MnII Sites Research Articles

Oxygen vacancy induction strategy to regulate the evolution of valence- and free-electrons for boosting visible photodegradation of tetracycline hydrochloride

The generation, migration and reaction paths of electrons are the key steps for photodegradation of pollutants. However, efficient operation of the above pathways is still challenging. Herein, by strong coordination and slow pyrolysis, we constructed a narrow band Zn-Mn bimetallic photoactive core-shell material (Mn@Zn-N-C, Eg = 3.38 eV) with abundant oxygen vacancies for enhancing the above electronic paths. The photodegradation experiments of tetracycline hydrochloride (TCH) showed that the formation and transfer of vacancy-induced free electrons in the synthesized Mn@Zn-N-C was the key to improve the photocatalytic activity. The DFT calculation results revealed that the photogenerated electrons can transfer along the Mn-O-Zn bridge in Mn@Zn-N-C, and promote the formation of MnIV, which directly capture the free electrons and reset itself to MnII site. In this case, the introduction of Mn would enhance the separation of h+ and e-. The adjacent vacancies and defects then also trapped the above free electrons and hinder the recombination of photogenerated carriers. Simultaneously, the localized valence electron transfer between the above redox pairs (Mn4+/Mn2+ and Zn2+/Zn0) also promoted the long-term stability of the photocatalytic process. In summary, using vacancy induction strategy to regulate the evolution of valence- and free-electrons is a promising method to improve the production-transfer-utilization efficiency of photogenerated carriers.

Cation Adaptive Structures Based on Manganese Cyanide Prussian Blue Analogues: Application of Powder Diffraction Data to Solve Complex, Unprecedented Stoichiometries and New Structure Types.

A Mn(II) salt and A+ CN- under anaerobic conditions react to form 2-D and 3-D extended structured compounds of Am MnII n (CN)m+2n stoichiometry. Here, the creation and characterization of this large family of compounds, for example AMnII 3 (CN)7 , A2 MnII 3 (CN)8 , A2 MnII 5 (CN)12 , A3 MnII 5 (CN)13 , and A2 MnII [MnII (CN)6 ], where A represents alkali and tetraalkylammonium cations, is reviewed. Cs2 MnII [MnII (CN)6 ] has the typical Prussian blue face centered cubic unit cell. However, the other alkali salts are monoclinic or rhombohedral. This is in accord with smaller alkali cation radii creating void space that is minimized by increasing the van der Waals stabilization energy by reducing ∠Mn-N≡C, which, strengthens the magnetic coupling and increases the magnetic ordering temperatures. This is attributed to the non-rigidity of the framework structure due the significant ionic character associated with the high-spin MnII sites. For larger tetraalkylammonium cations, the high-spin Mn sites lack sufficient electrostatic A+ ⋅⋅⋅NC stabilization and form unexpected 4- and 5-coordinated Mn sites within a flexible, extended framework around the cation; hence, the size, shape, and charge of the cation dictate the unprecedented stoichio-metry and unpredictable cation adaptive structures. Antiferromagnetic coupling between adjacent MnII sites leads to ferrimagnetic ordering, but in some cases antiferromagnetic coupling of ferrimagnetic layers are compensated and synthetic antiferromagnets are observed. The magnetic ordering temperatures for ferrimagnetic A2 MnII [MnII (CN)6 ] with both octahedral high- and low-spin MnII sites increase with decreasing ∠Mn-N≡C. The crystal structures for all of the extended structured materials were obtained by powder diffraction.

Two Stable Sodalite-Cage-Based MOFs for Highly Gas Selective Capture and Conversion in Cycloaddition Reaction.

