Probing Polyoxometalate-Protein Interactions Using Molecular Dynamics Simulations.

The molecular interactions between the CeIV -substituted Keggin anion [PW11 O39 Ce(OH2 )4 ]3- (CeK) and hen egg-white lysozyme (HEWL) were investigated by molecular dynamics simulations. The analysis of CeK was compared with the CeIV -substituted Keggin dimer [(PW11 O39 )2 Ce]10- (CeK2 ) and the ZrIV -substituted Lindqvist anion [W5 O18 Zr(OH2 )(OH)]3- (ZrL) to understand how POM features such as shape, size, charge, or type of incorporated metal ion influence the POM⋅⋅⋅protein interactions. Simulations revealed two regions of the protein in which the CeK anion interacts strongly: cationic sites formed by Arg21 and by Arg45 and Arg68. The POMs chiefly interact with the side chains of the positively charged (arginines, lysines) and the polar uncharged residues (tyrosines, serines, aspargines) via electrostatic attraction and hydrogen bonding with the oxygen atoms of the POM framework. The CeK anion shows higher protein affinity than the CeK2 and ZrL anions, because it is less hydrophilic and it has the right size and shape for establishing interactions with several residues simultaneously. The larger, more negatively charged CeK2 anion has a high solvent-accessible surface, which is sub-optimal for the interaction, while the smaller ZrL anion is highly hydrophilic and cannot efficiently interact with several residues simultaneously.

Detailed Mechanism of Phosphoanhydride Bond Hydrolysis Promoted by a Binuclear Zr(IV)-Substituted Keggin Polyoxometalate Elucidated by a Combination of (31)P, (31)P DOSY, and (31)P EXSY NMR Spectroscopy.

A detailed reaction mechanism is proposed for the hydrolysis of the phosphoanhydride bonds in adenosine triphosphate (ATP) in the presence of the binuclear Zr(IV)-substituted Keggin type polyoxometalate (Et2NH2)8[{α-PW11O39Zr(µ-OH)(H2O)}2]·7H2O (ZrK 2:2). The full reaction mechanism of ATP hydrolysis in the presence of ZrK 2:2 at pD 6.4 was elucidated by a combination of (31)P, (31)P DOSY, and (31)P EXSY NMR spectroscopy, demonstrating the potential of these techniques for the analysis of complex reaction mixtures involving polyoxometalates (POMs). Two possible parallel reaction pathways were proposed on the basis of the observed reaction intermediates and final products. The 1D (31)P and (31)P DOSY spectra of a mixture of 20.0 mM ATP and 3.0 mM ZrK 2:2 at pD 6.4, measured immediately after sample preparation, evidenced the formation of two types of complexes, I1A and I1B, representing different binding modes between ATP and the Zr(IV)-substituted Keggin type polyoxometalate (ZrK). Analysis of the NMR data shows that at pD 6.4 and 50 °C ATP hydrolysis in the presence of ZrK proceeds in a stepwise fashion. During the course of the hydrolytic reaction various products, including adenosine diphosphate (ADP), adenosine monophosphate (AMP), pyrophosphate (PP), and phosphate (P), were detected. In addition, intermediate species representing the complexes ADP/ZrK (I2) and PP/ZrK (I5) were identified and the potential formation of two other intermediates, AMP/ZrK (I3) and P/ZrK (I4), was demonstrated. (31)P EXSY NMR spectra evidenced slow exchange between ATP and I1A, ADP and I2, and PP and I5, thus confirming the proposed reaction pathways.

Phosphate Ester Bond Hydrolysis Promoted by Lanthanide-Substituted Keggin-type Polyoxometalates Studied by a Combined Experimental and Density Functional Theory Approach.

