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Spectroscopic and DFT Study on the Interaction System of Vanadium with L-Proline in Aqueous Solution Birong Zeng,† Tonghao Shen,‡ Anan Wu,‡ Shuhui Cai,§ Xianyong Yu,§ Xin Xu,*,‡ and Zhong Chen*,§,‡ Department of Materials Science and Engineering, and Department of Physics, Fujian Key Laboratory of Plasma and Magnetic Resonance, Xiamen UniVersity, Xiamen 361005, China and Department of Chemistry, State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen UniVersity, Xiamen 361005, China ReceiVed: February 3, 2010; ReVised Manuscript ReceiVed: March 10, 2010
To gain more insight into the interactions between vanadium and L-proline, the dynamic transformation of the reaction system of vanadium with L-proline and the coordination structures of the vanadium-containing products in 0.15 mol/L NaCl ionic medium mimicking physiological conditions were explored by multinuclear (1H, 13C, 51V) NMR, ESR, ESI-MS as well as density functional theory (DFT) calculations. Spectroscopic evidence and computational results showed that a monoperoxovanadium species [VO(O2)(L-proline)2]- was a major product, where L-proline coordinated to vanadium via nitrogen and oxygen atoms in a bidentate manner to form a distorted pentagonal bipyramidal structure. The species [VO(O2)(L-proline)2]- underwent chemical changes in solution at room temperature, finally leading to the reduction of vanadium(V) to vanadium(IV) and the formation of [VO(L-proline)2]. In the tetrahedral structure of the reduction product [VO(L-proline)2], L-proline also coordinated to vanadium in a bidentate manner. Such an investigation may be helpful for a better understanding of vanadium complexes as insulin-enhancing agents for the treatment of diabetes. Introduction Over the last several decades, a large body of evidence has accumulated to suggest that vanadium, whereas it is a trace element in biochemical systems, plays an important role in metalloenzymes and impacts insulin regulations.1-3 The biochemical activities of vanadium and its compounds are thought to be related to the chemical similarity of vanadate to phosphate, which allows vanadium compounds to interact with numerous enzymes in living organisms by either inhibiting or activating them.4 One of these interactions is the interference of vanadium with the glucose uptake, which probably involves the inhibition of tyrosine phosphatase (PTP1B) enzymes. However, the inhibitory mechanisms are not completely known yet.5 To gain detailed insight into the vanadium-ligand interactions that are essential for the activity of PTP1B inhibitors, it is important to study the solution chemistry of the vanadium-containing systems, including the complete speciation in the interaction process. In the past decades, many works have been done on the reaction systems of vanadium with kinds of ligands, including many amino acids.6-13 All of these studies are helpful for a better understanding of the pharmacological properties of vanadium complexes. However, the structures of vanadium products in solution are not described in detail in these studies. Among the selected amino acids, the reactions of vanadium with L-proline have not been reported. It is known that PTP1B is a protein with responsibility for the negative regulations of both * To whom correspondence should be addressed. E-mail: chenz@ xmu.edu.cn;
[email protected]. Tel: +86-592-2181712. Fax: +86-5922189426. † Department of Materials Science and Engineering, Fujian Key Laboratory of Plasma and Magnetic Resonance. ‡ State Key Laboratory of Physical Chemistry of Solid Surfaces. § Department of Physics, Fujian Key Laboratory of Plasma and Magnetic Resonance.
