Sun Un

Université Paris-Sud 11, Orsay, Île-de-France, France

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Publications (66)363.3 Total impact

  • Sun Un · Eduardo Marcos Bruch ·
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    ABSTRACT: Manganous phosphates have been postulated to play an important role in cells as antioxidants. In situ Mn(II) electron–nuclear double resonance (ENDOR) spectroscopy has been used to measure their speciation in cells. The analyses of such ENDOR spectra and the quantification of cellular Mn(II) phosphates has been based on comparisons to in vitro model complexes and heuristic modeling. In order to put such analyses on a more physical and theoretical footing, the Mn(II)–31P hyperfine interactions of various Mn(II) phosphate complexes have been measured by 95 GHz ENDOR spectroscopy. The dipolar components of these interactions remained relatively constant as a function of pH, esterification, and phosphate chain length, while the isotropic contributions were significantly affected. Counterintuitively, although the manganese–phosphate bonds are weakened by protonation and esterification, they lead to larger isotropic values, indicating higher unpaired-electron spin densities at the phosphorus nuclei. By comparison, extending the phosphate chain with additional phosphate groups lowers the spin density. Density functional theory calculations of model complexes quantitatively reproduced the measured hyperfine couplings and provided detailed insights into how bonding in Mn(II) phosphate complexes modulates the electron-spin polarization and consequently their isotropic hyperfine couplings. These results show that various classes of phosphates can be identified by their ENDOR spectra and provide a theoretical framework for understanding the in situ 31P ENDOR spectra of cellular Mn(II) complexes.
    Inorganic Chemistry 10/2015; DOI:10.1021/acs.inorgchem.5b01864 · 4.76 Impact Factor
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    ABSTRACT: Pulse electron-electron double resonance (PELDOR) is a versatile technique for probing the structures and functions of complex biological systems. Despite the recent interest in high-spin metal-ions for high field/frequency applications, PELDOR measurements of Mn(II) remain relatively underexplored. Here we present Mn(II)-Mn(II) PELDOR distance measurements at 94 GHz on polyproline II (PPII) helices doubly spin-labeled with Mn(II)DOTA, which are distinguished by their small zero-field interaction. The measured Mn-Mn distances and distribution profiles were in good agreement with the expected values from molecular models. Additional features in the frequency-domain spectra became apparent at certain combinations of detect and pump frequencies. Spin-Hamiltonian calculations showed that they likely arose from contributions from the pseudo-secular component of the dipolar interaction that were found to be non-negligible for Mn(II)DOTA. However, the influence of the pseudo-secular component on the distance distribution profiles apparently was limited. The results show the potential of Mn(II)DOTA spin labels for high-field PELDOR distance measurements in proteins and other biological systems.
    Physical Chemistry Chemical Physics 08/2015; 17(36). DOI:10.1039/C5CP03487F · 4.49 Impact Factor
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    ABSTRACT: D. radiodurans accumulates large quantities of Mn(II), which is believed to form low molecular weight complexes with phosphate and metabolites that protect D. radiodurans from radiation damage. The concentration of Mn(II) species in D. radiodurans during exponential and stationary phase were determined using high-field EPR and biochemical techniques. In the exponential growth phase cells a large fraction of the manganese was in the form of Mn(II)-orthophosphate complexes. By contrast, the intracellular concentrations of these compounds in stationary phase cells was less than 16 μM, while Mn superoxide dismutase was 320 μM and another, yet unidentified, Mn(II) protein 250 μM. Stationary cells were found to be equally resistant to irradiation as the exponential cells in spite of having significant lower Mn(II)-orthophosphate concentrations. Gamma irradiation induced no changes in the Mn(II) speciation. During stationary growth phase D. radiodurans favours the production of the two Mn-proteins over low molecular weight complexes suggesting that the latter were not essential for radio-resistance at this stage of growth
    Metallomics 03/2015; 7(5). DOI:10.1039/C5MT00009B · 3.59 Impact Factor
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    ABSTRACT: The ability to characterize the structure of metal centers beyond their primary ligands is important to understanding their chemistry. High magnetic-field pulsed electron double resonance detected NMR (ELDOR-NMR) is shown to be a very sensitive approach to measuring the multinuclear NMR spectra of the nuclei surrounding Mn(II) ions. Resolved spectra of intact organisms with resonances arising from 55Mn, 31P, 1H, 39K, 35Cl, 23Na and 14N nuclei surrounding Mn2+centers were obtained. Naturally abundant cellular 13C could be routinely measured as well. The amplitudes of the 14N and 2H ELDOR-NMR spectra were found to be linearly dependent on the number of nuclei in the ligand sphere. The evolution of the Mn(II) ELDOR-NMR spectra as a function excitation time was found be best described by a saturation phenomenon rather than a coherently driven process. Mn(II) ELDOR-NMR revealed not only details about the immediate ligands to the Mn(II) ions, but also more distant nuclei providing a view of their extended structures. This will be important for understanding the speciation and chemistry of the manganese complexes as well as other metals found in organisms.