Stable metal-organic frameworks, containing periodically arranged nanosized cages or pores and active Lewis acid-base sites, are considered ideal candidates for efficient heterogeneous catalysis. Herein, based on the light of reticular chemistry design principles, the ingenious assembly of two pyridine N-rich multifunctional triangular linkers, H3TBA [3,5-di (1h-tetrazol-5-yl) benzoic acid] and H2TZI [5-(1H-tetrazol-5-yl)isophthalic acid], with MnII formed PCP-33(Mn) and PCP-34(Mn), respectively. PCP-33(Mn) and PCP-34(Mn) are typical sod topology zeolitic metal-organic frameworks (ZMOFs) with hierarchical tetragonal micropores and metal organic polyhedral sodalite-like cages. The inner walls of these cages are modified by open metal sites MnII and Lewis acid-base sites of halide ions and N pyridine atoms. The characteristics of the cages' structures make two MOFs exhibit high surface area and a small window, which promote their outstanding gas capture ability (C2H2, 131.8 cm3 g-1; CO2, 77.9 cm3 g-1 at 273 K) and selective separation performance (C2H2/CH4, 226.2, CO2/CH4, 50.3 at 298 K), and are also suitable as catalytic reactors for metal/solvent-free chemical fixation of CO2 with epoxides to achieve high-efficiency CO2 conversion. Furthermore, they are greatly recyclable for several cycles while retaining their structural rigidity and catalytic activity.

Exchange Coupling in Binuclear Manganese and Cobalt Complexes with the Tetraimido Sulfate Anion [S(NtBu)4]2.

The (hetero)bimetallic complexes [Cl2Mn(NtBu)2S(tBuN)2Mn{ClLi(THF)3}2] (1), [(acac)Co(NtBu)2S(tBuN)2Co(acac)] (2), and [(acac)Co(NtBu)2S(tBuN)2Li(THF)2] (3), with THF = tetrahydrofuran and acac = acetylacetonate [H2C(C(O)Me)2], were synthesized and investigated for their magnetic properties. While the two different MnII sites in 1 gave a weak coupling of J = -1.00 cm-1, we could observe an appreciable antiferromagnetic coupling of J = -6.08 cm-1 between the two CoII cations in 2, proving the tetraimido sulfate anion to be a challenging but promising linker to enhance magnetic communication between paramagnetic centers. The heterobimetallic complex 3 seems a versatile platform for magnetically interesting d/d, f/d, or f/f mixed-metal complexes.

Ferrimagnetic Ordering and Anomalous Stoichiometry Observed for the Cubic, Extended 3D Prussian Blue Analogues (NEt3 Me)2 MnII 5 (CN)12 and (NEt2 Me2 )2 MnII 5 (CN)12 : A Cation-Adaptive Structure.

The reactions of MnII (O2 CCH3 )2 with NEt3 Me+ CN- and NEt2 Me2 + CN- form (NEt3 Me)2 MnII 5 (CN)12 (1) and (NEt2 Me2 )2 MnII 5 (CN)12 (2), respectively. Structure model-building and Rietveld refinement of high-resolution synchrotron powder diffraction data revealed a cubic [a=24.0093 Å (1), 23.8804 Å (2)] 3D extended structural motif with adjacent tetrahedral and octahedral MnII sites in a 3:2 ratio. Each tetrahedral MnII site is surrounded by four low-spin octahedral MnII sites, and each octahedral MnII site is surrounded by six high-spin tetrahedral MnII sites; adjacent sites are antiferromagnetically coupled in 3D. Compensation does not occur, and magnetic ordering as a ferrimagnet is observed at Tc =13 K for 2 based on the temperature at which remnant magnetization, Mr (T)→0. The hysteresis has an unusual constricted shape with inflection points around 50 and 1.2 kOe with a 5 K coercivity of 16 Oe and remnant magnetization, Mr , of 2050 emuOe mol-1 . The unusual structure and stoichiometry are attributed to the very ionic nature of the high-spin N-bonded MnII ion, which enables the maximization of the attractive van der Waals interactions through minimization of void space via a reduced ∠ MnNC. This results in an additional example of the Ax MnII y (CN)x+2y (x=0, y=1; x=1, y=3; x=2, y=1; x=2, y=2; x=2, y=3; x=3, y=5; and x=4, y=1) family of compounds possessing an unprecedented stoichiometry and lattice motif that are cation adaptive structured materials.

Silica-Supported MnII Sites as Efficient Catalysts for Carbonyl Hydroboration, Hydrosilylation, and Transesterification.

