepared from reaction of 1 equiv of V2O5 with 1 equiv of H2dipic and 2 equiv of tetramethylammonium hydroxide. If necessary, the pH of the solution was adjusted to pH 3-5 before the solution was placed at 4 °C for crystallization. After a few days the crystals were filtered, resulting in a white fluffy material after recrystallization with an overall yield of 50%. 51V and 1H NMR spectroscopic properties and UV-visible absorption spectra were identical to those reported for NH4[VO2dipic] with the exception of the extra proton peak for TMA[VO2dipic].17,22} No significant differences were observed between samples prepared with differing countercations. Sample Preparation and Characterization by 51V NMR Spectroscopy. Aqueous solutions were prepared as a mixture of 20% deuterium oxide/80% water. Stock solutions were prepared of TMAB (3.0 M), NH4Br (3.0 M), and KCl (3.0 M). Samples for 51V NMR studies were prepared by mixing solid [VO dipic]- with 2 TMAB, NH4Br, or KCl salt (0.1-3.0 M) stock solutions, to form solutions 5.0 mM in the vanadium complex. Solutions were then adjusted to pH ) 5.0 ((0.1) with HCl or NaOH (or DCl and NaOD), respectively. The pH was measured on an Orion 210A pH meter accurate to 0.01 pH unit. Unlike branched surfactant systems,7 CTAB reverse micelles require a cosurfactant, such as n-pentanol, to form. CTAB/n-pentanol (1:5 ratio of surfactant to cosurfactant) reverse micelles in cyclohexane were prepared by adding a stock solution containing 185 mM [VO2dipic]-/D2O to solutions of CTAB/n-pentanol/cyclohexane to form reverse micelles with varying w0 values.24 Formation and sizes of the reverse micelles were determined by dynamic light scattering (Protein Solutions, DynaPro) with cross-correlation analysis.25 Comparison solutions of TMAB/methanol (1:5 ratio) were prepared with 5 mM [VO2dipic]-/D2O stock solution and adjusted to the desired pH values. Sample Preparation for Pulsed Field Gradient 1H NMR Studies. Aqueous solutions were prepared in 100% D2O to allow preferential monitoring of the 1H on the [VO2dipic]- complex and TMA+ methyl protons. We investigated two different vanadium samples: 10 mM [VO2dipic]- with 0.1 M TMAB in D2O, and 10 mM [VO2dipic]- with 1.0 M TMAB in D2O. The control samples investigated contained pure reagents in D2O, for example, 0.1 M TMAB, 1.0 M TMAB, and 10 mM [VO2dipic]-. Experiments were carried out in H2O in place of D2O; because they had higher sensitivity, we report only results from D2O solutions here. The solutions were prepared from solid [VO2dipic]and the TMAB stock solutions. The pH was subsequently adjusted as needed. (24) Corbeil, E. M.; Levinger, N. E. Langmuir 2003, 19, 7264. (25) Atkins, P.; de Paula, J. Physical Chemistry, 7th ed.; W. H. Freeman and Company: New York, 2002.

6252

Langmuir, Vol. 21, No. 14, 2005

Table 1.

51V

NMR Peak Positions, Line Widths, and T1 Relaxation Times for Samples Containing 5 mM [VO2dipic]- in Aqueous Solutions of the Salt Indicated

sample TMABa

NH4Bra

KCla

methanol/TMABb

methanolc

Stover et al.

concn (M)

avg peak position (ppm)

experimental line width (Hz)d

lifetime (ms)

calcd line width (Hz)

3.0 2.0 1.0 0.5 0.1 3.0 2.0 1.0 0.5 0.1 3.0 2.0 1.0 0.5 0.1 2.0/10.0 1.0/5.0 0.5/2.5 0.1/0.5 10.0 5.0 2.5 0.5

-531.1 ( 0.3 -531.3 ( 0.3 -531.7 ( 0.1 -531.8 ( 0.2 -531.9 ( 0.5 -532.0 ( 0.1 -532.0 ( 0.1 -531.8 ( 0.1 -531.8 ( 0.2 -531.8 ( 0.1 -534 ( 2 -534 ( 2 -534 ( 2 -534 ( 2 -534 ( 2 -531.5 ( 0.4 -532.2 ( 0.3 -532.0 ( 0.1 -532.1 ( 0.1 -532.8 -532.5 -532.5 -532.1

310 ( 40 270 ( 20 230 ( 20 210 ( 20 190 ( 10 182 ( 4 179 ( 4 178 ( 2 175 ( 4 171 ( 5 220 ( 10 201 ( 3 198 ( 3 185 ( 5 172 ( 4 420 ( 10 320 ( 10 239 ( 8 205 ( 2 280c 260c 220c 190c