    The Journal of Physical Chemistry B 03/2015; 119(43). DOI:10.1021/acs.jpcb.5b01624 · 3.30 Impact Factor
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    ABSTRACT: The manganese(ii) speciation in intact cells of D. radiodurans, E. coli, S. cerevisiae and Arabidopsis thaliana seeds was measured using high-field electron paramagnetic resonance techniques. The majority of the Mn(ii) ions in these organisms were six-coordinate, bound predominately by water, phosphates and nitrogen-based molecules. The relative distribution of the different phosphates in bacteria and S. cerevisiae was the same and dominated by monophosphate monoesters. Mn(ii) was also found bound to the phosphate backbone of nucleic acids in these organisms. Phosphate ligation in Arabidopsis seeds was dominated by phytate. The extent of nitrogen ligation in the four organisms was also determined. On average, the Mn(ii) in D. radiodurans had the most nitrogen ligands followed by E. coli. This was attributed to higher concentrations of Mn(ii) bound to proteins in these species. Although constitutively expressed in all four organisms, MnSOD was only detected in D. radiodurans. As previously reported, D. radiodurans also accumulates a second abundant Mn containing protein species. The high concentration of proteinaceous Mn(ii) is a unique feature of D. radiodurans.
    Metallomics 11/2014; 7(1). DOI:10.1039/c4mt00265b · 3.59 Impact Factor
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    ABSTRACT: TrmFO is a tRNA methyltransferase using methylenetetrahydrofolate (CH2THF) and a flavin adenine dinucleotide hydroquinone as cofactors. We have recently shown that TrmFO from Bacillus subtilis stabilizes a TrmFO-CH2-FADH adduct and an ill-defined neutral flavin radical. The adduct contains a unique N-CH2-S moiety, with a methylene group bridging N5 of the isoalloxazine ring and the sulfur of an active site cysteine (Cys53). In the absence of tRNA substrate, this species is remarkably stable but becomes catalytically competent, using the methylene group as the source of methyl, for tRNA methylation following tRNA addition. Here we demonstrate that this dormant methylating agent can be activated at low pH and propose that this process is triggered upon tRNA addition. The reaction proceeds via protonation of Cys53, cleavage of the C-S bond and generation of a highly reactive [FADH(N5)= CH2]+ iminium intermediate, proposed as the actual tRNA methylating agent. This mechanism is fully supported by DFT calculations. The radical present in TrmFO is characterized here by optical and EPR/ENDOR spectroscopies together with DFT calculations and shown to be the one-electron oxidized product of the TrmFO-CH2-FADH adduct. It is also relatively stable and its decomposition is facilitated at high pH. These results provide new insights into the structure and reactivity of the unique flavin-dependent methylating agent used by this class of enzymes.