Manganese, the third most abundant transition-metal element after iron and titanium, has recently been demonstrated to be an effective homogeneous catalyst in numerous reactions. Herein, the preparation of silica-supported MnII sites is reported using Surface Organometallic Chemistry (SOMC), combined with tailored thermolytic molecular precursors approach based on Mn2 [OSi(OtBu)3 ]4 and Mn{N(SiMe3 )2 }2 ⋅THF. These supported MnII sites, free of organic ligands, efficiently catalyze numerous reactions: hydroboration and hydrosilylation of ketones and aldehydes as well as the transesterification of industrially relevant substrates.

Synthesis, structural, magnetic and thermal properties of Mn2As2O5, the first pyro-arsenite of a first-row transition metal

Single crystals of manganese pyro-arsenite [systematic name: dimanganese(II) pentaoxidodiarsenate(III)], Mn2As2O5, were grown under hydrothermal conditions. Mn2As2O5 adopts a novel structure type as revealed by single crystal X-ray diffraction from a crystal twinned by pseudo-merohedry. The crystal structure comprises of five independent MnII sites (two located on inversion centres, three in general sites) and two pairs of [As2O5]4-pyro-anions made up from corner-sharing trigonal-pyramidal [AsO3] units in general positions. Four of the five MnII sites exhibit distorted octahedral [MnO6] coordination polyhedra whereas one has five oxygen ligands in a distorted square-pyramidal environment. The manganese-oxygen polyhedra and [As2O5]4-pyro-anions share corners and edges under formation of a three-dimensional network with channels extending along [001]. The 4s2 electron lone pairs at the AsIII atoms are stereochemically active and point into the centre of the channels. Results of Raman spectroscopic studies are in agreement with the structural features of the pyro-arsenite anion. Magnetic susceptibility measurements on a powder sample confirm all Mn atoms to be in oxidation state ​+ ​II. An antiferromagnetic ordering below 14.3 ​K occurs that most likely points to a magnetic structure supporting a weak ferromagnetic component (‘weak ferromagnetism’).

Anomalous Stoichiometry and Antiferromagnetic Ordering for the Extended Hydroxymanganese(II) Cubes/Hexacyanometalate-Based 3D-Structured [MnII 4 (OH)4 ][MnII (CN)6 ](OH2 )6 ⋅H2 O.

The reaction of MnII (O2 CMe)2 and NaCN or LiCN in water forms a light green insoluble material. Structural solution and Rietveld refinement of high-resolution synchrotron powder diffraction data for this unprecedented, complicated compound of previously unknown composition revealed a new alkali-free ordered structural motif with [MnII 4 (μ3 -OH)4 ]4+ cubes and octahedral [MnII (CN)6 ]4- ions interconnected in 3D by MnII -N≡C-MnII linkages. The composition is {[MnII (OH2 )3 ][MnII (OH2 )]3 }(μ3 -OH)4 ][MnII (μ-CN)2 (CN)4 ]⋅H2 O=[MnII 4 (μ3 -OH)4 (OH2 )6 ][MnII (μ-CN)2 (CN)4 ]⋅H2 O, which is further simplified to [Mn4 (OH)4 ][Mn(CN)6 ](OH2 )7 (1). 1 has four high-spin (S=5/2) MnII sites that are antiferromagnetically coupled within the cube and are antiferromagnetically coupled to six low-spin (S=1/2) octahedral [MnII (CN)6 ]4- ions. Above 40 K the magnetic susceptibility, χ(T), can be fitted to the Curie-Weiss expression, χ ∝(T-θ)-1 , with θ=-13.4 K, indicative of significant antiferromagnetic coupling and 1 orders as an antiferromagnet at Tc =7.8 K.

Anomalous Stoichiometry, 3-D Bridged Triangular/Pentagonal Layered Structured Artificial Antiferromagnet for the Prussian Blue Analogue A3MnII5(CN)13 (A = NMe4, NEtMe3). A Cation Adaptive Structure.