Hydrolytic cleavage of 4-nitrophenyl phosphate (NPP), a commonly used DNA model substrate, was examined in the presence of series of lanthanide-substituted Keggin-type polyoxometalates (POMs) [Me2NH2]11[CeIII(PW11O39)2], [Me2NH2]10[CeIV(PW11O39)2] (abbreviated as (CeIV(PW11)2), and K4[EuPW11O39] by means of NMR and luminescence spectroscopies and density functional theory (DFT) calculations. Among the examined complexes, the Ce(IV)-substituted Keggin POM (CeIV(PW11)2) showed the highest reactivity, and its aqueous speciation was fully determined under different conditions of pD, temperature, concentration, and ionic strength by means of 31P and 31P diffusion-ordered NMR spectroscopy. The cleavage of the phosphoester bond of NPP in the presence of (CeIV(PW11)2) proceeded with an observed rate constant kobs = (5.31 ± 0.06) × 10-6 s-1 at pD 6.4 and 50 °C. The pD dependence of NPP hydrolysis exhibits a bell-shaped profile, with the fastest rate observed at pD 6.4. The formation constant (Kf = 127 M-1) and catalytic rate constant (kc = 19.41 × 10-5 s-1) for the NPP-Ce(IV)-Keggin POM complex were calculated, and binding between CeIV(PW11)2 and the phosphate group of NPP was also evidenced by the change of the chemical shift of the 31P nucleus in NPP upon addition of the POM complex. DFT calculations revealed that binding of NPP to the parent catalyst CeIV(PW11)2 is thermodynamically unlikely. On the contrary, formation of complexes with the monomeric 1:1 species, CeIVPW11, is considered to be more favorable, and the most stable complex, [CeIVPW11(H2O)2(NPP-κO)2]7-, was found to involve two NPP ligands coordinated to the CeIVcenter of CeIVPW11 in the monodentate fashion. The formation of such species is considered to be responsible for the hydrolytic activity of CeIV(PW11)2 toward phosphomonoesters. On the basis of these findings a principle mechanism for the hydrolysis of NPP by the POM is proposed.

Multinuclear diffusion NMR spectroscopy and DFT modeling: a powerful combination for unraveling the mechanism of phosphoester bond hydrolysis catalyzed by metal-substituted polyoxometalates.

A detailed reaction mechanism is proposed for the hydrolysis of the phosphoester bonds in the DNA model substrate bis(4-nitrophenyl) phosphate (BNPP) in the presence of the Zr(IV)-substituted Keggin type polyoxometalate (Et2NH2)8[{α-PW11O39Zr(µ-OH)(H2O)}2]⋅7 H2O (ZrK 2:2) at pD 6.4. Low-temperature (31)P DOSY spectra at pD 6.4 gave the first experimental evidence for the presence of ZrK 1:1 in fast equilibrium with ZrK 2:2 in purely aqueous solution. Moreover, theoretical calculations identified the ZrK 1:1 form as the potentially active species in solution. The reaction intermediates involved in the hydrolysis were identified by means of (1)H/(31)P NMR studies, including EXSY and DOSY NMR spectroscopy, which were supported by DFT calculations. This experimental/theoretical approach enabled the determination of the structures of four intermediate species in which the starting compound BNPP, nitrophenyl phosphate (NPP), or the end product phosphate (P) is coordinated to ZrK 1:1. In the proposed reaction mechanism, BNPP initially coordinates to ZrK 1:1 in a monodentate fashion, which results in hydrolysis of the first phosphoester bond in BNPP and formation of NPP. EXSY NMR studies showed that the bidentate complex between NPP and ZrK 1:1 is in equilibrium with monobound and free NPP. Subsequently, hydrolysis of NPP results in P, which is in equilibrium with its monobound form.

Reactivity of Dimeric Tetrazirconium(IV) Wells-Dawson Polyoxometalate toward Dipeptide Hydrolysis Studied by a Combined Experimental and Density Functional Theory Approach.