    Biochemistry 11/2013; 52(49). DOI:10.1021/bi4013879 · 3.02 Impact Factor
  • Sun Un ·
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    ABSTRACT: A common feature of a large majority of the manganese metalloenzymes, as well as many synthetic biomimetic complexes, is the bonding between the manganese ion and imidazoles. This interaction was studied by examining the nature and structure of manganese(II) imidazole complexes in frozen aqueous solutions using 285 GHz high magnet-field continuous-wave electron paramagnetic resonance (cw-HFEPR) and 95 GHz pulsed electron-nuclear double resonance (ENDOR) and pulsed electron-double resonance detected nuclear magnetic resonance (PELDOR-NMR). The (55)Mn hyperfine coupling and isotropic g values of Mn(II) in frozen imidazole solutions continuously decreased with increasing imidazole concentration. ENDOR and PELDOR-NMR measurements demonstrated that the structural basis for this behavior arose from the imidazole concentration-dependent distribution of three six-coordinate and two four-coordinate species: [Mn(H2O)6](2+), [Mn(imidazole)(H2O)5](2+), [Mn(imidazole)2(H2O)4](2+), [Mn(imidazole)3(H2O)](2+), and [Mn(imidazole)4](2+). The hyperfine and g values of manganese proteins were also fully consistent with this imidazole effect. Density functional theory methods were used to calculate the structures, spin and charge densities, and hyperfine couplings of a number of different manganese imidazole complexes. The use of density functional theory with large exact-exchange admixture calculations gave isotropic (55)Mn hyperfine couplings that were semiquantitative and of predictive value. The results show that the covalency of the Mn-N bonds play an important role in determining not only magnetic spin parameters but also the structure of the metal binding site. The relationship between the isotropic (55)Mn hyperfine value and the number of imidazole ligands provides a quick and easy test for determining whether a protein binds an Mn(II) ion using histidine residues and, if so, how many are involved. Application of this method shows that as much as 40% of the Mn(II) ions in Deinococcus radiodurans are ligated to two histidines (Tabares, L. C.; Un, S. J. Biol. Chem2013, in press).
    Inorganic Chemistry 03/2013; 52(7). DOI:10.1021/ic302415s · 4.76 Impact Factor
  • Leandro C Tabares · Sun Un ·
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    ABSTRACT: High magnetic field high frequency electron paramagnetic resonance techniques were used to measure in situ Mn(II) speciation in Deinococcus radiodurans, a radiation-resistant bacteria capable of accumulating high concentrations of Mn(II). It was possible to identify and quantify the evolution of Mn(II) species in intact cells at various stages of growth. Aside from water, 95-GHz high field electron nuclear double resonance showed that the Mn(II) ions are bound to histidines and phosphate groups, mostly from fructose-1,6-bisphosphate but also inorganic phosphates and nucleotides. During stationary growth phase, 285-GHz continuous wave EPR measurements showed that histidine is the most common ligand to Mn(II) and that significant amounts of cellular Mn(II) in D. radiodurans are bound to peptides and proteins. As much as 40% of the total Mn(II) was in manganese superoxide dismutase, and it is this protein and not smaller manganese complexes, as has been suggested recently, that is probably the primary defense against superoxide.
    Journal of Biological Chemistry 01/2013; 288(7). DOI:10.1074/jbc.C112.444992 · 4.57 Impact Factor
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    ABSTRACT: The main cofactors that determine the photosystem II (PSII) oxygen evolution activity are borne by the D1 and D2 subunits. In the cyanobacterium Thermosynechococcus elongatus, there are three psbA genes coding for D1. Among the 344 residues constituting D1, there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2, and 27 between PsbA2 and PsbA3. Here, we present the first study of PsbA2-PSII. Using EPR and UV-visible time-resolved absorption spectroscopy, we show that: (i) the time-resolved EPR spectrum of Tyr(Z)(•) in the (S(3)Tyr(Z)(•))' is slightly modified; (ii) the split EPR signal arising from Tyr(Z)(•) in the (S(2)Tyr(Z)(•))' state induced by near-infrared illumination at 4.2 K of the S(3)Tyr(Z) state is significantly modified; and (iii) the slow phases of P(680)(+) reduction by Tyr(Z) are slowed down from the hundreds of μs time range to the ms time range, whereas both the S(1)Tyr(Z)(•) → S(2)Tyr(Z) and the S(3)Tyr(Z)(•) → S(0)Tyr(Z) + O(2) transition kinetics remained similar to those in PsbA(1/3)-PSII. These results show that the geometry of the Tyr(Z) phenol and its environment, likely the Tyr-O···H···Nε-His bonding, are modified in PsbA2-PSII when compared with PsbA(1/3)-PSII. They also point to the dynamics of the proton-coupled electron transfer processes associated with the oxidation of Tyr(Z) being affected. From sequence comparison, we propose that the C144P and P173M substitutions in PsbA2-PSII versus PsbA(1/3)-PSII, respectively located upstream of the α-helix bearing Tyr(Z) and between the two α-helices bearing Tyr(Z) and its hydrogen-bonded partner, His-190, are responsible for these changes.