The size of the organic cation dictates both the composition and the extended 3-D structure for hybrid organic/inorganic Prussian blue analogues (PBAs) of A aMnII b(CN) a+2 b (A = cation) stoichiometry. Alkali PBAs are typically cubic with both MC6 and M'N6 octahedral coordination sites and the alkali cation content depends on the M and M' oxidation states. The reaction of MnII(O2CCH3)2 and A+CN- (A = NMe4, NEtMe3) forms a hydrated material of A3MnII5(CN)13 composition. A3MnII5(CN)13 forms a complex, 3-D extended structural motif with octahedral and rarely observed square pyramidal and trigonal bipyramidal MnII sites with a single layer motif of three pentagonal and one triangular fused rings. A complex pattern of MnIICN chains bridge the layers. (NMe4)3MnII5(CN)13 possesses one low-spin octahedral and four high-spin pentacoordinate MnII sites and orders as an antiferromagnet at 11 K due to the layers being bridged and antiferromagnetically coupled by the nonmagnetic cyanides. These are rare examples of intrinsic, chemically prepared and controlled artificial antiferromagnets and have the advantage of having controlled uniform spacing between the layers as they are not physically prepared via deposition methods. A3Mn5(CN)13 (A = NMe4, NEtMe3) along with [NEt4]2MnII3(CN)8, [NEt4]MnII3(CN)7, and Mn(CN)2 form stoichiometrically related A aMnII b(CN) a+2 b ( a = 0, b = 1; a = 2, b = 3; a = 1, b = 3; and a = 3, b = 5) series possessing unprecedented stoichiometries and lattice motifs. These unusual structures and stoichiometries are attributed to the very ionic nature of the high-spin N-bonded MnII ion that enables the maximization of the attractive van der Waals interactions via minimization of void space via a reduced ∠MnNC. This A aMnII b(CN) a+2 b family of compounds are referred to as being cation adaptive in which size and shape dictate both the stoichiometry and structure.

Coercivity enhancement and magnetic property evaluation of Bi doped Mn2Sb

Abstract Mn2Sb is a well-known ferrimagnetic material, which has been investigated thoroughly to understand the effect of doping on its magnetic properties. Present work reports the synthesis of Bi doped Mn2Sb employing arc-melting and melt spinning followed by characterization with a view to establish a correlation in the structure and magnetic properties. Bi doping in Mn2Sb lattice produces a significant change in the latter’s magnetic properties, giving rise to a saturation magnetization of 27.2 emu/g (27.2 Am2/Kg) and a high coercivity value of 3.4 kOe (279.2 kA/m), in comparison to Mn2Sb, which has a coercivity of 0.1 kOe (7.96 kA/m). Theoretical studies were carried out to understand the changes in magnetic properties. Ab-initio electronic and atomic structure calculations show that Bi atom occupies equally the MnI and MnII sites and this leads to ferromagnetic ordering, which results in enhancement of the saturation magnetisation. The coercivity mechanism has also been studied in detail; the large coercivity is attributed to the nucleation mechanism achieved via the inclusion of diamagnetic Bi particles in Mn2Sb.

Selective Catalytic Olefin Epoxidation with MnII-Exchanged MOF-5

Partial substitution of ZnII by MnII in Zn4O(terephthalate)3 (MOF-5) leads to a distorted all-oxygen ligand field supporting a single MnII site, whose structure was confirmed by Mn K-edge X-ray absorption spectroscopy. The MnII ion at the MOF-5 node engages in redox chemistry with a variety of oxidants. With tBuSO2PhIO, it produces a putative MnIV-oxo intermediate, which upon further reaction with adventitious hydrogen is trapped as a MnIII–OH species. Most intriguingly, the intermediacy of the high-spin MnIV–oxo species is likely responsible for catalytic activity of the MnII-MOF-5 precatalyst, which in the presence of tBuSO2PhIO catalyzes oxygen atom transfer reactivity to form epoxides from cyclic alkenes with >99% selectivity. These results demonstrate that MOF secondary building units serve as competent platforms for accessing terminal high-valent metal–oxo species that consequently engage in catalytic oxygen atom transfer chemistry owing to the relatively weak ligand fields provided by the SBU.