0.92 ( 0.06 1.11 ( 0.08 1.42 ( 0.03 1.59 ( 0.02 1.98 ( 0.07 1.83 ( 0.03

350 ( 50 290 ( 10 224 ( 2 200.9 ( 0.6 160 ( 13 174

2.16 ( 0.04 1.39 ( 0.02

147 230

2.09 ( 0.09

152

a Error bars reported reflect the standard deviation of five experimental measurements. b Error bars reported reflect the standard deviation of three experimental measurements. c Measurements were not repeated. d In pure water the linewidth is 165 Hz. 51V NMR Spectroscopy. The 51V NMR spectra were recorded on a 300 MHz Varian spectrometer at 78.9 MHz. The onedimensional spectra were acquired with a spectral window of 83.6 MHz, 30° pulse, and an acquisition time of 0.096 s with no relaxation delay. 51V NMR chemical shifts were referenced against an external sample of VOCl3. Except for T1 experiments, a 20 Hz exponential line broadening was applied before Fourier transformation. Spectra were collected at 295 K ((2 K). 51V T1 values were measured in an inverse recovery experiment. For these experiments, the pulse sequence consisted of a 180° pulse, a variable delay τ that allows for longitudinal recovery, a 90° read pulse, data acquisition, and then a relaxation time of 96 ms. The total time (acquisition time + delay time) was greater than 5T1. The 90° pulse differed for each sample and was determined prior to each lifetime measurement. The data were processed with the Varian VNMRG.1C software. Pulsed Field Gradient 1H NMR Spectroscopy. The 1H NMR measurements were performed on a Varian Inova 500 MHz spectrometer equipped with a 20 A/channel Highland-L200 threeaxis gradient amplifier and a Varian 5-mm HCN three-axis pulsed field gradient (PFG) probe. The combination provides a z gradient strength, g, of up to 0.65 T‚m-1 (65 G‚cm-1). A slice-selective, stimulated echo pulse program that incorporates bipolar gradient pulses and a spin-lock purge pulse (Antalek, 2002) was used. The gradient pulses were 1 ms in duration. The diffusion evolution time was ∼200 ms. The gradient strength varied over 16 values from about 0.04 to 0.60 T‚m-1, chosen such that g2 steps were equally spaced. For all experiments, the sweep width was 8 kHz, the Fourier number was 64K, 64 transients were acquired, and an acquisition time pulse delay of 11.1 s was used. All data analysis was performed with Varian VNMR, version 6.1C.

III. Results Both 51V NMR and 1H NMR spectroscopies are excellent probes of the structure and properties of the [VO2dipic]complex.20 The quadrupolar nature of the 51V nucleus means it is exquisitely sensitive to its environment.20 Variation in the chemical shift, line width, and spinlattice relaxation time sensitively probe properties of the complex in solution.20 Here, we report results from 51V NMR and 1H PFG-NMR measurements that we have used to investigate these systems. 51V NMR Spectroscopic Studies on Model Systems. The stability of the [VO2dipic]- complex depends critically on pH.20 At pH 5 the complex is very stable at a 1:1 ratio

Figure 2. 51V NMR spectra of [VO2dipic]- in 3.0 M aqueous solutions of TMAB, KCl, and NH4Br. The aqueous solution is 185 mM in [VO2dipic]-, making the overall concentration of the vanadium nucleus in the suspension 5 mM.

of vanadium to ligand; the chemical shift for the complex is -532 ppm and the line width in pure water is ∼165 Hz.23 For the experiments reported here, 51V NMR spectra of [VO2dipic]- solutions were recorded in the presence of varying concentrations of NH4Br, KCl, and TMAB. Representative NMR spectra of the [VO2dipic]- in 3.0 M aqueous salt solutions are shown in Figure 2. The 51V chemical shift at -532 ppm does not change significantly as the salt concentrations vary, but the line width of the [VO2dipic]- signal increases with increasing salt concentration (see Table 1). Line widths for the [VO2dipic]- in the various aqueous solutions are shown graphically in Figure 3 as a function of salt concentration. These results show that the solutions containing TMA+ causes the most significant broadening in the [VO2dipic]- spectrum, from 165 Hz in pure water to 313.8 Hz in 3.0 M aqueous TMAB. Chemical shifts, line widths, and lifetimes for the [VO2dipic]- signal in aqueous solutions are given in Table 1.


Langmuir, Vol. 21, No. 14, 2005 6253

Figure 3. Comparison of 51V NMR line widths with [VO2dipic]in aqueous solutions of TMAB (9), KCl (b), and NH4Br (2). The aqueous solution is 185 mM in [VO2dipic]-, making the overall concentration of the vanadium nucleus in the suspension 5 mM. Error bars represent the standard deviation of five experimental runs. If the error is small, the symbol covers the error bar.