Detailed kinetic studies on the hydrolysis of glycylglycine (Gly-Gly) in the presence of the dimeric tetrazirconium(IV)-substituted Wells-Dawson-type polyoxometalate Na14[Zr4(P2W16O59)2(µ3-O)2(OH)2(H2O)4] · 57H2O (1) were performed by a combination of (1)H, (13)C, and (31)P NMR spectroscopies. The catalyst was shown to be stable under a broad range of reaction conditions. The effect of pD on the hydrolysis of Gly-Gly showed a bell-shaped profile with the fastest hydrolysis observed at pD 7.4. The observed rate constant for the hydrolysis of Gly-Gly at pD 7.4 and 60 °C was 4.67 × 10(-7) s(-1), representing a significant acceleration as compared to the uncatalyzed reaction. (13)C NMR data were indicative for coordination of Gly-Gly to 1 via its amide oxygen and amine nitrogen atoms, resulting in a hydrolytically active complex. Importantly, the effective hydrolysis of a series of Gly-X dipeptides with different X side chain amino acids in the presence of 1 was achieved, and the observed rate constant was shown to be dependent on the volume, chemical nature, and charge of the X amino acid side chain. To give a mechanistic explanation of the observed catalytic hydrolysis of Gly-Gly, a detailed quantum-chemical study was performed. The theoretical results confirmed the nature of the experimentally suggested binding mode in the hydrolytically active complex formed between Gly-Gly and 1. To elucidate the role of 1 in the hydrolytic process, both the uncatalyzed and the polyoxometalate-catalyzed reactions were examined. In the rate-determining step of the uncatalyzed Gly-Gly hydrolysis, a carboxylic oxygen atom abstracts a proton from a solvent water molecule and the nascent OH nucleophile attacks the peptide carbon atom. Analogous general-base activity of the free carboxylic group was found to take place also in the case of polyoxometalate-catalyzed hydrolysis as the main catalytic effect originates from the -C═O···Zr(IV) binding.

Highly Amino Acid Selective Hydrolysis of Myoglobin at Aspartate Residues as Promoted by Zirconium(IV)-Substituted Polyoxometalates.

SDS-PAGE/Edman degradation and HPLC MS/MS showed that zirconium(IV)-substituted Lindqvist-, Keggin-, and Wells-Dawson-type polyoxometalates (POMs) selectively hydrolyze the protein myoglobin at Asp-X peptide bonds under mildly acidic and neutral conditions. This transformation is the first example of highly sequence selective protein hydrolysis by POMs, a novel class of protein-hydrolyzing agents. The selectivity is directed by Asp residues located on the surface of the protein and is further assisted by electrostatic interactions between the negatively charged POMs and positively charged surface patches in the vicinity of the cleavage site.

Regioselective hydrolysis of human serum albumin by Zr(IV)-substituted polyoxotungstates at the interface of positively charged protein surface patches and negatively charged amino acid residues.

Complexes comprising the Lewis acidic Zr(IV) metal and protein binding polyoxotungstate ligands of Lindqvist-, Keggin- and Wells-Dawson-type were found to region selectively hydrolyze human serum albumin at four distinct positions. Higher reactivities were found for structures with higher polyoxometalate charges and the cleavage positions were found in protein regions of mixed charge. Both findings suggest an electrostatic nature of the observed reactivity.

Molecular origin of the hydrolytic activity and fixed regioselectivity of a Zr(IV) -substituted polyoxotungstate as artificial protease.

A multitechnique approach has been applied in order to identify the thermodynamic and kinetic parameters related to the regioselective hydrolysis of human serum albumin (HSA) promoted by the Wells-Dawson polyoxometalate (POM), K15 H[Zr(α2 -P2 W17 O61 )2 ]. Isothermal titration calorimetry (ITC) studies indicate that up to four POM molecules interact with HSA. While the first interaction site is characterized by a 1:1 binding and an affinity constant of 2×10(8) M(-1) , the three remaining sites are characterized by a lower global affinity constant of 7×10(5) M(-1) . The higher affinity constant at the first site is in accordance with a high quenching constant of 2.2×10(8) M(-1) obtained for fluorescence quenching of the Trp214 residue located in the only positively charged cleft of HSA, in the presence of K15 H[Zr(α2 -P2 W17 O61 )2 ]. In addition, Eu(III) luminescence experiments with an Eu(III) -substituted POM analogue have shown the replacement of water molecules in the first coordination sphere of Eu(III) due to binding of the metal ion to amino acid side chain residues of HSA. All three interaction studies are in accordance with a stronger POM dominated binding at the positive cleft on the one hand, and interaction mainly governed by metal anchoring at the three remaining positions, on the other hand. Hydrolysis experiments in the presence of K15 H[Zr(α2 -P2 W17 O61 )2 ] have demonstrated regioselective cleavage of HSA at the Arg114Leu115, Ala257Asp258, Lys313Asp314 or Cys392Glu393 peptide bonds. This is in agreement with the interaction studies as the Arg114Leu115 peptide bond is located in the positive cleft of HSA and the three remaining peptide bonds are each located near an upstream acidic residue, which can be expected to coordinate to the metal ion. A detailed kinetic study has evidenced the formation of additional fragments upon prolonged reaction times. Edman degradation of the additional reaction products has shown that these fragments result from further hydrolysis at the initially observed cleavage positions, indicating a fixed selectivity for K15 H[Zr(α2 -P2 W17 O61 )2 ].