insulin- and leptin-simulated signal transduction. In PTP1B, two proline molecules are contained in the WPD-loop (defined as Thr177-Pro185), which has structural flexibility and is important for inhibitor binding.14,15 If proline could interact with vanadium, then the WPD-loop’s conformation would be regulated by the binding between vanadium and proline, thus causing a change in the activity of PTP1B. Therefore, the interactions of vanadium with proline and the coordination structure of vanadium-proline complexes in solution are worth studying. It is well known that 51V NMR spectroscopy is a good technique for characterizing vanadium species in solution.16-19 51 V NMR chemical shifts are very sensitive to the coordination sphere of metal center so that information on the coordination structure of vanadium complexes can be obtained via 51V NMR.20,21 In this article, the reactions in the system of vanadium with L-proline were investigated by NMR spectroscopy. NaCl medium (0.15 M) was used as solvent to model the human physiological environment. ESR and ESI-MS were used as a supplemental tool for the analysis of reaction products.22,23 DFT calculations were performed to rationalize the experimental observations.24-27 Through the combined use of these methods, the transformation process of the interaction system was achieved, and the solution structures of vanadium-L-proline complexes were determined. Experimental Methods Materials and Preparations. The compounds D2O, H2O2 (30%), NaCl, NH4VO3, and L-proline were commercial products (Sinopharm Chemical Reagent Company) used without further purification. The system of vanadium with L-proline was prepared as follows: NH4VO3 and H2O2 were first mixed in a molar ratio of 1:1 and then dissolved in 0.15 mol/L NaCl ionic medium. Finally, L-proline was added to this solution. The
10.1021/jp1010378 2010 American Chemical Society Published on Web 03/25/2010
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reaction mixture might be diluted to an appropriate concentration for spectroscopic measurements. NMR Measurements. Spectral data were collected on a Bruker AV 300 MHz spectrometer. DSS (trimethylsilylpropanesulfonic acid sodium salt) was used as an internal reference for the 1H and 13C chemical shifts. The 51V chemical shifts were measured relative to the external standard VOCl3 with upfield shift being considered as negative. The sample of the interaction solution was freshly prepared. Half an hour later, a series of 51 V, 1H and 13C spectra were recorded in turn. The acquisition times were 15, 5, and 240 min for 51V, 1H and 13C spectra, respectively. This cycle was repeated for ∼2880 min. ESR Spectroscopy. ESR was used to measure the vanadium(IV) compounds and to monitor the reaction process in the system of vanadium with L-proline. The spectra were recorded at room temperature (25 °C) in a capillary tube in a Bruker EMX 10/12 spectrometer. ESI-MS. Experiments were performed on a Finnigan MAT LCQ instrument. The solutions were introduced into the instrument by direct injection. The concentration of the vanadium solution was 1.0 × 10-2 mol/L in H2O. Theoretical Calculations. All geometries were optimized at the B3LYP28-31/6-31+G(d) level of theory. Analytic vibrational frequencies calculations at the same level were performed to ensure that each optimized geometry was a true local minimum. For the calculations of the nuclear magnetic shielding tensor, OPBE functional32-35 in conjunction with the 6-311+G(2d,p) basis set36 was used. The GIAO technique was employed to circumvent the gauge origin problem.37 Solvation effects were taken into account by using polarizable continuum model (PCM).38 All calculations were carried out using Gaussian 03 program suite.39 Results and Discussion Spectroscopic Characterization. NMR and ESR were employed to study the dynamic transformation and the vanadium species in the interaction system of vanadium with L-proline. Vanadate in the presence of hydrogen peroxide may form a number of products, including mono-, di-, tri-, and tetraperoxodivanadates, depending on the pH, solutes proportion, and concentration of the mixture solution.40-42 In this study, the concentration of the vanadium is 20 mmol/L with a molar ratio of vanadate/hydrogen peroxide/L-proline 1:1:3, forming a red solution with pH 6.7. Under such conditions, the interaction among the solutes in the system gave rise to one major species and five minor species with relative low concentrations, corresponding to a strong peak at -630 ppm (compound 1) and five weak peaks in the range of -400 to -800 ppm in the 51V NMR spectrum shown in Figure 1a. On the basis of the report of Howarth and coworkers,41 the three resonance peaks at -422, -498, and -513 ppm were assigned to decavanadate with a formula of [V10O28]6- (compound 2). The peak at -645 ppm (compound 3, 2% of the total amount of vanadium) was assigned to a byproduct in the system, which will be further explained later. The peaks at -694, -708, and -760 ppm were assigned to diperoxovanadates [V(OH)2(O2)2(H2O)2]- (compound 4), [HV(OH)2(O2)2(H2O)2] (compound 5), and dimer species [{VO(O2)2}2(OH)]3- (compound 6), respectively.43 From the integral areas ratio of all peaks in 51V NMR spectrum, we can see that the sum of decavanadate, diperoxovanadates, and dimer species accounts for 46% of the total amount of vanadium. The peak at -630 ppm (52% of the total amount of vanadium) was supposed to come from V(V)-L-proline complex. According to the ligand concentration in the reaction system, we can deduce
Zeng et al.