Figure 4. Comparison of 51V NMR line widths with [VO2dipic]in an aqueous solution of TMAB found experimentally (9) and calculated from lifetime measurements (b). The aqueous solution is 185 mM in [VO2dipic]-, making the overall concentration of the vanadium nucleus in the suspension 5 mM. Error bars represent the standard deviation of three experimental runs.

51 V NMR Longitudinal Relaxation Times. Comparing the T1 longitudinal relaxation data for 51V nucleus in [VO2dipic]- in the presence of varying salt concentrations provides a second method for gauging the degree of interaction between [VO2dipic]- and the three different salts TMAB, KCl, and NH4Br at varying concentration. The line width (full width at half-height), ∆1/2, can be calculated from eq 1 and T2*:26

data for the KCl and the NH4Br solutions of [VO2dipic]show similar trends as shown in Table 1. Pulsed Field Gradient NMR. Pulsed field gradient (PFG) NMR measures the rate of diffusion of a species, yielding the translational diffusion coefficient. As such, PFG NMR experiments can reveal the degree of association of the salts with the [VO2dipic]- by monitoring the effects on the 1H NMR signal intensity as the molecule diffuses through a gradient magnetic field.27 From a PFG NMR measurement, we can determine the translational diffusion of various species in solution. Our experiments measured diffusion characteristics for HOD (translational diffusion for D2O is estimated from the signals generated by the HOD), TMAB, and [VO2dipic]- in deuterated aqueous solution. The expected diffusion constants are calculated from the Stokes-Einstein relation43 given by

∆1/2 )

1 πT2*

(1)

T2* times include contributions from transverse relaxation and inhomogeneous broadening:22

1 1 1 ) + T2* T2 T2,inhom

(2)

In the system explored here, T2,inhom is long, so that T2* ≈ T2. In addition, the [VO2dipic]- with its small molar mass tumbles at a rate significantly faster than the frequency operating in the 51V NMR experiment. Accordingly, this system can be described within the extreme motional narrowing limit where T2 ≈ T1, and because inhomogeneous contributions to the line width are quite small for the quadrupolar 51V, it follows that T2* is equal to T1. In this limit T1, the longitudinal relaxation time, replaces T2* in eq 1.26 Figure 4 shows the line widths calculated from eqs 1 and 2 in the motional narrowing limit in comparison with the measured spectral line widths for [VO2dipic]- in the TMA+ aqueous solutions as a function of salt concentration. Within the experimental error, the line widths predicted from the spin-lattice relaxation experiments agree with the measured 51V NMR line widths. Since the line width is predominantly governed by dipolar and quadrupolar effects, and because T2 must be less than or equal to T1, this suggests that longitudinal relaxation broadening dominates the observed line widths with little contribution from processes governed by T2*, such as chemical exchange. Analogous (26) van de Ven, F. J. M. Multidimensional NMR in Liquids: Basic Principles and Experimental Methods; VCH Publishers: New York, 1995.

DT )

kBT 6πηRH

(3)

In this equation kB is the Boltzmann constant, T is the temperature, η is the viscosity, and RH is the hydrodynamic radius of the particle. Diffusion constants of various components in the solutions calculated from the 1H PFG NMR data are given in Table 2. Overall, we observe no systematic variation in the HOD peaks from any of the solutions. We observe small changes in the TMA+ signals in the presence of the [VO2dipic]- compound, but they are not experimentally significant. As the concentration of [VO2dipic]- is 10-100 times lower than the TMA+ concentration, we do not expect to be able to observe the [VO2dipic]- impact on the TMA+ signal. There are also small decreases in the diffusion constant for the [VO2dipic]- in the presence of the TMA+ cations. This difference in diffusion of [VO2dipic]- in water and in the presence of TMA+ may reflect the interaction between the two species. These observations are consistent with the TMA+ cations associating with the [VO2dipic]molecules and with the idea that interaction between the two species slows down the diffusion through the solution. (27) Price, W. S. Concepts Magn. Reson. 1997, 9, 299.

6254


Stover et al.

Table 2. Diffusion Constants Calculated from Pulsed Field Gradient 1H NMR Measurements diffusion constant/10-9 (m2 s)a

a

sample

D 2O

D2O 0.1 M TMAB/D2O 10 mM [VO2dipic]-/D2O 10 mM [VO2dipic]-/0.1 M TMAB/D2O 10 mM [VO2dipic]-/1.0 M TMAB/D2O

1.90 1.81 1.79 1.89 1.71

[VO2dipic]-

TMAB 0.92

0.62 0.66 0.56

0.94 0.86

Errors are (0.05 × 10-9 m2 s.