    Journal of Biological Chemistry 02/2012; 287(16):13336-47. DOI:10.1074/jbc.M112.340323 · 4.57 Impact Factor
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    ABSTRACT: The flavoprotein TrmFO catalyzes the C5 methylation of uridine 54 in the TΨC loop of tRNAs using 5,10-methylenetetrahydrofolate (CH(2)THF) as a methylene donor and FAD as a reducing agent. Here, we report biochemical and spectroscopic studies that unravel the remarkable capability of Bacillus subtilis TrmFO to stabilize, in the presence of oxygen, several flavin-reduced forms, including an FADH(•) radical, and a catalytic intermediate endowed with methylating activity. The FADH(•) radical was characterized by high-field electron paramagnetic resonance and electron nuclear double-resonance spectroscopies. Interestingly, the enzyme exhibited tRNA methylation activity in the absence of both an added carbon donor and an external reducing agent, indicating that a reaction intermediate, containing presumably CH(2)THF and FAD hydroquinone, is present in the freshly purified enzyme. Isolation by acid treatment, under anaerobic conditions, of noncovalently bound molecules, followed by mass spectrometry analysis, confirmed the presence in TrmFO of nonmodified FAD. Addition of formaldehyde to the purified enzyme protects the reduced flavins from decay by probably preventing degradation of CH(2)THF. The absence of air-stable reduced FAD species during anaerobic titration of oxidized TrmFO, performed in the absence or presence of added CH(2)THF, argues against their thermodynamic stabilization but rather implicates their kinetic trapping by the enzyme. Altogether, the unexpected isolation of a stable catalytic intermediate suggests that the flavin-binding pocket of TrmFO is a highly insulated environment, diverting the reduced FAD present in this intermediate from uncoupled reactions.
    Biochemistry 06/2011; 50(23):5208-19. DOI:10.1021/bi1019463 · 3.02 Impact Factor
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    ABSTRACT: The interaction of indigenous radicals of humic acid (HA) with metal cations has been studied using high magnetic field (10.5T-285 GHz) electron paramagnetic resonance (HFEPR) spectroscopy. Strong [HA]-[metal] interaction was observed in the case of heavy metals, Cd(2+), Pb(2+), and Sr(2+), leading to formation of covalent bonds with the radicals of HA. On the contrary, alkaline earth metal ions, such as Mg(2+), generate only electrostatic interaction. The two types of indigenous radicals that exist in all HAs are influenced by the metal cations in a unified manner. This provides evidence that the two types of indigenous radicals in HAs originate from a unique, phenolic, moiety in HA. Mg(2+) ions dramatically changed the pH profile of the two radical types of HA, downshifting their interconversion pK(a) by ca. 3 pH units. This is the first experimental observation of the effect of metals on the H-dissociation of the radical centers in HAs.
    Environmental Science and Technology 09/2010; 44(18):7011-6. DOI:10.1021/es101708f · 5.33 Impact Factor
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    ABSTRACT: One of the most puzzling questions of manganese and iron superoxide dismutases (SODs) is what is the basis for their metal-specificity. This review summarizes our findings on the Mn(II) electronic structure of SODs and related synthetic models using high-field high-frequency electron paramagnetic resonance (HFEPR), a technique that is able to achieve a very detailed and quantitative information about the electronic structure of the Mn(II) ions. We have used HFEPR to compare eight different SODs, including iron, manganese and cambialistic proteins. This comparative approach has shown that in spite of their high structural homology each of these groups have specific spectroscopic and biochemical characteristics. This has allowed us to develop a model about how protein and metal interactions influence protein pK, inhibitor binding and the electronic structure of the manganese center. To better appreciate the thermodynamic prerequisites required for metal discriminatory SOD activity and their relationship to HFEPR spectroscopy, we review the work on synthetic model systems that functionally mimic Mn-and FeSOD. Using a single ligand framework, it was possible to obtain metal-discriminatory "activity" as well as variations in the HFEPR spectra that parallel those found in the proteins. Our results give new insights into protein-metal interactions from the perspective of the Mn(II) and new steps towards solving the puzzle of metal-specificity in SODs.