Integrating 31P DOSY NMR spectroscopy and molecular mechanics as a powerful tool for unraveling the chemical structures of polyoxomolybdate-based amphiphilic nanohybrids in aqueous solution.

Novel organic-inorganic hybrids of various sizes were generated by reaction of 1,8-octanediphosphonic acid (ODP) and (NH4)6Mo7O24 in aqueous solution. The formation of rodlike hybrids with variable numbers of covalently bound ODP and polyoxomolybdate (POM) units can be tuned as a function of increasing (NH4)6Mo7O24 concentration at fixed ODP concentration. The chemical structure of the ODP/POM hybrids was characterized by (1)H, (31)P, and (95)Mo NMR spectroscopy. Heteronuclear (31)P DOSY (diffusion- ordered NMR spectroscopy) and molecular mechanics (MM) calculations were applied to determine the size and shape of the nanosized hybrids generated at various ODP/POM ratios. For this purpose, the structures of ODP/POM hybrids with variable numbers of ODP and POM units were optimized by MM and then approximated as cylinder-shaped objects by using a recently described mathematical algorithm. The thus-obtained cylinder length and diameter were further used to calculate the expected diffusion coefficients of the ODP/POM hybrids. Comparison of the calculated and experimentally determined diffusion coefficients led to the most probable ODP/POM hybrid length for each sample composition. The (31)P DOSY results show that the length of the hybrids increases with increasing POM concentration and reaches a maximum corresponding to an average of 8 ODP/7 POM units per chain at a sample composition of 20 mM ODP and 14 mM POM. With excess POM, above the latter concentration, the formation of shorter-chain hybrids terminated by Mo7 clusters at one or both ends was evidenced on further increasing the POM concentration. The results demonstrate that the combination of (31)P DOSY and MM, although virtually unexplored in POM chemistry, is a powerful innovative strategy for the detailed characterization of nanosized organic-inorganic POM-based hybrids in solution.

Molecular interactions between serum albumin proteins and Keggin type polyoxometalates studied using luminescence spectroscopy.

The interaction between the plenary Keggin H3PW12O40, lacunary Keggin K7PW11O39 and the Eu(III)-substituted Keggin K4EuPW11O39 (Eu-Keggin) type polyoxometalates (POMs), and the proteins human and bovine serum albumin (HSA and BSA) was studied using steady state and time-resolved Eu(III) luminescence and tryptophan (Trp) fluorescence spectroscopy. The excitation spectrum of the Eu-Keggin POM is dominated by a ligand-to-metal charge transfer band at 291 nm. In the absence of proteins, the number of water molecules coordinated in the first coordination sphere of the Eu(III) center of Eu-Keggin was determined to be 4, indicating that Eu(III) occurs as a 1 : 1 isomer in solution. In the presence of HSA or BSA, the number of coordinated water molecules decreased to 0 and 1, respectively, suggesting interaction between the Eu-Keggin POM and the protein surface. As a result of this interaction, a five-fold increase of the hypersensitive (5)D0 → (7)F2 transition in the luminescence intensity was observed for the Eu-Keggin-HSA complex. The association constants were calculated to be 1.5 × 10(2) M(-1) and 2.0 × 10(3) M(-1) for the Eu-Keggin-HSA and Eu-Keggin-BSA complexes, respectively. Tryptophan fluorescence quenching studies were performed and the quenching constants were calculated using a Stern-Volmer analysis. The obtained values of the quenching constants were 6.1 × 10(4) M(-1) and 2.0 × 10(6) M(-1) for the Eu-Keggin-HSA and Eu-Keggin-BSA complexes, respectively. The surface map of both proteins shows that the cavity containing the tryptophan has a positive surface potential, providing a specific binding site at the surface of albumin proteins for the negatively charged POM.