Table 3. 51V NMR Chemical Shifts, Line Widths, and T1 Longitudinal Relaxation Times for Samples Containing 5 mM [VO2dipic]- in the Quaternary D2O/CTAB/n-pentanol/Cyclohexane Reverse Micellar Solutions as a Function of Water Content, w0 w0 5 7.5 15 20 30

reverse micelle radius (nm)a

peak position (ppm)

line width (Hz)

T1b (ms)

calcd line width (Hz)

3.0 ( 0.7 3.9 ( 0.9 4.0 ( 0.7 7.3

-530 -528 ( 1 -529 ( 1 -529.8 ( 0.6 -528.4 ( 0.1

1190 1130 ( 70 1130 ( 80 1160 ( 30 1170 ( 50

0.33 0.35 0.37 0.37

970 910 870 870

a Reverse micelle sizes are measured by dynamic light scattering. b T measurements for the reverse micelles push the instrumental 1 limits of the NMR spectrometer; hence we expect the error on these values may be as much as 50%. At the very least, these numbers represent an upper limit to the T1 times.

51

V NMR Spectroscopic Studies on Reverse Micelles. While interactions of positively charged TMA+ ions in a dispersed aqueous phase with the [VO2dipic]- complex are interesting, the ultimate goal of these studies is to understand the nature of the interface in reverse micelles, how metal complexes interact with these interfaces, and how these interactions are different from bulk solution interactions. To that end, we have examined the interaction of [VO2dipic]- at interfaces in water/CTAB/n-pentanol/ cyclohexane reverse micelles. We chose conditions for our CTAB solutions where reverse micelles have been reported to form.28 In these reverse micelles, the CTAB positively charged polar headgroup resides at the inner lipid interface beside the water pool. The TMA+ cation selected for the dispersed ion studies was specifically chosen for its structural similarity to the headgroup of the CTAB surfactant and its greater solubility in aqueous solution. Representative 51V NMR spectra obtained for [VO2dipic]in CTAB reverse micelles and in the presence of TMAB are shown in Figure 5. The spectrum in CTAB reveals

Figure 5. 51V NMR spectra of [VO2dipic]- in deuterium oxide/ CTAB/n-pentanol/cyclohexane quaternary microemulsions forming reverse micelles. For comparison, we also show the spectrum of [VO2dipic]- in tetramethylammonium bromide. The aqueous solution is 185 mM in [VO2dipic]-, making the overall concentration of the vanadium nucleus in the suspension 5 mM.

both a change in chemical shift and dramatic line broadening for the 51V signal. We attempted to measure the T1 longitudinal relaxation times for the [VO2dipic]complex in reverse micelles. However, the signals obtained were noisy and very near the lower limit of the NMR detection, precluding quantitative results. We place an upper limit of 0.4 ms on the lifetime of [VO2dipic]- inside the reverse micelles. The chemical shifts and line widths for the [VO2dipic]- in the quaternary microemulsion samples are given for a range of micelle sizes, w0 ) 5, 7.5, 15, 20, and 30, in Table 3. In contrast to the studies in dispersed aqueous salt solutions, 51V NMR signals of [VO2dipic]- also display a small chemical shift change in addition to line broadening that we have observed in other reverse micelle samples. Except for a small difference for the very smallest size reverse micelles, w0 ) 5, we observed no variation in chemical shift or line width as a function of w0. This is consistent with interpretation that the compound associates with the micellar surface rather than migrating into the expanding water pool in the larger reverse micelles. Because we use n-pentanol with the CTAB to form reverse micelles, to directly compare the reverse micelle results with results from the TMAB experiments, we have measured the 51V NMR line widths for [VO2dipic]- in salt solutions containing methanol as a model for the npentanol cosurfactant in the reverse micelles. We chose methanol as the model alcohol for several reasons. First, methanol has high solubility in water. Second, we want to model interactions of the [VO2dipic]- with an alcohol group because the -OH portion should be more accessible to the vanadium complex because the pentanol OH protrudes into the reverse micellar water pool. Third, complexation with methanol manifests as line broadening of vanadate NMR signals, which is a sensitive indicator of intermolecular interaction. The relative line widths obtained from aqueous solutions containing TMAB, methanol, and a methanol/TMAB mixture are shown in Figure 6. Caution must be used in interpreting the data from these experiments because alcohols are known to interact with vanadate to form esters,29,30 and alcohols (28) Cuccovia, I. M.; Dias, L. G.; Maximiano, F. A.; Chaimovich, H. Langmuir 2001, 17, 1060. (29) Gresser, M. J.; Tracey, A. S. J. Am. Chem. Soc. 1985, 107, 4215. (30) Crans, D. C.; Schelble, S. M.; Theisen, L. A. J. Org. Chem. 1991, 56, 1266.