Figure 1. 51V NMR spectra of a 0.15 M NaCl D2O solution of the interaction system of vanadium with L-proline recorded after (a) 30, (b) 300, (c) 570, (d) 840, (e) 1110, and (f) 1680 min of preparation.
TABLE 1: 51V NMR Chemical Shifts for Species Observed in the System of Vanadium with L-Proline species numbering compound 2 compound 4 compound 5 compound 6 compound 7 compound 10 compounds 1, 3, 8, 9
δ(51V)/ppm
formula 6-
[V10O28]
-
[V(OH)2(O2)2(H2O)2] HV(OH)2(O2)2(H2O)2 [{VO(O2)2}2(OH)]3[V4O12]4[V5O15]5[VO(O2)(H2O)x(L-proline)y]n(x ) 0, 1, 2; y ) 1, 2; n ) 0, 1)
-422, -498, -513 -694 -706 -760 -578 -584 -630, -645, -662, -664
that every vanadium-L-proline product molecule has two ligands.44 This deduction is supported by the 13C and 1H NMR spectra of the reaction system and further confirmed by DFT calculations (explained later in the article). Because stability is an important aspect for vanadium compounds,45,46 we explored the stabilities of the vanadium species in the system of vanadium with L-proline by measuring the variations of the 51V, 1H and 13C NMR spectra with the increase in interaction time. The time-dependent 51V NMR spectra are given in Figure 1. It shows that compound 1 at -630 ppm was predominant, and each of compounds 2-6 give weak signal intensities in the period of 30-45 min after sample preparation. It can be seen that this interaction system at 300 min (Figure 1b) had the same vanadium species as those observed in Figure 1a. The amount of compound 1 (-630 ppm) still accounted for 52% of the total amount of vanadium. However, the sum of decavanadate, diperoxovanadates, and dimer species decreased to 42%, and the amount of compound 3 (-645 ppm) increased from 2 to 6%. At 570 min, compounds 7 (-578 ppm), 8 (-662 ppm), and 9 (-664 ppm) appeared, whereas compounds 4-6 almost disappeared (Figure 1c). Compound 1 accounted for 68% of the total amount of vanadium, and the sum of the integral areas of the peaks at -645, -662, and -664 ppm accounted for 10%. At 840 min, compound 1 lessened visibly (Figure 1d). Only compounds 7 and 10 existed in the solution, whereas all others including compound 1 disappeared entirely in the period of 1110-1125 min (Figure 1e). Obviously, the total amount of vanadium(V) decreased. The peaks at -578 and -584 ppm were assigned to tetramer [V4O12]4- and pentamer [V5O15]5-.3 The peaks at -662
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Figure 2. ESR spectra of (a) a 2.0 mM VOSO4 solution and (b) the system of vanadium with L-proline after 2880 min of preparation. Figure 5. Full 13C NMR spectrum of a 0.15 M NaCl D2O solution of the interaction system of vanadium with L-proline recorded in the reaction period of 30-270 min.
TABLE 2: 13C and 1H NMR Chemical Shifts of Free and Coordinated L-Proline in the Interaction System δ (coordinated δ (free L-proline)/ppm
Figure 3. Variation of the relative intensity of ESR signals of the system of vanadium with L-proline with time.