Peptide bond hydrolysis catalyzed by the Wells-Dawson Zr(α2-P2W17O61)2 polyoxometalate.

In this paper we report the first example of peptide hydrolysis catalyzed by a polyoxometalate complex. A series of metal-substituted Wells-Dawson polyoxometalates were synthesized, and their hydrolytic activity toward the peptide bond in glycylglycine (GG) was examined. Among these, the Zr(IV)- and Hf(IV)-substituted ones were the most reactive. Detailed kinetic studies were performed with the Zr(IV)-substituted Wells-Dawson type polyoxometalate K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O which was shown to act as a catalyst for the hydrolysis of the peptide bond in GG. The speciation of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O which is highly dependent on the pD, concentration, and temperature of the solution, was fully determined with the help of (31)P NMR spectroscopy and its influence on the GG hydrolysis rate was examined. The highest reaction rate (k(obs) = 9.2 (±0.2) × 10(-5) min(-1)) was observed at pD 5.0 and 60 °C. A 10-fold excess of GG was hydrolyzed in the presence of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O proving the principles of catalysis. (13)C NMR data suggested the coordination of GG to the Zr(IV) center in K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O via its N-terminal amine group and amide carbonyl oxygen. These findings were confirmed by the inactivity of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O toward the N-blocked analogue acetamidoglycylglycinate and the inhibitory effect of oxalic, malic, and citric acid. Triglycine, tetraglycine, and pentaglycine were also fully hydrolyzed in the presence of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O yielding glycine as the final product of hydrolysis. K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O also exhibited hydrolytic activity toward a series of other dipeptides.

Polyoxomolybdate promoted hydrolysis of a DNA-model phosphoester studied by NMR and EXAFS spectroscopy.

Hydrolysis of (p-nitrophenyl)phosphate (NPP), a commonly used phosphatase model substrate, was examined in molybdate solutions by means of (1)H, (31)P, and (95)Mo NMR spectroscopy and Mo K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. At 50 °C and pD 5.1 the cleavage of the phosphoester bond in NPP proceeds with a rate constant of 2.73 × 10(-5) s(-1) representing an acceleration of nearly 3 orders of magnitude as compared to the hydrolysis measured in the absence of molybdate. The pD dependence of k(obs) exhibits a bell-shaped profile, with the fastest cleavage observed in solutions where [Mo(7)O(24)](6-) is the major species in solution. Mixing of NPP and [Mo(7)O(24)](6-) resulted in formation of these two intermediate complexes that were detected by (31)P NMR spectroscopy. Complex A was characterized by a (31)P NMR resonance at -4.27 ppm and complex B was characterized by a (31)P NMR resonance at -7.42 ppm. On the basis of the previous results from diffusion ordered NMR spectroscopy, performed with the hydrolytically inactive substrate phenylphosphonate (PhP), the structure of these two complexes was deduced to be (NPP)(2)Mo(5)O(21)(4-) (complex A) and (NPP)(2)Mo(12)O(36)(H(2)O)(6)(4-) (complex B). The pH studies point out that both complexes are hydrolytically active and lead to the hydrolysis of phosphoester bond in NPP. The NMR spectra did not show evidence of any paramagnetic species, excluding the possibility of Mo(VI) reduction to Mo(V), and indicating that the cleavage of the phosphomonoester bond is purely hydrolytic. The Mo K-edge XANES region also did not show any sign of Mo(VI) to Mo(V) reduction during the hydrolytic reaction. (95)Mo NMR and Mo K-edge EXAFS spectra measured during different stages of the hydrolytic reaction showed a gradual disappearance of [Mo(7)O(24)](6-) during the hydrolytic reaction and appearance of [P(2)Mo(5)O(23)](6-), which was the final complex observed at the end of hydrolytic reaction.

Direct observation of molecular-level template action leading to self-assembly of a porous framework.