Langmuir, Vol. 21, No. 14, 2005 6255

IV. Discussion

Figure 6. Comparison of 51V NMR line widths with [VO2dipic]in aqueous solutions of TMAB only (9), methanol only (b), and a TMAB/methanol solution (2). The aqueous solution is 185 mM in [VO2dipic]-, making the overall concentration of the vanadium nucleus in the suspension 5 mM. The ratio of methanol to TMAB is 5:1. The error bars represent the average of two measurements.

can also form ternary complexes with other vanadium complexes.31 Often, the presence of such esters is detected by additional peaks in the [VO2dipic]- 51V NMR spectrum or by line broadening. Neither the aqueous methanol, TMAB/methanol mixtures, nor the CTAB reverse micelles exhibit additional signals in the 51V NMR spectrum in the presence of [VO2dipic]-. In addition, while other vanadium complexes such as [VOdea] or [VOtea] will form ternary complexes with alcohols in organic solutions,32 there is no direct evidence for ternary complex formation of [VO2dipic]- with methanol or other alcohols in any of our studies. Thus, while it is likely that [VO2dipic]- interacts with methanol and the alcohol cosurfactants in the reverse micelles, it is unlikely that it forms esters or ternary complexes in these solutions. It is clear from the data in Figure 6 and Table 1 that the methanol affects how the [VO2dipic]- molecule interacts with the cations and that this environment differs from the solutions of dispersed salt molecules. Because specific properties of the micellar systems could potentially impact the interaction of the vanadium compounds at the interface, we explored the role of the cosurfactant and supporting nonpolar solvents while maintaining a constant micelle size, w0 ) 15. Specifically, we varied the cosurfactant from chain length by shortening it by use of 1-butanol and lengthening it by use of 1-decanol. In addition, the organic solvent isooctane was exchanged with cyclohexane, which leads to smaller reverse micelles. The extent of the interaction was measured by 51V NMR spectroscopy, while dynamic light scattering confirmed the formation of the reverse micelles. Little change was observed in the chemical shift for [VO2dipic]- in these different micelles. A small and statistically insignificant difference was observed in the line width as the composition of the micelle changed from n-pentanol to 1-butanol and the organic solvent changed from isooctane to cyclohexane. (31) Elvingson, K.; Keramidas, A. D.; Crans, D. C.; Pettersson, L. Inorg. Chem. 1998, 37, 6153. (32) Crans, D. C.; Chen, H. J.; Anderson, O. P.; Miller, M. M. J. Am. Chem. Soc. 1993, 115, 6769.

Interactions of [VO2dipic]- with Cations in Aqueous Solution. The changes in 51V NMR line width as a function of the systems was a key result of these experiments. Line width increases in 51V NMR spectra arise from various sources. Due to the quadrupolar nature of the nucleus, vanadium complexes are highly sensitive to their environment. They respond to changes in the local electric field as a result of species coordinated to or in the vicinity of the vanadium atom. Typically, symmetrical vanadium compounds display sharp features while ligand coordination often results in broader lines.33 The changing [VO2dipic]- line widths observed demonstrate that the various salt solutions influence the complex’s properties. Increasing line width generally reflects changes in dynamic or relaxation parameters of the V compound. At the pH value of 5 used here, the [VO2dipic]- complex displays substantial stability and most of the dipicolinate ligand remains coordinated to the vanadium ion at any point in time.23 Thus, the line width increases observed in these experiments should be attributed to environmental effects on the vanadium complexes. The largest increase in line width was observed in the most concentrated TMAB environment, with a 310 Hz line width; see Figure 3 and Table 1. Increasing line widths, albeit less dramatic, were observed with increasing KCl and NH4Br solutions as well. We note that TMA+ is most effective at increasing [VO2dipic]- line widths. Changes in the longitudinal relaxation times, T1, allow us to correlate observed changes in line width with relaxation times of the vanadium complex in the presence of the salts. These results provide insight into the interactions between species in dispersed aqueous solutions and the role of the interfacial environment in the reverse micelles. Relaxation of the vanadium nucleus is dominated by quadrupole interactions.23 In vanadium oxoanions and complexes, motional properties place it in the limit of extreme motional narrowing when T1 approximates T2.33-35 If no other factors contribute to line broadening then the broadening observed in the 51V NMR spectra will follow the results for T1; the line width is inversely proportional to the lifetime of the complex, and the shorter the lifetime, the broader the signal will be.35 In the experiments reported here, we measured signal broadening as a function of dispersed salts TMAB, NH4Br or KCl and as a function of the salt concentration. Figure 4 shows representative data for TMAB; results are given in Table 1. Except for the measurement at 0.1 M TMAB, the calculated and measured line widths are within experimental error of each other. In other words, the observed increases in line width display a strong correlation with the line widths calculated from lifetime data, confirming our hypothesis that line width increases are due to environmental effects. The 51V NMR line widths and lifetimes of the [VO2dipic]complex in aqueous salt solutions demonstrate that the complex interacts with cations in aqueous solution. What is less clear is the exact nature of the interaction between [VO2dipic]- and the cations. The ionic radii of K+ and NH4+ ions, 1.33 and 1.43 Å,36 respectively, are small enough that they could easily coordinate between the oxo groups (see Figure 1).21 We interpret the observed increasing line width with increasing concentration as (33) Butler, A.; Eckert, H. J. Am. Chem. Soc. 1989, 111, 2802. (34) Lee, H. C.; Oldfield, E. J. Am. Chem. Soc. 1989, 111, 1584. (35) Crans, D. C.; Rithner, C. D.; Theisen, L. A. J. Am. Chem. Soc. 1990, 112, 2901. (36) Weast, R. C.; Astle, M. J. CRC Handbook of Chemistry and Physics, 63rd ed.; CRC Press: Boca Raton, FL, 1982.