Lattice distortion tuning of the metamagnetic phase transition in tetragonal Cu2Sb-type Mn1.95V0.05Sb alloy

The lattice distortion of tetragonal Cu2Sb-type Mn1.95V0.05Sb alloy was realized by Li interstitial doping or hydrostatic pressure. The metamagnetic transition temperature was decreased and the thermal hysteresis was widened by either depressurization or increasing interstitial Li content. Its effects on magnetic entropy changes (∆S) and critical field had been studied. Thanks to moderate hydrostatic pressure, the beneficial enhancement of maximum of ∆S was achieved in tetragonal Cu2Sb-type Mn1.95V0.05Sb alloy. Our experiment demonstrates that the interatomic distance between MnII-MnII sites and the electronic density surrounding MnII sites are critical for metamagnetic phase transition.

Self-assembly of tetrabromoterephthalic acid with different metal system: Diversity in dimensionalities, structures and gas adsorption

A new series of coordination compounds of different dimensionalities, {Cd2(TBTH)2(TBT)(DMF)6} (1), {Cu(TBT)(DMF)2}n (2), {Ni(TBT)(DMF)2(EtOH)2}n (3) and {Mn(TBT)(DMF)}n (4) have been synthesized under similar reaction conditions and stoichiometry using tetrabromoterephthalic acid (TBTH2) as a building block. Single crystal X-ray diffraction study reveals 1 is a dimeric discrete complex whereas 2 and 3 are found to be 1D coordination polymers and all the three compounds extend to three dimensions by non-covalent (H-bonding and O⋯Br) interactions. Compound 4 has a 3D framework structure where {Mn(CO2)}n chains are cross-linked by TBT linkers and DMF molecule also acts as a bidentate bridging ligand. The difference in dimensionalities can be correlated to the metal ions (CdII, CuII, NiII and MnII) having different ionic radius. Compound 4 does not contain any guest solvent molecule but removal of bridging DMF molecules creates a 1D channel with dimension of 2.1×4.2Å2. Adsorption studies reveal that the desolvated compound 4 (4′) with unsaturated MnII sites can uptake an appreciable amount of CO2 at 195K (15wt.%), and H2 at 77K (∼0.4wt.%) although surface area is not significant.

Mechanism and Energetics of Secondary Oxidation Reactions in the Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates

Electronic structure calculations employing hybrid exchange DFT methods under periodic boundary conditions are applied to unravel the mechanism and energetics of the secondary oxidation reactions of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates. Secondary oxidations are favored by the activated nature of the primary oxidative products, alcohols, which have weaker C–H bonds than the corresponding hydrocarbon molecules. Our model accounts for the secondary oxidations of terminal alcohols; a double H-abstraction from these compounds, first by MnIII complexes with radical-type oxo-based ligands, and then by framework O atoms nearest neighbor to Mn, yields aldehyde molecules or ketones if nonterminal alcohols are oxidized. These aldehydes are then transformed first into carbonyl-containing α-hydroperoxide derivatives through H-abstraction, addition of O2, and a subsequent H-transfer. These hydroperoxides can then only be decomposed by the action of MnII sites which, depending on the stereochemistry of the hydroperoxide adsorption onto the Mn sites, yield the carboxylic acid final products directly or after a new H-abstraction. The knowledge gained in our study allows us to propose a mechanism for the secondary oxidation of ketones to give carboxylic acids, a route that involves cleavage of C–C bonds adjacent to the carbonyl group and gives place to carboxylic acids of shorter chain-length than the initial hydrocarbon, products commonly found during the experimental reaction.

Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates (III): Propagation Mechanism

We apply electronic structure methods based on hybrid-exchange DFT functionals under periodic boundary conditions to study the catalytic aerobic oxidation of hydrocarbons in Mn-doped aluminophosphates. In particular, we focus on the mechanism of the propagation reactions. Hydrocarbon oxidation is achieved via a succession of H abstraction, O2 addition, and desorption reactions occurring on MnIII···OX complexes (X = H, CH2CH3, or OCH2CH3). The complexes MnIII···OH and MnIII···OCH2CH3 result from the decomposition of CH3CH2OOH by preactivated MnII sites, whereas MnIII···OOCH2CH3 is formed in preactivation or propagation routes. The radical nature of the oxo-type ligands (OX) allows for the homolytic H abstraction from new hydrocarbon molecules, leading to XO–H (HO–H, CH3CH2O–H and CH3CH2OO–H) and to CH3CH2· radicals that are stabilized by interaction with the H atoms transferred. Subsequent stereospecific O2 additions yield free peroxo radicals CH3CH2OO· that undergo a propagation subcycle to produce furthe...

Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates (II): Hydroperoxide Decomposition

Electronic structure methods based on hybrid-exchange functionals in Density Functional Theory (DFT) and periodic boundary conditions have been applied to study the reaction mechanism of the aerobic oxidation of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates. In this paper we examine the decomposition of hydroperoxide intermediates (ROOH). The reaction takes place on MnII acid sites, charge-balanced by a proton on a nearest neighbor framework oxygen, resulting from the preactivation step. In this stage, the MnII sites catalyze the homolytic decomposition of the hydroperoxide molecules to produce MnIII and oxo-containing radical species, stabilized by complexation to MnIII, yielding in addition alcohol and water molecules. Two parallel reaction pathways have been identified for this process, through alkoxy (RO·) or hydroxy (HO·) radical-like intermediates. The occurrence of the two mechanisms depends on the stereochemistry of the initial adsorption of ROOH onto the active site, which takes...

Identification of DypB from Rhodococcus jostii RHA1 as a Lignin Peroxidase

Rhodococcus jostii RHA1, a polychlorinated biphenyl-degrading soil bacterium whose genome has been sequenced, shows lignin degrading activity in two recently developed spectrophotometric assays. Bioinformatic analysis reveals two unannotated peroxidase genes present in the genome of R. jostii RHA1 with sequence similarity to open reading frames in other lignin-degrading microbes. They are members of the Dyp peroxidase family and were annotated as DypA and DypB, on the basis of bioinformatic analysis. Assay of gene deletion mutants using a colorimetric lignin degradation assay reveals that a ΔdypB mutant shows greatly reduced lignin degradation activity, consistent with a role in lignin breakdown. Recombinant DypB protein shows activity in the colorimetric assay and shows Michaelis-Menten kinetic behavior using Kraft lignin as a substrate. DypB is activated by Mn(2+) by 5-23-fold using a range of assay substrates, and breakdown of wheat straw lignocellulose by recombinant DypB is observed over 24-48 h in the presence of 1 mM MnCl(2). Incubation of recombinant DypB with a β-aryl ether lignin model compound shows time-dependent turnover, giving vanillin as a product, indicating that C(α)-C(β) bond cleavage has taken place. This reaction is inhibited by addition of diaphorase, consistent with a radical mechanism for C-C bond cleavage. Stopped-flow kinetic analysis of the DypB-catalyzed reaction shows reaction between the intermediate compound I (397 nm) and either Mn(II) (k(obs) = 2.35 s(-1)) or the β-aryl ether (k(obs) = 3.10 s(-1)), in the latter case also showing a transient at 417 nm, consistent with a compound II intermediate. These results indicate that DypB has a significant role in lignin degradation in R. jostii RHA1, is able to oxidize both polymeric lignin and a lignin model compound, and appears to have both Mn(II) and lignin oxidation sites. This is the first detailed characterization of a recombinant bacterial lignin peroxidase.

Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates(I): Preactivation of the Mn Sites

We apply state of the art electronic structure techniques, based on hybrid exchange-functionals in DFT and periodic boundary conditions, to unravel the reaction mechanism responsible for the initial stages of the aerobic oxidation of hydrocarbons catalyzed by Mn-doped aluminophosphates. In this preactivation step of the catalyst, which precedes the catalytic propagation cycle in which the final oxidation products (alcohol, aldehyde, and carboxylic acid) are formed, the MnIII ions initially present in the activated (calcined) catalyst are transformed by interaction with one alkane and one O2 molecule into new Mn-bearing species: a reduced MnII site and a MnIII···peroxo complex, which are active for the subsequent propagation cycle. The preactivation step has a high activation energy, calculated as 135 kJ/mol, explaining the long induction time observed experimentally. Our results further show that MnIII sites are able to produce the hydroperoxide intermediate from the reactants; however, this intermediate can be transformed into the oxidative products only through reduced MnII sites. The latter are formed from MnIII in the preactivation step, via a H-abstraction from the hydrocarbon, also yielding an alkyl radical (R•) that subsequently adds O2 in a stereospecific way to form a free peroxo radical, ROO•. Migration of ROO• in the AlPO nanopores frees MnII for the propagation cycle and forms MnIII···ROO• complexes also needed for propagation. We demonstrate the essential role of MnIII active sites at the initial stages of the reaction for activating the hydrocarbon molecules; such hydrocarbon activation catalyzed by Mn requires much lower activation energies than through noncatalytic pathways, where the hydrocarbon is activated by O2 alone.

Kinetic and crystallographic studies of a redesigned manganese-binding site in cytochrome c peroxidase

Manganese peroxidase (MnP) from the white rot fungus Phanerochaete chrysosporium contains a manganese-binding site that plays a critical role in its function. Previously, a Mn(II)-binding site was designed into cytochrome c peroxidase (CcP) based on sequence homology (Yeung et al. in Chem. Biol. 4:215-222, 1997; Gengenbach et al. in Biochemistry 38:11425-11432, 1999). Here, we report a redesign of this site based on X-ray structural comparison of MnP and CcP. The variant, CcP(D37E, V45E, H181E), displays 2.5-fold higher catalytic efficiency (k (cat)/K (M)) than the variant in the original design, mostly due to a stronger K (M) of 1.9 mM (vs. 4.1 mM). High-resolution X-ray crystal structures of a metal-free form and a form with Co(II) at the designed Mn(II) site were also obtained. The metal ion in the engineered metal-binding site overlays well with Mn(II) bound in MnP, suggesting that this variant is the closest structural model of the Mn(II)-binding site in MnP for which a crystal structure exists. A major difference arises in the distances of the ligands to the metal; the metal-ligand interactions in the CcP variant are much weaker than the corresponding interactions in MnP, probably owing to partial occupancy of metal ion at the designed site, difference in the identity of metal ions (Co(II) rather than Mn(II)) and other interactions in the second coordination sphere. These results indicate that the metal ion, the ligands, and the environment around the metal-binding site play important roles in tuning the structure and function of metalloenzymes.

Binding of manganese(II) to a tertiary stabilized hammerhead ribozyme as studied by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is used to study the binding of MnII ions to a tertiary stabilized hammer-head ribozyme (tsHHRz) and to compare it with the binding to the minimal hammerhead ribozyme (mHHRz). Continuous wave EPR measurements show that the tsHHRz possesses a single high-affinity MnII binding site with a KD of < or =10 nM at an NaCl concentration of 0.1 M. This dissociation constant is at least two orders of magnitude smaller than the KD determined previously for the single high-affinity MnII site in the mHHRz. In addition, whereas the high-affinity MnII is displaced from the mHHRz upon binding of the aminoglycoside antibiotic neomycin B, it is not from the tsHHRz. Despite these pronounced differences in binding, a comparison between the electron spin echo envelope modulation and hyperfine sublevel correlation spectra of the minimal and tertiary stabilized HHRz demonstrates that the structure of both binding sites is very similar. This suggests that the MnII is located in both ribozymes between the bases A9 and G10.1 of the sheared G . A tandem base pair, as shown previously and in detail for the mHHRz. Thus, the much stronger MnII binding in the tsHHRz is attributed to the interaction between the two external loops, which locks in the RNA fold, trapping the MnII in the tightly bound conformation, whereas the absence of long-range loop-loop interactions in the mHHRz leads to more dynamical and open conformations, decreasing MnII binding.