The molecular steps involved in the self-assembly of Cu(3)(BTC)(2) (BTC=1,3,5-benzenetricarboxylic acid) metal-organic frameworks that enclose Keggin-type H(3)PW(12)O(40) heteropolyacid molecules were unraveled by using solution (17)O, (31)P, and (183)W NMR spectroscopy, small-angle X-ray scattering, near-IR spectroscopy, and dynamic light scattering. In aqueous solution, complexation of Cu(2+) ions with Keggin-type heteropolyacids was observed. Cu(2+) ions are arranged around the Keggin structure so that linking through benzenetricarboxylate groups results in the formation of the Cu(3)(BTC)(2) MOF structure HKUST-1. This is a unique instance in which a templating mechanism that relies on specific molecular-level matching and leads to explicit nanoscale building units can be observed in situ during formation of the synthetic nanoporous material.

Hydrolytic cleavage of an RNA-model phosphodiester catalyzed by a highly negatively charged polyoxomolybdate [Mo7O24]6- cluster.

Hydrolysis of 2-hydroxypropyl-4-nitrophenyl phosphate (HPNP), a commonly used RNA model substrate, was examined in molybdate solutions by means of (1)H, (31)P, and (95)Mo NMR, Raman, and Mo K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. (1)H and (31)P NMR spectroscopy indicate that at 50 degrees C and pD 5.9 the cleavage of the phosphodiester bond in HPNP proceeds with a rate constant of 6.62 x 10(-6) s(-1), giving a cyclic phosphate ester and p-nitrophenol as the only products of hydrolysis. The NMR spectra did not show evidence of any paramagnetic species, excluding the possibility of Mo(VI) reduction to Mo(V), and indicating that the cleavage of the phosphodiester bond is purely hydrolytic. The Mo K-edge XANES region also did not show any sign of Mo(VI) to Mo(V) reduction during the hydrolytic reaction. The pD dependence of k(obs) exhibits a bell-shaped profile, with the fastest cleavage observed at pD 5.9. Comparison of the rate profile with the concentration profile of polyoxomolybdates shows a striking overlap of the k(obs) profile with the concentration of heptamolybdate, suggesting that the highly negatively charged [Mo(7)O(24)](6-) is the hydrolytically active species. Kinetic experiments at pD 5.9 using a fixed amount of [Mo(7)O(24)](6-) and increasing amounts of HPNP revealed slight signs of curvature at 25 molar excess of HPNP. The data fit the general Michaelis-Menten reaction scheme, permitting the calculation of the catalytic rate constant k(2) (3.02 x 10(-4) s(-1)) and K(m) (1.06 M). Variable temperature (31)P NMR spectra of a reaction mixture revealed broadening of the HPNP (31)P resonance upon increase of temperature, implying the dynamic exchange process between free and bound HPNP at higher temperatures. Addition of salts resulted in the inhibition of HPNP hydrolysis, as well as addition of dimethyl phosphate, suggesting competition for the binding to [Mo(7)O(24)](6-). The hydrolysis of 10 equiv of HPNP could be achieved in the presence of 1 equiv of [Mo(7)O(24)](6-), and the multiple turnovers demonstrate that the reaction is catalytic. (31)P NMR and Mo K-edge EXAFS spectra measured during different stages of the hydrolysis indicated that under catalytic conditions a partial conversion of [Mo(7)O(24)](6-) into [P(2)Mo(5)O(23)](6-) occurs.

Questioning the paradigm of metal complex promoted phosphodiester hydrolysis: [Mo7O24](6-) polyoxometalate cluster as an unlikely catalyst for the hydrolysis of a DNA model substrate.

The first example of a phosphodiester bond cleavage promoted by a highly negatively charged polyoxometalate cluster has been discovered: the hydrolysis of the phosphodiester bond in a DNA model substrate bis(p-nitrophenyl)phosphate (BNPP) is promoted by the heptamolybdate anion [Mo7O24](6-) with rates which represent an acceleration of nearly four orders of magnitude compared to the uncatalyzed cleavage.

Phosphoesterase activity of polyoxomolybdates: diffusion ordered NMR spectroscopy as a tool for obtaining insights into the reactivity of polyoxometalate clusters.