6256


evidence that the K+ and NH4+ ions reside near the partial negatively charged oxygen atoms on the [VO2dipic]-. However, the TMA+ ion is substantially larger than either K+ or NH4+ ion with an ionic radius of 2.2 Å, making it too big to ion-pair.37 It also possesses substantially greater lipophilic character from its methyl groups as well as weaker solvation by the surrounding water. Thus, one could imagine it interacting with [VO2dipic]- on either the VO2- side or the aromatic side of the dipicolinate ligand. Because the trends we observe are the same for the NMR spectra and T1 times of the [VO2dipic]- regardless of cation size or hydrophobicity, this suggests that TMA+ interacts with the VO2- side of the complex, similar to the K+ and NH4+ cations. Alternative interpretations could be invoked to explain our observations. In an aqueous environment containing a range of other components, it is possible for the [VO2dipic]- complex to interact and exchange its dipicolinate ligand.23 Most specifically, for all the dispersed aqueous environments probed here, we observed a chemical shift for the [VO2dipic]- identical to the shift observed for the complex in pure water, that is, -532 ppm. We believe that this is unlikely. While the dipicolinate ligand forms a 1:1 complex with vanadate,19 its complexes with other metals contain multiple dipicolinate ligands38 or ternary complexes with the dipicolinate and another ligand.39 In addition, while ternary complexes of vanadium with dipicolinate and peroxo or hydroxylamine ligands have been reported,11 these ligands’ properties differ substantially from those of the dispersed ions and alcohols under study here. Furthermore, conditions present at the pH value of 5 used in these studies minimized the lability of the [VO2dipic]- complex.23 Finally, in previous measurements, values for T1 do not generally correlate to line widths, because in most V(V) complexes ligand exchange is observed.23,30,35 The correlation between T1 and line width observed in these experiments is not consistent with dipicolinate ligand exchange but indicates a situation where environmental effects dominate. Thus we feel confident in our interpretation of the results. The line width broadening and lifetime shortening was significantly greater for the TMA+ ion than for the other two cations, K+ and NH4+. Potassium and ammonium ions are strongly solvated and will possess a complete water solvation sphere. In contrast, TMA+ should be less solvated, making easier for the ion to approach the vanadium complex. From a consideration of relative concentrations, the [VO2dipic]- complex should be virtually completely surrounded by the large TMA+ cations in the 3.0 M TMAB solution, leaving the complex very little room to maneuver without bumping into a TMA+ ion. In contrast, the smaller cations present in the KCl or NH4Br solutions have less direct interaction with the [VO2dipic]complex, and as a result a smaller line width increase is observed. However, differences in the effects of K+ and NH4+, which have very similar van der Waals and ionic radii, suggest that size is not the only factor of importance in this interaction. Indeed the NH4+ ion is much more effective in interacting with the water than K+ because it can also serve as an H-bond donor. While TMA+ and NH4+ are both ammonium ions, the TMA+ lacks the ability to form hydrogen bonds. Thus, the higher propensity of the TMA+ to associate with the [VO2dipic]- complex may (37) Eastoe, J.; Chatfield, S.; Heenan, R. Langmuir 1994, 10, 1650. (38) Yang, L. Q.; Crans, D. C.; Miller, S. M.; la Cour, A.; Anderson, O. P.; Kaszynski, P. M.; Godzala, M. E.; Austin, L. D.; Willsky, G. R. Inorg. Chem. 2002, 41, 4859. (39) Sengupta, S. K.; Sahni, S. K.; Kapoor, R. N. Synth. React. Inorg. Met. Org. Chem. 1983, 13, 117.