    Biochimica et Biophysica Acta 10/2009; 1804(2):308-17. DOI:10.1016/j.bbapap.2009.09.027 · 4.66 Impact Factor
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    ABSTRACT: A high-field electron paramagnetic resonance (HFEPR) study of oxalate decarboxylase (OxdC) is reported. OxdC breaks down oxalate to carbon dioxide and formate and possesses two distinct manganese(II) binding sites, referred to as site-1 and -2. The Mn(II) zero-field interaction was used to probe the electronic state of the metal ion and to examine chemical/mechanistic roles of each of the Mn(II) centers. High magnetic-fields were exploited not only to resolve the two sites, but also to measure accurately the Mn(II) zero-field parameters of each of the sites. The spectra exhibited surprisingly complex behavior as a function of pH. Six different species were identified based on their zero-field interactions, two corresponding to site-1 and four states to site-2. The assignments were verified using a mutant that only affected site-1. The speciation data determined from the HFEPR spectra for site -2 was consistent with a simple triprotic equilibrium model, while the pH dependence of site-1 could be described by a single pK(a). This pH dependence was independent of the presence of the His-tag and of whether the preparations contained 1.2 or 1.6 Mn per subunit. Possible structures of the six species are proposed based on spectroscopic data from model complexes and existing protein crystallographic structures obtained at pH 8 are discussed. Although site-1 has been identified as the active site and no role has been assigned to site-2, the pronounced changes in the electronic structure of the latter and its pH behavior, which also matches the pH-dependent activity of this enzyme, suggests that even if the conversion of oxalate to formate is carried out at site-1, site-2 likely plays a catalytically relevant role.
    The Journal of Physical Chemistry B 08/2009; 113(26):9016-25. DOI:10.1021/jp9021807 · 3.30 Impact Factor
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    ABSTRACT: The blue or Type 1 (T1) copper site of Paracoccuspantotrophus pseudoazurin exhibits significant absorption intensity in both the 450 and 600 nm regions. These are sigma and pi S(Cys) to Cu(2+) charge transfer (CT) transitions. The temperature dependent absorption, EPR, and resonance Raman (rR) vibrations enhanced by these bands indicate that a single species is present at all temperatures. This contrasts the temperature dependent behavior of the T1 center in nitrite reductase [S. Ghosh, X. Xie, A. Dey, Y. Sun, C. Scholes, E. Solomon, Proc. Natl. Acad. Sci. 106 (2009) 4969-4974] which has a thioether ligand that is unconstrained by the protein. The lack of temperature dependence in the T1 site in pseudoazurin indicates the presence of a protein constraint similar to the blue Cu site in plastocyanin where the thioether ligand is constrained at 2.8 A. However, plastocyanin exhibits only pi CT. This spectral difference between pseudoazurin and plastocyanin reflects a coupled distortion of the site where the axial thioether in pseudoazurin is also constrained, but at a shorter Cu-S(Met) bond length. This leads to an increase in the Cu(2+)-S(Cys) bond length, and the site undergoes a partial tetragonal distortion in pseudoazurin. Thus, its ground state wavefunction has both sigma and pi character in the Cu(2+)-S(Cys) bond.
