Dioxygen Activation by Enzymes with Mononuclear Non-Heme Iron Active Sites

Scandium ion-enhanced oxidative dimerization and N-demethylation of N,N-dimethylanilines by a non-heme iron(IV)-oxo complex

Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [Fe(IV)(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by [Fe(IV)(O)(N4Py)](2+) is markedly enhanced by the presence of scandium triflate, Sc(OTf)(3) (OTf = CF(3)SO(3)(-)), when TMB is further oxidized to the radical cation (TMB(•+)). In contrast, we have observed the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by [Fe(IV)(O)(N4Py)](2+) is also markedly enhanced by the presence of Sc(OTf)(3). In the case of para-substituted DMA derivatives with electron-donating substituents, radical cations of DMA derivatives are initially formed by Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), giving demethylated products. Binding of Sc(3+) to [Fe(IV)(O)(N4Py)](2+) enhances the Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), whereas binding of Sc(3+) to DMA derivatives retards the electron-transfer reaction. The complicated kinetics of the Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+) are analyzed by competition between binding of Sc(3+) to DMA derivatives and to [Fe(IV)(O)(N4Py)](2+). The binding constants of Sc(3+) to DMA derivatives increase with the increase of the electron-donating ability of the para-substituent. The rate constants of Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), which are estimated from the binding constants of Sc(3+) to DMA derivatives, agree well with those predicted from the driving force dependence of the rate constants of Sc(3+) ion-coupled electron transfer from one-electron reductants to [Fe(IV)(O)(N4Py)](2+). Thus, oxidative dimerization of DMA and N-demethylation of para-substituted DMA derivatives proceed via Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+).

Microbial degradation of aromatic compounds - from one strategy to four

Aromatic compounds are both common growth substrates for microorganisms and prominent environmental pollutants. The crucial step in their degradation is overcoming the resonance energy that stabilizes the ring structure. The classical strategy for degradation comprises an attack by oxygenases that hydroxylate and finally cleave the ring with the help of activated molecular oxygen. Here, we describe three alternative strategies used by microorganisms to degrade aromatic compounds. All three of these methods involve the use of CoA thioesters and ring cleavage by hydrolysis. However, these strategies are based on different ring activation mechanisms that consist of either formation of a non-aromatic ring-epoxide under oxic conditions, or reduction of the aromatic ring under anoxic conditions using one of two completely different systems.

Role of microbial enzymes in the bioremediation of pollutants: a review

A large number of enzymes from bacteria, fungi, and plants have been reported to be involved in the biodegradation of toxic organic pollutants. Bioremediation is a cost effective and nature friendly biotechnology that is powered by microbial enzymes. The research activity in this area would contribute towards developing advanced bioprocess technology to reduce the toxicity of the pollutants and also to obtain novel useful substances. The information on the mechanisms of bioremediation-related enzymes such as oxido-reductases and hydrolases have been extensively studied. This review attempts to provide descriptive information on the enzymes from various microorganisms involved in the biodegradation of wide range of pollutants, applications, and suggestions required to overcome the limitations of their efficient use.

X-ray absorption spectroscopy structural investigation of early intermediates in the mechanism of DNA repair by human ABH2

Human ABH2 repairs DNA lesions by using an Fe(II)- and αKG-dependent oxidative demethylation mechanism. The structure of the active site features the facial triad of protein ligands consisting of the side chains of two histidine residues and one aspartate residue that is common to many non-heme Fe(II) oxygenases. X-ray absorption spectroscopy (XAS) of metallated (Fe and Ni) samples of ABH2 was used to investigate the mechanism of ABH2 and its inhibition by Ni(II) ions. The data are consistent with a sequential mechanism that features a five-coordinate metal center in the presence and absence of the α-ketoglutarate cofactor. This aspect is not altered in the Ni(II)-substituted enzyme, and both metals are shown to bind the cofactor. When the substrate is bound to the native Fe(II) complex with α-ketoglutarate bound, a five-coordinate Fe(II) center is retained that features an open coordination position for O(2) binding. However, in the case of the Ni(II)-substituted enzyme, the complex that forms in the presence of the cofactor and substrate is six-coordinate and, therefore, features no open coordination site for oxygen activation at the metal.