C-1 C-2 C-3 C-4 C-5 H-2 H-3 H-4 H-5 a
Figure 4. 13C NMR spectra of the C-2 atom of L-proline in the interaction system in the reaction period of (a) 30-270, (b) 300-570, (c) 840-1080, (d) 1110-1380, (e) 1440-1680, and (f) 2880-3120 min.
and -664 ppm were assigned to a vanadium-L-proline species different from complex 1.44 At 1680 min, no signal was acquired (Figure 1f) in the 51V NMR spectrum, indicating that all vanadium(V) has been deoxidized to paramagnetic vanadium(IV). The vanadate species observed in our experiments and their corresponding 51V NMR peak assignments are summarized in Table 1. Because 51V NMR is limited to measuring the vanadium(V) species while vanadium(IV) cannot be resolved, ESR was used to confirm the NMR study. The ambient temperature ESR spectrum of a solution containing 2.0 mM VOSO4 shows a signal with the characteristic eight-line pattern of VO2+(IV) (Figure 2a). When reaction time was shorter than 840 min, the ESR spectra of the interaction system had no signal. Later, a characteristic eight-line signal emerged with intensity that increased gradually. It depicts the formation of the vanadium(IV) complex, in agreement with the NMR results. The fine structure
174.6 61.2 28.9 23.7 46.0 4.11 (t, 1H) 2.34 (m, 2H) 2.01 (m, 2H) 3.35 (m, 2H)
L-proline)/ppm
185.0 65.0 30.0 26.7 55.6
181.8 64.4 29.9 25.8 51.6 3.95 2.22 2.15 3.78 3.58
|∆δ|/ppma 10.4, 7.2 3.8, 3.2 1.1, 1.0 3.0, 2.1 9.6, 5.6 0.16 0.12 0.14 0.42, 0.22
∆δ ) δcoordinated ligand - δfree ligand.
of the ESR signal of vanadium(IV) complex in the interaction system (Figure 2b) shows distinguishable difference from VO2+ signal. The parameters g and A0 are 2.055 and 111.4 G for VO2+, whereas they are 2.023 and 105.5 G for V(IV)-L-proline, respectively. This difference indicates that L-proline interacts with vanadium(IV) center. Furthermore, plotting the intensity of the ESR signal as a function of time allows the behavior shown in Figure 3 to be observed. The ESR signal is zero at the beginning and then gradually increases with the start of reduction, reaching its maximum intensity when reduction reaction finishes. Finally, it forms a plateau. Besides 51V NMR and ESR, 13C NMR is another effective tool to track the interaction process. 13C NMR spectra of the C-2 atom of L-proline in the interaction system recorded at different reaction time are shown in Figure 4. In the period of 30-270 min, two new sets of L-proline signals emerged besides the free ligand, and their chemical shifts were in the downfield
Figure 6. (a) Numbering of carbon atoms in L-proline, (b) the most stable structure of [VO(O2)(L-proline)2]-, and (c) the most stable structure of [VO(L-proline)2].
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TABLE 3: Reaction Equations and Related Energy Variations of the System of Vanadium With L-Proline ∆G (298 K) kJ/mol no.
