A (31)P-NMR study of the H(+)-MoO(4)(2-)-(HP)O(3)(2-)-HPO(4)(2-)-(C(6)H(5)P)O(3)(2-)-(CH(3)P)O(3)(2-) system at low Mo(tot)/P(tot) ratio - Formation of mixed-hetero X2M5-type polyanions.

Formation of X2M5-type heteropolyanions in the aqueous mixed-hetero H(+)-MoO(4)(2-)-(HP)O(3)(2-)-HPO(4)(2-)-(C(6)H(5)P)O(3)(2-)-(CH(3)P)O(3)(2-) system has been studied by (31)P-NMR at [Mo](tot)/[P(all)](tot) = 1.5 at 298(1) K in 0.600 M Na(Cl) ionic medium. The -log[H(+)] range 1.4-6.2 has been covered. The pK(a) values of methylphosphonate were determined from (31)P-NMR chemical shift data in the range of 0 < -log[H(+)] < 13.2 as H(CH(3)P)O(3)(-) 7.30 and H(2)(CH(3)P)O(3) 2.05. All four-component H(+)-MoO(4)(2-)-P1-P2 subsystems (P1 and P2 are two of the four phosphorus-containing components), except H(+)-MoO(4)(2-)-(HP)O(3)(2-)-HPO(4)(2-), which has been studied previously, were examined instead of the full six-component system in order to simplify the evaluation of the NMR spectra. The concentrations were adjusted to [Mo](tot) = 60 mM and [P1](tot) = [P2](tot) = 20 mM in the measurements. All conceivable P2M5 and (P1)(P2)M5 heteropolyanions were identified. The following novel X2M5-type polyanions were found and their formation constants are given in logarithmic values with 3sigma in parentheses: (CH(3)P)(2)Mo(5)O(21)(4-) 69.68(7), (HP)(C(6)H(5)P)Mo(5)O(21)(4-) 67.09(4), (C(6)H(5)P)PMo(5)O(22)(5-) 63.23(5), H(C(6)H(5)P)PMo(5)O(22)(4-) 68.11(4), (HP)(CH(3)P)Mo(5)O(21)(4-) 67.85(4), (CH(3)P)PMo(5)O(22)(5-) 64.14(7), H(CH(3)P)PMo(5)O(22)(4-) 68.89(4) and (C(6)H(5)P)(CH(3)P)Mo(5)O(21)(4-) 69.18(4). The equilibrium conditions are illustrated in distribution diagrams. Phosphite proved to bind more weakly in X2M5-type polyoxometallates than phosphate or phosphonates, which are almost equally favoured in complexation.

Speciation in aqueous vanadate-ligand and peroxovanadate-ligand systems.

In the present focused review, the speciation studies of aqueous vanadate-ligand (L) and peroxovanadate-L systems are addressed. The paper focuses solely on the systems studied at our department in the context of potential insulin-enhancing effects, including the following ligands: imidazole, alanylhistidine, alanylserine, lactate, picolinate, citrate, phosphate, maltol, and uridine. We summarise the results of detailed and thorough potentiometric (glass electrode) and (51)V NMR (Bruker AMX-500MHz) spectroscopic studies, performed at 25 degrees C in 0.150 M Na(Cl), a medium representing human blood. The importance of experimental conditions is discussed and illustrated. A detailed overview of our methodology, based on combining potentiometric and (51)V integral and chemical shift data by means of the computer program LAKE, is also given. We list the important steps of equilibrium analysis and the kinds of information available from different sets of NMR spectra. The ligand picolinate is chosen to exemplify our working method, but conclusions are drawn from all systems, reviewing trends and common features. An overview of all systems is given in two tables, including e.g. types and number of species formed. Previously unpublished modelling results at physiological conditions are also shown for all peroxovanadate-ligand systems.

Speciation in the aqueous peroxovanadate-maltol and (peroxo)vanadate-uridine systems.