Chlorophenols and other related derivatives of environmental concern: properties, distribution and microbial degradation processes

Chlorophenols are chlorinated aromatic compound structures and are commonly found in pesticide preparations as well as industrial wastes. They are recalcitrant to biodegradation and consequently persistent in the environment. A variety of chlorophenols derivatives compounds are highly toxic, mutagenic and carcinogenic for living organisms. Biological transformation by microorganisms is one of the key remediation options that can be exploited to solve environmental pollution problems caused by these notorious compounds. The key enzymes in the microbial degradation of chlorophenols are the oxygenases and dioxygenases. These enzymes can be engineered for enhanced degradation of highly chlorinated aromatic compounds through directed evolution methods. This review underscores the mechanisms of chlorophenols biodegradation with the view to understanding how bioremediation processes can be optimized for cleaning up chloroaromatic contaminated environments.

If C-H bonds could talk: selective C-H bond oxidation

C-H oxidation has a long history and an ongoing presence in research at the forefront of chemistry and interrelated fields. As such, numerous highly useful articles and reviews have been written on this subject. Logically, these are generally written from the perspective of the scope and limitations of the reagents employed. This Minireview instead attempts to emphasize chemoselectivity imposed by the nature of the substrate. Consequently, many landmark discoveries in the field of C-H oxidation are not discussed, but hopefully the perspective taken herein will allow C-H oxidation reactions to be more readily incorporated into synthetic planning.

Spectroscopic and magnetic studies of wild-type and mutant forms of the Fe(II)- and 2-oxoglutarate-dependent decarboxylase ALKBH4

The Fe(II)/2OG (2-oxoglutarate)-dependent dioxygenase superfamily comprises proteins that couple substrate oxidation to decarboxylation of 2OG to succinate. A member of this class of mononuclear non-haem Fe proteins is the Escherichia coli DNA/RNA repair enzyme AlkB. In the present work, we describe the magnetic and optical properties of the yet uncharacterized human ALKBH4 (AlkB homologue). Through EPR and UV-visible spectroscopy studies, we address the Fe-binding environment of the proposed catalytic centre of wild-type ALKBH4 and an Fe(II)-binding mutant. We could observe a novel unusual Fe(III) high-spin EPR-active species in the presence of sulfide with a g(max) of 8.2. The Fe(II) site was probed with NO. An intact histidine-carboxylate site is necessary for productive Fe binding. We also report the presence of a unique cysteine-rich motif conserved in the N-terminus of ALKBH4 orthologues, and investigate its possible Fe-binding ability. Furthermore, we show that recombinant ALKBH4 mediates decarboxylation of 2OG in absence of primary substrate. This activity is dependent on Fe as well as on residues predicted to be involved in Fe(II) co-ordination. The present results demonstrate that ALKBH4 represents an active Fe(II)/2OG-dependent decarboxylase and suggest that the cysteine cluster is involved in processes other than Fe co-ordination.

X-ray absorption spectroscopy and reactivity of thiolate-ligated Fe(III)-OOR complexes