reaction equation +
in solution
in vacuo
+
1
VO2 + H2O2 h VO(O2) + H2O
2
[VO(O2)(H2O)3]+ + H2O h VO(O2)[(H2O)4]+
3
[VO(O2)(H2O)3]+ + 2L-prolineh
-4.28
3.17 -44.6
-164.8
[VO(O2)(L-proline)2]- + 2H+ + 3H2O 3-A1
[VO(O2)(H2O)3]+ + L-proline-h [VO(O2)(H2O)2(L-proline)] + H2O
-30.2
-139.2
3-A2
[VO(O2)(H2O)2(L-proline)] + L-proline-h
-14.4
-25.5
-37.0
-144.2
-7.6
-20.6
-
[VO(O2)(L-proline)2] + 2H2O 3-B1
[VO(O2)(H2O)3]+ + L-proline-h [VO(O2)(H2O)(L-proline)] + 2H2O
3-B2
[VO(O2)(H2O)(L-proline)] + L-proline-h -
[VO(O2)(L-proline)2] + H2 3-C1
[VO(O2)(H2O)3]+ + L-proline-h [VO(O2)(L-proline)] + 3H2O
-29.0
-141.0
3-C2
[VO(O2)(L-proline)] + L-proline-h
-15.6
-23.8
-32.2
-136.7
[VO(O2)(L-proline)2]4
[VO(O2)(H2O)4]+ + 2L-prolineh -
+
[VO(O2)(L-proline)2] + 2H + 4H2O region relative to the free ligand (Figure 4a). Their average intensities increased and reached a maximum in the period of 300-570 min (Figure 4b). The solution was clear and red during this period. As time further lapsed, the intensities of the two new sets of signals decreased (Figure 4c) and then disappeared finally (Figure 4d-f). When the color of the solution changed to blue, vanadium(IV) species formed. At this moment, all 13C and 1H NMR signals had very broad half-height widths due to the paramagnetic environment, which made it hard to determine the peaks of V(IV)-L-proline product. These results correlate well with the ESR observation. Figure 5 shows the full 13C NMR spectrum in the reaction period of 30-270 min. It can be seen that besides the 13C signals from free L-proline, there are two new sets of L-proline signals. Because the product yield of compound 1 is 25 times more than that of compound 3, the two new sets of 13C signals were assigned to compound 1. They have equal integral areas. This indicates that there are two L-proline molecules coordinated to vanadium center in compound 1 with different coordination environments. The spectral assignments are listed in Table 2. (See Figure 6a for the atomic labeling of L-proline.) All 13C chemical shifts of five carbon atoms in coordinated L-proline molecule significantly move to low field relative to free ligand. The order of variation amount is C-1 > C-5 > C-2 > C-4 > C-3. Because vanadium(V) would make much more change to the electronic density of the atom linked to or adjacent to it, reflected by the variation of chemical shift, the larger change of C-1, C-5, and C-2 chemical shifts due to the coordination suggests that the possible coordinating groups of L-proline are carboxyl and amino groups. Table 2 also lists the 1H NMR spectral assignments. The atom H in the imino group -NH- is reactive, and it is easily exchanged by deuterium, so its signal could not be observed in the 1H NMR spectra. The chemical shift changes of hydrogen atoms in L-proline between the free one and coordinated one have the same order as that of carbon atoms
TABLE 4: Optimized Geometric Data of [VO(O2)(L-proline)2]- and [VO(L-proline)2]a,b [VO(O2)(L-proline)2]bond length VdO1 V-O2 V-O3 V-O4 V-O6 O2-O3 V-N1′ V-N2′
[VO(L-proline)2]
bond angle 1.608 1.879 1.898 2.169 2.098 1.426 2.178 2.169
O1-V-O2 O1-V-O3 O2-V-O3 O1-V-O4 O1-V-O6 O1-V-N1′ O1-V-N2′ N1′-V-N2′
bond length 103.3 101.9 44.3 167.7 89.6 95.0 99.9 147.4
VdO V-O1 V-N1′ V-O2 V-N2′
bond angle 1.584 1.941 2.129 1.941 2.129
O1-V-N1′ O2-V-N2′ O1-V-N2′ O2-V-N1′ OdV-O1 OdV-O2 OdV-N1′ OdV-N2′ O1-V-O2 N1′-V-N2′
a Distances are in angstroms and angles are in degrees. Figure 6b,c for atomic labeling.