Stover et al.

reflect the increased hydrophobicity of TMA+ due to its greater steric bulk (compared to the ammonium and potassium ions) and not to hydrogen-bonding interactions. Further evidence that the TMA+ ion interacts with the [VO2dipic]- complex can be measured from pulsed field gradient (PFG) 1H NMR experiments. The PFG NMR experiments measure the degree of translational diffusion of a species. The Stokes-Einstein description of diffusion (eq 3) predicts that a molecule with larger molecular volume diffuses at a slower rate. Thus we expect that if the [VO2dipic]- complex associates strongly with another species, such as the TMA+ ion, a reduced diffusion rate will reflect that association. Table 2 data indicate only small or no differences in diffusion for the D2O in all the samples. This is not surprising as the concentration of water is 3-6 orders of magnitude higher than concentration of solutes. Similarly, since the TMA+ concentrations were 10-100 times larger than the concentration of the vanadium complex, the change in TMA+ signal is expected to be small and therefore below the detection limit. However, we observe a slight decreasing trend in the vanadium complex diffusion coefficient in the presence of TMA+ (see Table 2), suggesting that the ion slows down the translational motion. The most likely explanation for the change in diffusion coefficient is association between the cationic TMA+ and anionic [VO2dipic]-. Interactions in Reverse Micelles. Data for the [VO2dipic]- complex inside the CTAB reverse micelles compared with the [VO2dipic]- TMAB data demonstrate that the reverse micellar environment impacts [VO2dipic]more than the ionic interactions seen in the aqueous environments with dispersed ions. As given in Table 3, neither line width nor peak position varies between the differing micelle environments. In contrast, by comparison of data in Table 3 with that in Table 1, the 51V NMR peak positions and line widths for the five differently sized reverse micelles, w0 5, 7.5, 15, 20, and 30, differ from all of the dispersed salt systems. In addition, the T1 times are substantially smaller than in the aqueous systems; indeed, they are so small that we only place an upper limit on them. This is consistent with increasing [VO2dipic]- complex association with the lipid interface and a stronger perturbation from the interfacial electric field. The result of these effects is a faster relaxation of the 51V nucleus. Assuming that the system is in the limit of extreme motional narrowing,33,34 the line widths predicted from the T1 values are narrower than the measured line widths. This is expected when the [VO2dipic]- complex is firmly associated with the lipid interface, because under these conditions, the system is no longer within the motional narrowing limit and thus the substitution of the longitudinal relaxation, T1, for the spin-spin relaxation, T2*, in eq 1 is no longer valid. In either orientation, the [VO2dipic]- complex interacts strongly with the lipid interface, and this interaction is the source of the spectral differences observed between the dispersed aqueous solutions and the organized lipid interfaces. Because CTAB requires a cosurfactant to form reverse micelles, there is a possibility that the cosurfactant alone is responsible for some of the differences between the dispersed aqueous environments and the reverse micelles. In the reverse micelles explored here, the cosurfactant was a straight alcohol, namely, n-pentanol. Through comparisons of dispersed systems including alcohols, we can gauge the degree to which the alcohol hydroxyl group impacts [VO2dipic]- in the reverse micellar interior. The data shown in Figure 6 and given in Table 1 indicate that alcohol present in the aqueous solution leads to increased


Figure 7. Two possible orientations for the [VO2dipic]- complex at the CTAB/pentanol interface. In one orientation the hydrophobic effects are dominating (a); in the other orientation the Coulombic effects are dominating (b). 51

V NMR line widths. At the same time, even at the highest alcohol concentrations, 10.0 M, when the concentration in solution should be similar to the local concentrations in the reverse micellar interfacial region where the intramicellar [VO2dipic]- complex most likely resides, the reverse micellar environment results in more broadening and spectral shifting than we observe in the TMABmethanol solutions. Thus, we believe that the reverse micellar interior presents a substantially different environment than found in a solution with dispersed ions and alcohol molecules. Along with a large increase in the line width, the 51V NMR chemical shifts measured for the [VO2dipic]- complex in the reverse micelles appear at lower field, by about 2-3 ppm. The shift suggests that the [VO2dipic]- interacts with the interface of the reverse micelle, possibly partitioning into the interface, rather than being located in the middle of the reverse micellar water pool. This is to be expected in the smaller reverse micelles because there the micelle interior is quite small and there is less space for the complex to move away from the interface. However, similarities between the 51V NMR spectra in all the reverse micellar solutions indicate that even in the larger reverse micelles where the [VO2dipic]- complex would be able to dive to the core of the reverse micelle, an environment similar to bulk water, it appears to reside at or in the interface. In part, this is to be expected because the [VO2dipic]- possesses a negative charge while the trimethylammonium headgroups at the inner surface of the reverse micelles are positively charged. This is consistent with other results that show spectroscopic probes are observed to partition to the interfacial region rather than residing in the aqueous core.2 While the [VO2dipic]- complex clearly resides at the reverse micellar interface, it could reside at the interface in different orientations or partition into the interfacial region. Figure 7 depicts two likely locations for the vanadium complex at the interface. One shows an orientation where the hydrophobic effect dominates, causing the complex to partition into the interface (Figure 7a), while the other is dominated by Coulombic attraction, leaving the complex in the interfacial water layer (Figure 7b). We would expect both these locations to result in