The reaction of a series of thiolate-ligated iron(II) complexes [Fe(II)([15]aneN(4))(SC(6)H(5))]BF(4) (1), [Fe(II)([15]aneN(4))(SC(6)H(4)-p-Cl)]BF(4) (2), and [Fe(II)([15]aneN(4))(SC(6)H(4)-p-NO(2))]BF(4) (3) with alkylhydroperoxides at low temperature (-78 °C or -40 °C) leads to the metastable alkylperoxo-iron(III) species [Fe(III)([15]aneN(4))(SC(6)H(5))(OOtBu)]BF(4) (1a), [Fe(III)([15]aneN(4))(SC(6)H(4)-p-Cl)(OOtBu)]BF(4) (2a), and [Fe(III)([15]aneN(4))(SC(6)H(4)-p-NO(2))(OOtBu)]BF(4) (3a), respectively. X-ray absorption spectroscopy (XAS) studies were conducted on the Fe(III)-OOR complexes and their iron(II) precursors. The edge energy for the iron(II) complexes (∼7118 eV) shifts to higher energy upon oxidation by ROOH, and the resulting edge energies for the Fe(III)-OOR species range from 7121-7125 eV and correlate with the nature of the thiolate donor. Extended X-ray absorption fine structure (EXAFS) analysis of the iron(II) complexes 1-3 in CH(2)Cl(2) show that their solid state structures remain intact in solution. The EXAFS data on 1a-3a confirm their proposed structures as mononuclear, 6-coordinate Fe(III)-OOR complexes with 4N and 1S donors completing the coordination sphere. The Fe-O bond distances obtained from EXAFS for 1a-3a are 1.82-1.85 Å, significantly longer than other low-spin Fe(III)-OOR complexes. The Fe-O distances correlate with the nature of the thiolate donor, in agreement with the previous trends observed for ν(Fe-O) from resonance Raman (RR) spectroscopy, and supported by optimized geometries obtained from density functional theory (DFT) calculations. Reactivity and kinetic studies on 1a- 3a show an important influence of the thiolate donor.

Substrate activation by iron superoxo intermediates

A growing number of non-heme-iron oxygenases and oxidases catalyze reactions for which the well-established mechanistic paradigm involving a single C-H-bond-cleaving intermediate of the Fe(IV)-oxo (ferryl) type [1(•)] is insufficient to explain the chemistry. It is becoming clear that, in several of these cases, Fe(III)-superoxide complexes formed by simple addition of O(2) to the reduced [Fe(II)] cofactor initiate substrate oxidation by abstracting hydrogen [2,3(•)]. This substrate-oxidizing entry route into high-valent-iron intermediates makes possible an array of complex and elegant oxidation reactions without the consumption of valuable reducing equivalents. Examples of this novel mechanistic strategy are discussed with the goal of bringing forth unifying principles.

Dioxygenase activity of epidermal lipoxygenase-3 unveiled: typical and atypical features of its catalytic activity with natural and synthetic polyunsaturated fatty acids

Epidermal lipoxygenase-3 (eLOX3) exhibits hydroperoxide isomerase activity implicated in epidermal barrier formation, but its potential dioxygenase activity has remained elusive. We identified herein a synthetic fatty acid, 9E,11Z,14Z-20:3ω6, that was oxygenated by eLOX3 specifically to the 9S-hydroperoxide. Reaction showed a pronounced lag phase, which suggested that eLOX3 is deficient in its activation step. Indeed, we found that high concentrations of hydroperoxide activator (e.g. 65 μM) overcame a prolonged lag phase (>1 h) and unveiled a dioxygenase activity with arachidonic acid; the main products were the 5-, 9-, and 7-hydroperoxyeicosatetraenoic acids (HPETEs). These were R/S mixtures (ranging from ∼50:50 to 73:27), and as the bis-allylic 7-HPETE can be formed only inside the enzyme active site, the results indicate there is oxygen availability along either face of the reacting fatty acid radical. That the active site oxygen supply is limited is implied from the need for continuous re-activation, as carbon radical leakage leaves the enzyme in the unactivated ferrous state. An Ala-to-Gly mutation, known to affect the positioning of O(2) in the active site of other lipoxygenase enzymes, led to more readily activated reaction and a significant increase in the 9R- over the 5-HPETE. Activation and cycling of the ferric enzyme are thus promoted using the 9E,11Z,14Z-20:3ω6 substrate, by continuous hydroperoxide activation, or by the Ala-to-Gly mutation. We suggest that eLOX3 represents one end of a spectrum among lipoxygenases where activation is inefficient, favoring hydroperoxide isomerase cycling, with the opposite end represented by readily activated enzymes in which dioxygenase activity is prominent.