    Journal of inorganic biochemistry 06/2009; 103(10):1307-13. DOI:10.1016/j.jinorgbio.2009.04.012 · 3.44 Impact Factor
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    ABSTRACT: The catalytic cycle of numerous enzymes involves the coupling between proton transfer and electron transfer. Yet, the understanding of this coordinated transfer in biological systems remains limited, likely because its characterization relies on the controlled but experimentally challenging modifications of the free energy changes associated with either the electron or proton transfer. We have performed such a study here in Photosystem II. The driving force for electron transfer from Tyr(Z) to P(680)(*+) has been decreased by approximately 80 meV by mutating the axial ligand of P(680), and that for proton transfer upon oxidation of Tyr(Z) by substituting a 3-fluorotyrosine (3F-Tyr(Z)) for Tyr(Z). In Mn-depleted Photosystem II, the dependence upon pH of the oxidation rates of Tyr(Z) and 3F-Tyr(Z) were found to be similar. However, in the pH range where the phenolic hydroxyl of Tyr(Z) is involved in a H-bond with a proton acceptor, the activation energy of the oxidation of 3F-Tyr(Z) is decreased by 110 meV, a value which correlates with the in vitro finding of a 90 meV stabilization energy to the phenolate form of 3F-Tyr when compared to Tyr (Seyedsayamdost et al. J. Am. Chem. Soc. 2006, 128,1569-1579). Thus, when the phenol of Y(Z) acts as a H-bond donor, its oxidation by P(680)(*+) is controlled by its prior deprotonation. This contrasts with the situation prevailing at lower pH, where the proton acceptor is protonated and therefore unavailable, in which the oxidation-induced proton transfer from the phenolic hydroxyl of Tyr(Z) has been proposed to occur concertedly with the electron transfer to P(680)(*+). This suggests a switch between a concerted proton/electron transfer at pHs < 7.5 to a sequential one at pHs > 7.5 and illustrates the roles of the H-bond and of the likely salt-bridge existing between the phenolate and the nearby proton acceptor in determining the coupling between proton and electron transfer.
    Journal of the American Chemical Society 04/2009; 131(12):4425-33. DOI:10.1021/ja808604h · 12.11 Impact Factor
  • Sun Un · Arezki Sedoud ·
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    ABSTRACT: The effect of chloride concentration on Mn2+ (S=5/2, I=5/2) ions in frozen aqueous solutions is studied by high-field high-frequency electron paramagnetic resonance (HFEPR). The usually six sharp lines characteristic of Mn2+ ions, arising from the m s =−1/2→1/2 transition, is modified by the addition of Cl− anions and the six resonances become much broader and more complex. This new feature likely arises from the ligation of one Cl− anion to a hydrated Mn2+ ion forming a [Mn(H2O)5Cl]− complex. This complex increases linearly with Cl− concentration with an association constant of K a, apparent=61M−1. The structure of the putative chloride complex was studied using density functional theory calculations and the expected zero-field interaction of such a manganese center was calculated using the superposition model. The predicted values were similar to those determined from the simulation of the spectrum of the m s =−5/3→−3/2 transition of the chloride complex. This effect of Cl− anions occurs at biologically relevant concentration and can be used to probe the Mn2+ ions in cellular and protein environments.
    Applied Magnetic Resonance 01/2009; 37(1):247-256. DOI:10.1007/s00723-009-0062-y · 1.17 Impact Factor
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    ABSTRACT: Superoxide dismutases (SODs) catalyze the disproportionation of superoxide to dioxygen and hydrogen peroxide. The active metal sites of iron and manganese superoxide dismutases are structurally indistinguishable from each other. Despite the structural homology, these enzymes exhibit a high degree of metal selective activity suggesting subtle redox tuning of the active site. The redox tuning model, however, up to now has been challenged by the existence of so-called cambialistic SODs that function with either metal ion. We have prepared and investigated two sets of manganese complexes in which groups of varying electron-withdrawing character, as measured by their Hammett constants sigma Para, have been introduced into the ligands. We observed that the Mn(III)/Mn(II) reduction potential for the series based on 4'-X-terpyridine ligands together with the corresponding values for the iron-substituted 4'-X-terpyridine complexes changed linearly with sigma Para. The redox potential of the iron and manganese complexes could be varied by as much as 600 mV by the 4'-substitution with the manganese complexes being slightly more sensitive to the substitution than iron. The difference was such that in the case where the 4'-substituent was a pyrrolidine group both the manganese and the iron complex were thermodynamically competent to catalytically disproportionate superoxide, making this particular ligand "cambialistic". Taking our data and those available from the literature together, it was found that in addition to the electron-withdrawing capacity of the 4'-substituents the overall charge of the Mn(II) complexes plays a major role in tuning the redox potential, about 600 mV per charge unit. The ion selectivity in Mn and FeSODs and the occurrence of cambialistic SODs are discussed in view of these results. We conclude that the more distant electrostatic contributions may be the source of metal specific enzymatic activity.