Langmuir, Vol. 21, No. 14, 2005 6257

substantial broadening, consistent with our results, due to heterogeneity and to increased microviscosity of the water layer at the interface.40 Previous 1H NMR studies on substituted porphyrins at aqueous micellar interfaces have suggested that a shift toward lower field indicated that the complex is associated with the interface, while an shift toward higher field would indicate that the complex penetrated the lipid interface.41,42 By a similar rationale, the shifts to lower field we observed in 51V NMR suggest that the [VO2dipic]- complex associates with the cetyltrimethylammonium cation CTA+ of the surfactant headgroup. Furthermore, we have measured 1H NMR spectra of these reverse micelle solutions containing [VO2dipic]-. These spectra corroborate the shift toward lower field observed for the [VO2dipic]- from 51V NMR. Interpreting these data in fashion similar to that for porphyrins,41,42 while somewhat surprising, would direct the aromatic and hydrophobic ligand portion into the water pool, as shown in Figure 7b. Thus, the chemical shifts observed strongly suggest that Coulomb attraction determines the orientation of the [VO2dipic]- at the interface. Considering their potential as a therapeutic for type 2 diabetes,10-12 understanding how these vanadium complexes interact with and ultimately cross interfaces is of significant interest and value. Vanadium compounds have been shown to traverse biological interfaces by both active and passive transport mechanisms.14,43 Although transport in biological systems may take place through proteinmediated mechanisms,13 recent studies with vanadium complexes have shown that passive transport mechanisms are important and comparably fast.15 While compounds transported through passive pathways are generally expected to be charge-neutral, researchers have shown that negatively charged compounds can be transported across membranes through passive mechanisms; in particular, a study probing a series of vanadium dipicolinate complexes with the vanadium in oxidation states III, IV, and V showed that all three complexes transversed the membrane with similar permeability constants.15 All the major species of these three complexes are negatively charged, although a nonnegligible fraction of the V(IV) complex exists in solution as a neutral species.19,20,23 At the same time, reagents that can form ion pairs with the charged solute increased permeability constants for all three complexes, suggesting that ion pairing increases membrane permeability to charged species.15 The results from our investigations of [VO2dipic]- interactions with dispersed ions and interfaces in reverse micelles impact interpretation of the passive transport data. We observe strong interaction between [VO2dipic]- and TMA+ ions as well as the ammonium headgroups in CTAB. Our results show that Coulomb attraction dominates the [VO2dipic]-/ cation interactions, suggesting that ion pairing is likely in these systems, which may provide information on how the complex anion interacts with the lipid interface and thus potentially how such ion pairing affects the transport across membranes, as reported recently in studies in Caco cells.15 V. Summary A range of NMR experiments performed on [VO2dipic]in aqueous and reverse micellar environments are re(40) Hasegawa, M.; Yamasaki, Y.; Sonta, N.; Shindo, Y.; Sugimura, T.; Kitahara, A. J. Phys. Chem. 1996, 100, 15575. (41) Vermathen, M.; Louie, E. A.; Chodosh, A. B.; Ried, S.; Simonis, U. Langmuir 2000, 16, 210. (42) Vermathen, M.; Chodosh, A. B.; Louie, E. A.; Simonis, U. J. Inorg. Biochem. 1999, 74, 328. (43) Zhang, Y.; Yang, X.; Wang, K.; Crans, D. C. (submitted for publication).

6258


ported. Together the 51V NMR line widths, T1 times, and 1 H pulsed field gradient NMR measurements demonstrate that [VO2dipic]- interacts with tetramethylammonium ions in solution. These interactions appear to be dominated by Coulomb interactions. Measurements of line widths and T1 times for [VO2dipic]- in water/CTAB/n-pentanol/ cyclohexane quaternary microemulsions forming reverse micelles suggest that the micellar environment provides a stronger perturbation to the [VO2dipic]- complex than observed in the dispersed systems, showing that perturbations observed at self-assembled interfaces do not arise from Coulomb interaction alone. Results confirm that Coulomb attraction dominates both interactions between [VO2dipic]- and dispersed cations and interactions of [VO2dipic]- at the reverse micellar inner interface and unexpectedly suggest that the hydrophobic portion of the

Stover et al.

dipicolinato ligand points toward the aqueous micellar interior. These experiments provide valuable results aimed at understanding the interaction of the insulinmimetic [VO2dipic]- complex at organized interfaces. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant 0314719. This work was also supported through the Colorado State University Office of the Vice President for Research and Information Technology. J.S. is an undergraduate researcher. We are grateful to Rochelle R. Arvizo for her role in the preliminary studies of these systems. We thank Mandy Maes for the preparation of the tetramethylammonium salt of the [VO2dipic]-. LA0508137

Interaction of Dipicolinatodioxovanadium(V) with Polyatomic Cations

Recommend Documents