Density Functional Theory Calculations on Fe-O and O-O Cleavage of Ferric Hydroperoxide Species: Role of axial ligand and spin state

Density Functional Theory (DFT) calculations are performed on thiolate bound hydroperoxide complexes. O-O and Fe-O cleavage reaction coordinates, relevant to the active sites of Cytocrome P450 and Superoxide Reductase enzymes, were investigated for both high and low spin states and for cis and trans orientations of the thiolate ligand with respect to the hydroperoxide ligand. The results indicate that the presence of a thiolate ligand produces significant elongation of the Fe-O bond and reduction of Fe-O vibrational frequency. While the fate of the O-O cleavage reaction is not significantly altered, the presence of a thiolate induces a heterolytic Fe-O cleavage irrespective of the spin state and orientation which is very different from results obtained with a trans ammine ligand.

Structural insights into maize viviparous14, a key enzyme in the biosynthesis of the phytohormone abscisic acid

The key regulatory step in the biosynthesis of abscisic acid (ABA), a hormone central to the regulation of several important processes in plants, is the oxidative cleavage of the 11,12 double bond of a 9-cis-epoxycarotenoid. The enzyme viviparous14 (VP14) performs this cleavage in maize (Zea mays), making it a target for the rational design of novel chemical agents and genetic modifications that improve plant behavior through the modulation of ABA levels. The structure of VP14, determined to 3.2-Å resolution, provides both insight into the determinants of regio- and stereospecificity of this enzyme and suggests a possible mechanism for oxidative cleavage. Furthermore, mutagenesis of the distantly related CCD1 of maize shows how the VP14 structure represents a template for all plant carotenoid cleavage dioxygenases (CCDs). In addition, the structure suggests how VP14 associates with the membrane as a way of gaining access to its membrane soluble substrate.

The Catalytic Function of Nonheme Iron (III) Complex for Hydrocarbon Oxidation

A detailed catalytic study of LFe(III)Cl (where L = 3-{2-[2-(3-hydroxy-1,3-diphenyl-allylideneamino)-ethylamino]-ethylimino}-1,3-diphenyl-propen-1-ol) for hydrocarbon oxidation was carried out, focusing on the role of solvent, atmospheric dioxygen, and oxidant on catalytic efficiency. The data showed that LFe(III)Cl catalyst was efficient in homogeneous hydrocarbon oxidations providing significant yields. Moreover, tert-BuOOH provided comparable oxidation yields with H(2)O(2), slightly favoring the formation of alcohols and ketones versus epoxides. Dioxygen intervened in the catalytic reaction, influencing the nature of oxidation products. The polarity of solvent strongly influenced the reaction rates and the nature of oxidation products. A mechanistic model is postulated assuming that LFe(III)Cl functions via the formation of iron-hydroperoxo-species, followed by a radical-based mechanistic path.

Steric hindrance effect of the equatorial ligand on Fe(IV)O and Ru(IV)O complexes: a density functional study

The geometric structures and mechanisms for hydrogen abstraction from cyclohexane for four high-valent complexes, [Fe IV(O)(TMC)(NCMe)]2+ (where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; 1-NCMe), the inverted isomer [Fe IV(NCMe)(TMC)(O)]2+ (2-NCMe), [Ru IV(O)(TMC)(NCMe)]2+ (the ruthenium analogue of 1-NCMe; 3-NCMe), and the inverted isomer [Ru IV(NCMe)(TMC)(O)]2+ (4-NCMe), were investigated using density functional theory. The axial NCMe ligand was found to be sterically more hindered in 2-NCMe than in 1-NCMe, which is in accord with the calculated results that the Fe-L axial distance is longer in the former. Both 1-NCMe and 2-NCMe are capable of hydrogen abstraction from cyclohexane via two-state reactivity patterns. In contrast, 3-NCMe and 4-NCMe react with cyclohexane by a single-state mechanism. The reaction pathways computed reveal that 2-NCMe is more reactive than 1-NCMe, in agreement with experimental results, whereas the reactivity of 3-NCMe and 4-NCMe shows little dependence on whether the oxo unit is syn or anti to the four N-methyl groups. Our analysis shows that along the reaction pathway for 2-NCMe in the triplet spin state, the NCMe ligand moves away from the iron center, and therefore the energy of the sigma *2 z (alpha-spin) orbital decreases and an electron is transferred to this orbital. Finally, we calculated the kinetic isotope effect and investigated the relationship between this effect and reaction barriers.