Diffusion ordered NMR spectroscopy (DOSY NMR) is shown to be an excellent tool for observing reactive transients in the hydrolysis of the phosphatase model substrate (p-nitrophenyl)phosphate (NPP) promoted by polyoxomolybdate.

Selective hydrolysis of oxidized insulin chain B by a Zr(IV)-substituted Wells-Dawson polyoxometalate.

We report for the first time on the selective hydrolysis of a polypeptide system by a metal-substituted polyoxometalate (POM). Oxidized insulin chain B, a 30 amino acid polypeptide, was selectively cleaved by the Zr(IV)-substituted Wells-Dawson POM, K15H[Zr(α2-P2W17O61)2]·25H2O, under physiological pH and temperature conditions in aqueous solution. HPLC-ESI-MS, LC-MS/MS, MALDI-TOF and MALDI-TOF MS/MS data indicate hydrolysis at the Phe1-Val2, Gln4-His5, Leu6-Cys(SO3H)7, and Gly8-Ser9 peptide bonds. The rate of oxidized insulin chain B hydrolysis (0.45 h(-1) at pH 7.0 and 60 °C) was calculated by fitting the integration values of its HPLC-UV signal to a first-order exponential decay function. (1)H NMR measurements show significant line broadening and shifting of the polypeptide resonances upon addition of the Zr(IV)-POM, indicating that interaction between the Zr(IV)-POM and the polypeptide takes place in solution. Circular dichroism (CD) measurements clearly prove that the flexible unfolded nature of the polypeptide was retained in the presence of the Zr(IV)-POM. The thermal stability of the Zr(IV)-POM in the presence of the polypeptide chain during the hydrolytic reaction was confirmed by (31)P NMR spectroscopy. Despite the highly negative charge of the Zr(IV)-POM, the mechanism of interaction appears to be dominated by a strong metal-directed binding between the positively charged Zr(IV) center and negatively charged amino acid side chains.

Eu(III) luminescence and tryptophan fluorescence spectroscopy as a tool for understanding interactions between hen egg white lysozyme and metal-substituted Keggin type polyoxometalates.

The interaction between the lacunary Keggin K7PW11O39, the Eu(III)-substituted Keggin K4EuPW11O39 (Eu-Keggin) and the Ce(IV)-substituted Keggin [Me2NH2]10[Ce(PW11O39)2] (Ce-Keggin) polyoxometalates (POMs), and the proteins hen egg white lysozyme (HEWL) and the structurally homologous α-lactalbumin (α-LA) was studied by steady state and time-resolved Eu(III) luminescence and tryptophan (Trp) fluorescence spectroscopy. The excitation spectrum of Eu-Keggin at lower concentrations ([Eu-Keggin]<100 µM) is dominated by a ligand-to-metal charge transfer band (291 nm). For higher concentrations ([Eu-Keggin]>250 µM) the (5)L6←(7)F0 transition becomes the most intense peak. In the absence of protein, the number of coordinated water molecules to the Eu(III) centre of Eu-Keggin is 4, indicating a 1:1 Eu(III):POM species. In the presence of phosphate buffer this number linearly decreases from 4 to 2 upon increasing phosphate buffer concentration. Upon addition of HEWL, there are no coordinated water molecules, suggesting interaction between Eu-Keggin and the protein surface. In addition, this interaction results in a more than threefold increase of the hypersensitive (5)D0→(7)F2 transition for the Eu-Keggin/HEWL mixture. The calculated association constant amounted to 2.2×10(2) M(-1) for the Eu-Keggin/HEWL complex. Tryptophan fluorescence quenching studies were performed and the quenching constants were calculated to be 9.1×10(4) M(-1), 4×10(4) M(-1) and 4.1×10(5) M(-1) for the lacunary Keggin/HEWL, the Eu-Keggin/HEWL and the Ce-Keggin/HEWL complexes, respectively. The number of bound POM molecules to HEWL was 1.04 for the lacunary Keggin POM, and 1.0 for Eu-Keggin, indicating the formation of a 1:1 POM/HEWL complex. The value of 1.38 for Ce-Keggin might indicate a transition from 1:1 to 1:2 interaction.