78.8 78.8 87.9 87.9 119.0 119.0 103.8 103.8 122.0 152.5 b
See
discussed above: |∆δ5| > |∆δ2| > |∆δ4| > |∆δ3|. The large variations of 13C and 1H chemical shifts upon complexation suggest a strong interaction of L-proline with the vanadium center. The possible coordination structure of complex 1 is shown in Figure 6b. DFT Calculations. In the absence of hydrogen peroxide, vanadium(V) is present in aqueous solution as vanadate or its derivatives depending on pH and reduction potential.2 It is known that VO2+ is usually regarded as a precursor for the formation of peroxovanadium derivatives.47 The reaction equation is shown in eq 1 (Table 3). The ESI-MS spectrum of the solution of vanadium and hydrogen peroxide show that the peaks at m/z 99, 153, and 171 are corresponding to [VO(O2)]+, [VO(O2)(H2O)3]+, and [VO(O2)(H2O)4]+, respectively. Theoretical calculations were carried out to obtain the relative energies for elucidating the reaction process (Table 3). The addition of one H2O molecule to [VO(O2)(H2O)3]+ to form
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TABLE 5: Calculated 13C Chemical Shifts of Free and Coordinated L-Proline (in ppm) C-1 C-2 C-3 C-4 C-5 a
δcalcd -proline
∆δprolinea
177.4 63.1 31.3 26.2 48.9
2.8 2.0 2.4 2.5 2.9
δcorrect-coordb
δcalcd-coord 178.2 68.8 34.1 30.6 58.2
175.2 68.1 33.9 30.9 53.7
175.4 66.8 31.7 28.4 55.3
172.4 66.1 31.5 28.1 50.8
∆δcoordc -9.6 1.85 1.7 1.7 -0.25
-9.4 1.85 1.6 2.3 -0.75
δproline ) δcalcd-proline - δexptl-proline. b δcorrect-coord ) δcalcd-coord - δproline. c δcoord ) δcorrect-coord - δexptl-coord
[VO(O2)(H2O)4]+ (eq 2) is a process with an reaction energy (∆G at 298 K) of 3.17 kJ/mol in PCM and -4.28 kJ/mol in gas phase. Both [VO(O2)(H2O)3]+ and [VO(O2)(H2O)4]+ were observed in the mass spectrum, so both of them were taken as reaction precursors for the formation of V(V)-L-proline complexes in solution. The postulated reaction equations and their relative energies are shown in eqs 3 and 4. The calculated results show that the coordination of L-proline to [VO(O2)(H2O)3/4]+ to form [VO(O2)(L-proline)2]- complex is a favorable exothermic reaction in both PCM and gas phase. The product [VO(O2)(L-proline)2]- is stable in thermodynamics. Because eq 3 has more exothermic energy than eq 4, eq 3 was further analyzed into three types of pathways, as shown in eqs 3-A1-3C2. The corresponding relative energies of each step were obtained. The 51V NMR results have shown that compound 1 (-630 ppm) is the major product and compounds 3 (-645 ppm), 8 (-662 ppm), and 9 (-664 ppm) are the minor species. Besides, compounds 1 and 3 are more favorable than compounds 8 and 9 in kinetics. Here, combined with the energy data, it can be concluded that compounds 3, 8, and 9 correspond to somebyproductsuchas[VO(O2)(H2O)(L-proline)],[VO(O2)(H2O)2(Lproline)], and [VO(O2)(L-proline)]. However, it is hard to fulfill the specific assignments further. The major product [VO(O2)(Lproline)2]- and the three byproducts could be found in the mass spectrum of the solution of vanadium and L-proline with m/z 327, 249, 231, and 213, respectively. To gain more insight into the coordination structures of the major V(V)-L-proline complex and its reduction product V(IV)-L-proline, DFT optimizations were also carried out. As suggested by the concentration data and the NMR results, there are two proline molecules in the major V(V)-L-proline complex, and each molecule coordinates to vanadium in a bidentate manner via the nitrogen atom and the oxygen atom of carboxylic group. To verify these suggestions, we