The speciation in the aqueous H(+)/H(2)VO(4)(-)/H(2)O(2)/maltol (Ma), H(+)/H(2)VO(4)(-)/uridine (Ur) and H(+)/H(2)VO(4)(-)/H(2)O(2)/Ur systems was determined in the physiological medium of 0.150 M Na(Cl) at 25 degrees C. A combination of quantitative (51)V NMR (Bruker AMX500) and potentiometric data (glass electrode) was collected and treated simultaneously by the computer program LAKE. In the quaternary maltol system, the two species VXMa(2)(-) and VX(2)Ma(2-) (where X denotes the peroxo ligand) were found in the pH region 5-10, in addition to all binary and ternary complexes. Their formation was fast. In the ternary uridine (H(+)/H(2)VO(4)(-)/Ur) subsystem, altogether three vanadate-uridine (V-Ur) species were found in the pH region 4-10, with compositions VUr(2-), V(2)Ur(2)(2-) and V(2)Ur(2)(3-). Equilibrium was fast except in weakly acidic solutions when slowly decomposing decavanadates formed. In the quaternary H(+)/H(2)VO(4)(-)/H(2)O(2)/Ur system, five additional species were found at pH > 7. They were of VXUr and VX(2)Ur compositions. Their formation was fast. Formation constants, compositions and (51)V NMR chemical shifts are given for all the species found in the systems, and equilibrium conditions are illustrated in distribution diagrams as well as the fit of the model to the experimental data. Biological and medical relevance of the species (in the treatment of diabetes) are also discussed, with pseudo-physiological conditions modelled.

An equilibrium analysis of the aqueous H+-MoO4(2-)-(HP)O(3)2- and H+-MoO4(2-)-(HP)O(3)2--HPO4(2-) systems.

The speciation in the phosphitomolybdate system, H+-MoO4(2-)-(HP)O(3)2-, has been determined from combined potentiometric and 31P NMR measurements in 0.600 M Na(Cl) medium at 298(1) K. Potentiometric titration data were collected in the ranges 2.5<-log[H+]<6.2, 40.0

Speciation in the aqueous H+/H2VO4-/H2O2/phosphate system.

The speciation in the aqueous H(+)/H(2)VO(4)(-)/phosphate (dihydrogen phosphate, P(-)) and H(+)/H(2)VO(4)(-)/H(2)O(2)/P(-) systems has been determined in the physiological medium of 0.150 M Na(Cl) at 25 degrees C. A combination of multinuclear NMR integral and chemical shift (Bruker AMX500) as well as potentiometric data (glass electrode) have been collected and treated simultaneously by the computer program LAKE. The pK(a)-values for phosphoric acid have been determined by potentiometric and (31)P NMR chemical shift data, and have been found to be 1.85 +/- 0.02, 6.69 +/- 0.02 and 11.58 +/- 0.07. The errors given are 3sigma. Altogether nine vanadate-phosphate species have been found in the ternary H(+)/H(2)VO(4)(-)/P(-) system in the pH region 1-11, with the following compositions: VP, VP(2) and V(14)P. Equilibrium is very slow in acidic solutions, requiring more than 3 months for the formation of V(14)P species. On the other hand, less than 15 min are needed for equilibration at neutral and alkaline pH. In the quaternary H(+)/H(2)VO(4)(-)/H(2)O(2)/P(-) system, four new species have been found in addition to all binary and ternary complexes. They are of VXP and VX(2)P compositions, where X denotes the peroxo ligand. (51)V and (31)P NMR chemical shifts, compositions and formation constants are given, and equilibrium conditions are illustrated in distribution diagrams as well as the fit of the model to the experimental data. Biological and medical relevance of the species is also discussed and physiological conditions are modelled.

Speciation in the aqueous H+/H2VO4-/H2O2/citrate system of biomedical interest.