Hydrolysis of the RNA model substrate catalyzed by a binuclear Zr(IV)-substituted Keggin polyoxometalate.

The reactivity and solution behaviour of the binuclear Zr(IV)-substituted Keggin polyoxometalate (Et2NH2)8[{α-PW11O39Zr(µ-OH)(H2O)}2]·7H2O (ZrK 2 : 2) towards phosphoester bond hydrolysis of the RNA model substrate 2-hydroxypropyl-4-nitrophenyl phosphate (HPNP) was investigated at different reaction conditions (pD, temperature, concentration, and ionic strength). The hydrolysis of the phosphoester bond of HPNP, followed by means of (1)H NMR spectroscopy, proceeded with an observed rate constant, kobs = 11.5(±0.42) × 10(-5) s(-1) at pD 6.4 and 50 °C, representing a 530-fold rate enhancement in comparison with the spontaneous hydrolysis of HPNP. (1)H and (31)P NMR spectra indicate that at these reaction conditions the only products of hydrolysis are p-nitrophenol and the corresponding cyclic phosphate ester. The pD dependence of kobs exhibits a bell-shaped profile, with the fastest rate observed at pD 6.4. The formation constant (Kf = 455 M(-1)) and catalytic rate constant (kc = 42 × 10(-5) s(-1)) for the HPNP-ZrK 2 : 2 complex, activation energy (Ea) of 63.35 ± 1.82 kJ mol(-1), enthalpy of activation (ΔH(‡)) of 60.60 ± 2.09 kJ mol(-1), entropy of activation (ΔS(‡)) of -133.70 ± 6.13 J mol(-1) K(-1), and Gibbs activation energy (ΔG(‡)) of 102.05 ± 0.13 kJ mol(-1) at 37 °C were calculated from kinetic experiments. Binding between ZrK 2 : 2 and the P-O bond of HPNP was evidenced by the change in the (31)P chemical shift and signal line-broadening of the (31)P atom in HPNP upon addition of ZrK 2 : 2. Based on (31)P NMR experiments and isotope effect studies, a mechanism for HPNP hydrolysis in the presence of ZrK 2 : 2 was proposed.

Amino acid side chain induced selectivity in the hydrolysis of peptides catalyzed by a Zr(IV)-substituted Wells-Dawson type polyoxometalate.

In this paper the reactivity of K15H[Zr(α2-P2W17O61)2]·25H2O (1), a Zr(IV)-substituted Wells-Dawson polyoxometalate, is examined towards a series of Gly-Aa, Aa-Gly or Aa-Ser dipeptides, in which the nature and the size of the Aa amino acid side chain were varied. The rate of peptide bond hydrolysis, determined by (1)H NMR experiments, in Gly-Aa dipeptides is strongly dependent on the molecular volume and the chemical structure of the Aa side chain. When the volume of the aliphatic side chain of the Aa residue in Gly-Aa increased, a clear decrease in the hydrolysis rate was observed. Replacing one α-H in the C-terminal Gly residue of Gly-Gly by a methyl group (Gly-Ala) resulted in a 6-fold reactivity decrease, pointing towards the importance of steric factors for efficient peptide bond hydrolysis. The rate constants for peptide bond hydrolysis in Gly-Aa dipeptides at pD 5.0 and 60 °C ranged from 208.0 ± 15.6 × 10(-6) min(-1) for Gly-Ser to 5.0 ± 1.0 × 10(-6) min(-1) for Gly-Glu, reflecting the influence of the different nature of the amino acid side chains on the hydrolysis rate. Faster hydrolysis was observed for peptides containing Ser and Thr since the hydroxyl group in their side chain is able to facilitate amide bond hydrolysis by promoting an N→O acyl rearrangement. Peptides containing positively charged side chains at pD 5.0 show enhanced hydrolysis rates as a result of the secondary electrostatic interactions with the negatively charged surface of the polyoxometalate, which stabilize the peptide-polyoxometalate complex. A slow hydrolysis rate was observed for Gly-Glu, because of the preferential coordination of the carboxylate group in the side chain of Glu to Zr(IV), which prevents coordination of the peptide carbonyl group and its activation towards hydrolysis.