Nickel ions inhibit histone demethylase JMJD1A and DNA repair enzyme ABH2 by replacing the ferrous iron in the catalytic centers

Iron- and 2-oxoglutarate-dependent dioxygenases are a diverse family of non-heme iron enzymes that catalyze various important oxidations in cells. A key structural motif of these dioxygenases is a facial triad of 2-histidines-1-carboxylate that coordinates the Fe(II) at the catalytic site. Using histone demethylase JMJD1A and DNA repair enzyme ABH2 as examples, we show that this family of dioxygenases is highly sensitive to inhibition by carcinogenic nickel ions. We find that, with iron, the 50% inhibitory concentrations of nickel (IC(50) [Ni(II)]) are 25 microm for JMJD1A and 7.5 microm for ABH2. Without iron, JMJD1A is 10 times more sensitive to nickel inhibition with an IC(50) [Ni(II)] of 2.5 microm, and approximately one molecule of Ni(II) inhibits one molecule of JMJD1A, suggesting that nickel causes inhibition by replacing the iron. Furthermore, nickel-bound JMJD1A is not reactivated by excessive iron even up to a 2 mm concentration. Using x-ray absorption spectroscopy, we demonstrate that nickel binds to the same site in ABH2 as iron, and replacement of the iron by nickel does not prevent the binding of the cofactor 2-oxoglutarate. Finally, we show that nickel ions target and inhibit JMJD1A in intact cells, and disruption of the iron-binding site decreases binding of nickel ions to ABH2 in intact cells. Together, our results reveal that the members of this dioxygenase family are specific targets for nickel ions in cells. Inhibition of these dioxygenases by nickel is likely to have widespread impacts on cells (e.g. impaired epigenetic programs and DNA repair) and may eventually lead to cancer development.

Syntheses and structures of cationic and neutral, homo- and heteroleptic tert-butoxides of the group 4 metals

The utility of tri-tert-butoxystannate as a chelating tridentate ligand for group 4 metals was investigated. The highly Lewis acidic metals degraded the stannate ion in a series of tert-butoxide abstraction steps to produce a variety of group 4 tert-butoxides. A total of 1 equiv of NaSn(O(t)Bu)(3) reacted with cis-MCl(4)(THF)(2) [M = Zr (1), Hf (2)] in THF solutions to furnish the salts fac-{[M(O(t)Bu)(3)(THF)(3)](SnCl(3))}, which are separated ion pairs featuring weakly coordinating trichlorostannate ions. Neutral complexes, namely, [M(O(t)Bu)(2)Cl(2)(THF)(2)] [M = Zr (3), Hf (4)], were isolated when 2/3 equiv of sodium stannate was used in these reactions. Titanium tetrachloride formed analogues neither of 1 and 2 nor of 3 and 4, but Ti(O(t)Bu)(3)Cl reacted with silver triflate to give [Ti(O(t)Bu)(2)(OTf)(2)(THF)(2)] (5). Anion exchange of triflate for trichlorostannate transformed 1 to the contact ion pair fac-[Zr(O(t)Bu)(3)OTf(THF)(2)] (6). A total of 2 equiv of NaSn(O(t)Bu)(3) reacted with cis-MCl(4)(THF)(2) to give the complexes fac-[Sn(mu-O(t)Bu)(3)M(O(t)Bu)(3)] [M = Zr (7), Hf (8)]. Tri-tert-butoxystannate may be used as a selective alkoxylating agent for group 4 metals, and it can be transferred to these metals intact if their Lewis acidity is appropriately attenuated as in fac-{[M(O(t)Bu)(3)(THF)(3)](SnCl(3))}. Single-crystal X-ray studies revealed distorted octahedral coordination geometries for all compounds (1-8), with 1, 2, 7, and 8 being crystallographically C(3) symmetric.