The speciation in the quaternary aqueous H+/H2VO4-/H2O2/citrate (Cit3-) and H+/H2VO4-/Cit3-/L-(+)-lactate (Lac-) systems has been determined at 25 degrees C in the physiological medium of 0.150 M Na(Cl). A combination of 51V NMR integral intensities and chemical shift (Bruker AMX500) as well as potentiometric data (glass electrode) have been collected and evaluated with the computer program LAKE, which is able to treat multimethod data simultaneously. The pKa-values for citric acid have been determined as 2.94, 4.34 and 5.61. Altogether six vanadate-citrate species have been found in the ternary H+/H2VO4-/Cit3- system in the pH region 2-10, only two of which are mononuclear. Reduction of vanadium(V) becomes more pronounced at pH < 2. Solutions, in which reduction occurred to any extent, were excluded from all calculations. In the quaternary H+/H2VO4-/H2O2/Cit3- system, eight complexes have been found in addition to all binary and ternary complexes over the pH region 2-10, including three mononuclear species. Equilibria in general are fast, but the significant and rapid decomposition of peroxide in acidic solutions limited the final model to pH > 4. In the quaternary H+/H2VO4-/Cit3-/Lac- system, two mixed-ligand species have been determined, with the compositions V2CitLac2- and V2CitLac3- (pKa = 5.0). To our knowledge, this is the first time such complexes have been reported for vanadium(V). 51V NMR chemical shifts, compositions and formation constants are given, and equilibrium conditions are illustrated in distribution diagrams as well as the fit of the model to the experimental data. When suitable, structural proposals are given, based on 13C NMR measurements and available literature data of related compounds.

Speciation in the aqueous H+/H2VO4-/H2O2/picolinate system relevant to diabetes research.

A detailed study of the quaternary aqueous H+/H2VO4-/H2O2/picolinate (Pi-) system has been performed at 25 degrees C in 0.150 M Na(Cl) medium using quantitative 51 V NMR (500 MHz) and potentiometric data (glass electrode). In the ternary H+/H2VO4-/Pi- system, six complexes have been found in the pH region 1-10. In the quaternary H+/H2VO4-/H2O2/Pi- system, eight additional complexes have been found. Generally, equilibria are fast in both systems. The rate of peroxide decomposition depends on the species in solution. Chemical shifts, compositions and formation constants for the species are given. Equilibrium conditions and the fit of the model to the experimental data are illustrated in distribution diagrams. Possible formation of mixed ligand species with imidazole, lactic acid and citric acid have been investigated and ruled out under the same experimental conditions. Structural proposals are given, based on 1)C NMR data and available crystal structures.

In vitro study of the insulin-mimetic behaviour of vanadium(IV, V) coordination compounds.

A representative set of vanadium(IV and V) compounds in varying coordination environments has been tested in the concentration range 1 to 10(-6) mM, using transformed mice fibroblasts (cell line SV 3T3), with respect to their short-term cell toxicity (up to 36 hours) and their ability to stimulate glucose uptake by cells. These insulin-mimetic tests have also been carried out with non-transformed human fibroblasts (cell line F26). The compounds under investigation comprise established insulin-mimetic species such as vanadate ([H(2)VO(4)](-)), [VO(acetylacetonate)(2)], [VO(2)(dipicolinate)](-) and [VO(maltolate)(2)], and new systems and coordination compounds containing OO, ON, OS, NS and ONS donor atom sets. A vitality test assay, measuring the reduction equivalents released in the mitochondrial respiratory chain by intracellular glucose degradation, is introduced and the results are counter-checked with (3)H-labelled glucose. Most compounds are toxic at the 1 mM concentration level, and most compounds are essentially non-toxic and about as effective as or more potent than insulin at concentrations of 0.01 mM and below. V(V) compounds tend to be less toxic than V(IV)compounds, and complexes containing thio functional ligands are somewhat more toxic than others. Generally, ON ligation is superior in insulin-mimetic efficacy to OO or O/ NS coordination, irrespective of the vanadium oxidation state. There is, however, no striking correlation between the nature of the ligand systems and the insulin-mimetic potency in these cell culture tests, encompassing 41 vanadium compounds, the results on 22 of which are reported in detail here. The syntheses and characteristics of various new compounds are provided together with selected speciation results. The crystal and molecular structures of [[VO(naph-tris)](2)] [where naph-tris is the Schiff base formed between o-hydroxynaphthaldehyde and tris(hydroxymethyl)amine] are reported. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00775-001-0311-5.