    Inorganic Chemistry 05/2008; 47(7):2897-908. DOI:10.1021/ic702428s · 4.76 Impact Factor
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    ABSTRACT: Humic substances, the largest source of carbon on Earth, contain indigenous stable free radicals that are involved in important biogeochemical environmental processes occurring in soil and water systems. Here, we present the first high-magnetic-field 285GHz electron paramagnetic resonance spectra for humic acids from various geographical origins. All humic acids irrespective of their origin contain two limiting types of indigenous stable radicals, types I and II, with distinct electronic structure. Type I, which prevails at acidic pH 5, is characterized by a g tensor with principal values gIx = 2.0032, gIy = 2.0032, and gIz = 2.0023. Type II, which prevails at alkaline pH 12, is characterized by gIIx = 2.0057, gIIy = 2.0055, and gIIz = 2.0023. The two limiting types are correlated in a unified reversible manner with pH, irrespective of the geographic origin of the HA. Both types of radical centers are consistent with pi-type radicals. They persist not only in liquid solutions but also in humic acid powders.
    The Journal of Physical Chemistry A 12/2007; 111(46):11860-6. DOI:10.1021/jp0717692 · 2.69 Impact Factor
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    ABSTRACT: The Mn(II)/Mn(III) redox potentials and Mn(II) zero-field interactions were measured for a series of Mn(II)(4'-X-terpy)2 complexes as a function of the electron-donating properties of the 4'-substituent. The two properties were found to be linearly related to each other. Density functional calculations demonstrated that the variation of both the redox potential and zero-field interaction depended on the charge on the center nitrogen of the terpy ligand that was modulated by the 4'-substituent. A similar relationship was found for Mn(II) complexes formed with the N,N -bis(2-ethyl-5-methylimidazol-4-ylmethyl)aminopropane ligand, indicating that the relationship between the Mn(II) zero-field interaction and the Mn(II)/Mn(III) redox potential was likely to be general.
    Journal of the American Chemical Society 12/2007; 129(45):13825-7. DOI:10.1021/ja076024o · 12.11 Impact Factor
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    ABSTRACT: Superoxide dismutases (SODs) are proteins specialized in the depletion of superoxide from the cell through disproportionation of this anion into oxygen and hydrogen peroxide. We have used high-field electron paramagnetic resonance (HFEPR) to test a two-site binding model for the interaction of manganese-SODs with small anions. Because tyrosine-34 was thought to act as a gate between these two sites in this model, a tyrosine to phenylalanine mutant of the superoxide dismutase from R. capsulatus was constructed. Although the replacement slightly reduced activity, HFEPR measurements demonstrated that the electronic structure of the Mn(II) center was unaffected by the mutation. In contrast, the mutation had a profound effect on the interactions of fluoride and azide with the Mn(II) center. It was concluded that the absence of tyrosine-34 prevented the close approach of these anions to the metal ion. This mutation also enhanced the formation of a hexacoordinated water-Mn(II)SOD complex at low temperatures. Together, these results showed that the role of Y34 is unlikely to involve redox tuning but rather is important in regulating the equilibria between the anionic substrate in solution and the two binding sites near the metal. These observations further supported the originally proposed mutually exclusive two-binding-site model.
    Biochemistry 09/2007; 46(32):9320-7. DOI:10.1021/bi700438j · 3.02 Impact Factor

Publication Stats

2k Citations
363.30 Total Impact Points


  • 2015
    • Université Paris-Sud 11
      Orsay, Île-de-France, France
    • Université Paris-Saclay
      Lutetia Parisorum, Île-de-France, France
  • 1996-2013
    • French National Centre for Scientific Research
      • • Institut de Chimie des Substances Naturelles
      • • Centre of Molecular Genetics
      Lutetia Parisorum, Île-de-France, France
  • 2002
    • Albert Einstein College of Medicine
      New York, New York, United States
  • 1997-1999
    • Institut de Génétique et de Biologie Moléculaire et Cellulaire
      Strasburg, Alsace, France
  • 1998
    • Atomic Energy and Alternative Energies Commission
      • Bioenergetics, Structural Biology, and Mechanisms (SB2SM/UMR 8221CNRS)
      Gif-sur-Yvette, Ile-de-France, France
  • 1992-1993
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States