optimized the structures of [VO(O2)(H2O)n(L-proline)] (n ) 0, 1, 2) and [VO(O2)(Lproline)2]- with all coordination possible. The results show that [VO(O2)(L-proline)2]- is the most stable state with the structure illustrated in Figure 6b. Such a bonding structure is in line with the deduction from the experimental observations. The nitrogen atoms of the two proline ligands are in the equatorial positions, cis to the peroxo oxygen atoms. The apical position is occupied by the oxygen of V ) O, and its opposite site is the oxygen atom from a -COO- group. The five oxygen and two nitrogen atoms, coordinated to the V atom, form a distorted pentagonal bipyramid. The key geometric parameters of the most stable isomer of [VO(O2)(L-proline)2]- are listed in Table 4. The η2peroxo ligand is coordinated asymmetrically with d(V-O2) ) 1.879 and d(V-O3) ) 1.898 Å. The V-N bond lengths are d(V-N1′) ) 2.178 and d(V-N2′) ) 2.169 Å. The elongation of d(V-O4(apical)), as compared with d(V-O6(equatorial)), is 0.071 Å. All of these are in accordance with geometries of other similar monoperoxo complexes.48 Similarly, the vanadium(IV) complex [VO(L-proline)2] was found to be most stable with the coordination fashion shown in Figure 6c. The two proline ligands symmetrically coordinate
to vanadium in a bidentate form via the nitrogen atom and the oxygen atom of the carboxylic group. The important geometric data are also listed in Table 4. The V-O(equatorial) and V-N bond lengths are d(V-O1) ) d(V-O2) ) 1.941 Å and d(V-N1′) ) d(V-N2′) ) 2.129 Å, respectively. 13 C NMR shielding tensors were calculated for all eight possible isomers of [VO(O2)(L-proline)2]-. Because the free energies of other isomers are at least 12.6 kJ/mol higher than the lowest one, only the most stable one is used for further analysis. 13 C NMR parameters were first calculated for L-proline and DSS. The differences of chemical shifts between calculated and experimental data were taken as rovibrational corrections for the coordinated proline. (See eq 5.) The calculated isotropic chemical shifts are given in Table 5.
δcorrect-coord ) δcalcd-coord - (δcalcd-proline - δexptl-proline) (5) As shown in Table 5, the calculated isotropic 13C NMR shifts agree well with the experimental values. It gives an overall mean absolute deviation (MAD) of 3.4 ppm. The largest errors occur at the carboxyl carbons. However, it is known that the computational data for carboxyl carbon is not as reliable as the others. If the carboxyl carbons were omitted in error analysis, then the MAD between the calculated values and experimental data was reduced to 0.9 ppm, leading strong support to the geometric assignment of the V(V)-L-proline complex. Conclusions L-proline was found to interact with vanadate in the system of vanadium with L-proline in 0.15 mol/L NaCl ionic medium. On the basis of the results of NMR, ESR, and ESI-MS measurements and DFT calculations, the major product is suggested to be a new monoperoxovanadate species [VO(O2)(Lproline)2]-. L-Proline coordinates to vanadium via nitrogen and oxygen atoms in a bidentate manner, and [VO(O2)(L-proline)2]adopts a distorted pentagonal bipyramidal coordination. This complex undergoes chemical changes in solution at room temperature, finally leading to the departure of the peroxy group (-O-O-) in the complex accompanied by the reduction of vanadium(V) to vanadium(IV). In the tetrahedral structure of the reduction product [VO(L-proline)2], L-proline also coordinated to vanadium in a bidentate manner. As we have mentioned in the