Modular behavior of tauD provides insight into the origin of specificity in alpha-ketoglutarate-dependent nonheme iron oxygenases

Taurine alpha-ketoglutarate dioxygenase (tauD) is one of the best-studied alpha-ketoglutarate (alphaKG)-dependent nonheme iron oxygenases. As with all oxygenases, a fine balance must be struck between generating a species sufficiently reactive for the required chemistry and controlling that species to prevent undesirable side reactions [Klinman JP (2007) Accts Chem Res 40:325-333]. In the case of tauD, the substrate oxidizing species has been shown to be a ferryl-oxo, and the introduction of deuterium at the reactive position of substrate results in an enormous kinetic isotope effect together with a partial uncoupling of oxygen activation from substrate oxidation [Price JC, Barr EW, Glass TE, Krebs C, Bollinger JM (2003) J Am Chem Soc 125:13008-13009]. We have generated a series of site-specific variants at a position that resides directly behind bound substrate (F159 to L, V, A, and G). Decreasing side-chain bulk diminishes the coupling of oxygen activation to C-H cleavage, which is further reduced by substrate deuteration. Despite this impact, oxygen activation remains completely coupled to the oxidative decarboxylation of alphaKG. The concentration of bis-Tris buffer impacts the extent of coupling of oxygen activation to C-H cleavage, implicating the buffer in the uncoupling pathway. These data indicate a critical role for residue 159 in substrate positioning and reaction in tauD and show that minor active-site perturbations in these enzymes could allow for changes in substrate reactivity while maintaining substrate triggering and oxygen binding/activation.

Synthesis, structure, spectra and reactivity of iron(III) complexes of imidazole and pyrazole containing ligands as functional models for catechol dioxygenases

A series of new 1 : 1 iron(iii) complexes of the type [Fe()Cl(3)], where is a tridentate 3N donor ligand, has been isolated and studied as functional models for catechol dioxygenases. The ligands (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethyl-amine (), N,N-dimethyl-N'-(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine () and N-(1-methyl-1H-imidazol-2-ylmethyl)-N'-phenylethane-1,2-diamine () are linear while the ligands tris(1-pyrazolyl)methane (), tris(3,5-dimethyl-1-pyrazolyl)methane () and tris(3-iso-propylpyrazolyl)methane () are tripodal ones. All the complexes have been characterized by spectral and electrochemical methods. The X-ray crystal structure of the dinuclear catecholate adduct [Fe()(TCC)](2)O, where TCC(2-) is a tetrachlorocatecholate dianion, has been successfully determined. In this complex both the iron(iii) atoms are bridged by a mu-oxo group and each iron(iii) center possesses a distorted octahedral coordination geometry in which the ligand is facially coordinated and the remaining coordination sites are occupied by the TCC(2-) dianion. Spectral studies suggest that addition of a base like Et(3)N induces the mononuclear complex species [Fe()(TCC)Cl] to dimerize forming a mu-oxo-bridged complex. The spectral and electrochemical properties of the catecholate adducts of the complexes generated in situ reveal that a systematic variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(iii) center and hence the interaction of the complexes with simple and substituted catechols. The 3,5-di-tert-butylcatecholate (DBC(2-)) adducts of the type [Fe()(DBC)Cl], where is a linear tridentate ligand (), undergo mainly oxidative intradiol cleavage of the catechol in the presence of dioxygen. Also, the extradiol-to-intradiol product selectivity (E : I) is enhanced upon removal of the coordinated chloride ion in these adducts to obtain [Fe()(DBC)(Sol)](+) and upon incorporating coordinated N-methylimidazolyl nitrogen in them. In contrast to the iron(iii) complexes of imidazole-based ligands, those of the tripodal pyrazole-based ligands yield major amounts of the oxidized product benzoquinone and small amounts of both intra- and extradiol products. One of the pyrazole arms coordinated in the equatorial plane of these sterically constrained complexes is substituted by a solvent molecule upon adduct formation with DBC(2-), which encourages molecular oxygen to attack this site leading to benzoquinone formation. The DBSQ/DBC(2-) redox potentials of both the imidazole- and pyrazole-based complexes fall in the narrow range of -0.186 to -0.214 V supporting this proposal.