Introduction, the WPD-loop has been regarded to be essential for the catalytic mechanism of PTP1B. When the two proline residues in the WPD-loop interact with vanadium in a bidentate manner, the WPD-loop may lose the function of forming a binding pocket for the residues in the active site of PTP1B, resulting in the regulations of insulin signal transduction. Even if vanadium(V) was reduced to vanadium(IV) in the cells, the vanadyl could also interact with proline in a bidentate manner, maintaining an inhibition of PTP1B to some extent. We expect that the information we obtained herein can help to
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develop potential PTP1B inhibitors with longer acting time. Further studies would be centered on the interconversion rates and the mechanism of the vanadium species formed in the interaction system and the competitive effect of different kind of amino acids. These would be of significance for the vanadium complexes used as insulin-enhancing agents for the treatment of diabetes. Acknowledgment. This work was supported by the Science Research Foundation of Ministry of Health & United Fujian Provincial Health and Education Project for Tackling the Key Research (WKJ2008-2-36), the National Natural Science Foundation of China (20525311, 20923004), and the Ministry of Science and Technology (2007CB815206). References and Notes (1) Thompson, K. H.; Orvig, C. J. Inorg. Biochem. 2006, 100, 1925– 1935. (2) Crans, D. C.; Smee, J. J.; Gaidamauskas, E.; Yang, L. Q. Chem. ReV. 2004, 104, 849–902. (3) Polenova, T.; Pooransingh-Margolis, N.; Renirie, R.; Wever, R.; Rehder, D. In Vanadium: The Versatile Metal; American Chemical Society: Washington, DC, 2007. (4) Tsiani, E.; Fantus, I. G. Trends Endocrinol. Metab. 1997, 8, 51– 58. (5) Hiromura, M.; Sakurai, H. Chem. BiodiVersity 2008, 5, 1615–1621. (6) Li, M.; Ding, W. J.; Baruah, B.; Crans, D. C.; Wang, R. L. J. Inorg. Biochem. 2008, 102, 1846–1853. (7) Rehder, D. J. Inorg. Biochem. 2008, 102, 1152–1158. (8) Gabriel, C.; Kaliva, M.; Venetis, J.; Baran, P.; Rodriguez-Escudero, I.; Voyiatzis, G.; Zervou, M.; Salifoglou, A. Inorg. Chem. 2009, 48, 476– 487. (9) Gorzsas, A.; Anderson, I.; Pettersson, L. J. Inorg. Biochem. 2009, 103, 517–526. (10) Lippold, I.; Becher, J.; Klemm, D.; Plass, W. J. Mol. Catal. A: Chem. 2009, 299, 12–17. (11) Lovat, S.; Mba, M.; Abbenhuis, H. C. L.; Vogt, D.; Zonta, C.; Licini, G. Inorg. Chem. 2009, 48, 4724–4728. (12) Smee, J. J.; Epps, J. A.; Ooms, K.; Bolte, S. E.; Polenova, T.; Baruah, B.; Yang, L.; Ding, W.; Li, M.; Willsky, G. R.; La Cour, A.; Anderson, O. P.; Crans, D. C. J. Inorg. Biochem. 2009, 103, 575–584. (13) Tracey, A. S. J. Inorg. Biochem. 2000, 80, 11–16. (14) Barford, D.; Flint, A. J.; Tonks, N. K. Science 1994, 263, 1397– 1404. (15) Jia, Z.; Barford, D.; Flint, A. J.; Tonks, N. K. Science 1995, 268, 1754–1758. (16) Ooms, K. J.; Bolte, S. E.; Smee, J. J.; Baruah, B.; Crans, D. C.; Polenova, T. Inorg. Chem. 2007, 46, 9285–9293. (17) Zeng, B. R.; Zhu, X. B.; Cai, S. H.; Chen, Z. Spectrochim. Acta, Part A 2007, 67, 202–207. (18) Tracey, A. S. Coord. Chem. ReV. 2003, 237, 113–121. (19) Zeng, B. R.; Zhu, X. B.; Yu, X. Y.; Cai, S. H.; Chen, Z. Spectrochim. Acta, Part A 2008, 69, 117–122. (20) Skibsted, J.; Jacobsen, C. J. H.; Jakobsen, H. J. Inorg. Chem. 1998, 37, 3083–3092. (21) Rehder, D. Coord. Chem. ReV. 2008, 252, 2209–2223. (22) Yu, X. Y.; Zhang, J.; Zeng, B. R.; Yi, P. G.; Cai, S. H.; Chen, Z. Spectrochim. Acta, Part A 2008, 71, 644–649.
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