Coordination Sphere Of Manganese Research Articles

Salen‑manganese complexes for controlling ROS damage: Neuroprotective effects, antioxidant activity and kinetic studies

A new manganese(III) complex [MnL1(DCA)(H2O)](H2O),1 [H2L1 is the chelating ligand N,N′-bis(2-hydroxy-3-methoxybenzylidene)-1,2-diaminopropane, and DCA is dicyanamide], has been prepared and characterized by different analytical and spectroscopic techniques. The tetragonally elongated octahedral geometry for the manganese coordination sphere was revealed by X-ray diffraction studies for 1. The antioxidant behavior of this complex and other manganese(III)-salen type complexes was tested through superoxide dismutase and catalase probes, and through the study of their neuroprotective effects in SH-SY5Y neuroblastoma cells. In this human neuronal model, these model complexes were found to improve cell survival in an oxidative stress model. During studies aimed to getting a better understanding of the kinetics of the processes involved in this antioxidant behavior, an important effect on the solvent in the kinetics of reaction of the complexes with H2O2 was revealed that suggests a change in the mechanism of reaction of the complexes. The kinetic data in methanol and buffered aqueous solutions correlate well with the results of the test of catalase activity, thus showing that the rate determining step in the catalytic cycle corresponds to the initial reaction of the complexes with H2O2.

Electrochemical Conversion of the Lignin Model Veratryl Alcohol to Veratryl Aldehyde Using Manganese(III)-Schiff Base Homogeneous Catalysts

Lignin and other colored structures need to be bleached after the Kraft process in the pulp industry. Development of environmentally-safe bleaching catalysts or electrocatalysts constitutes an attractive strategy for selective removal of lignin. Seven manganese(III)-complexes with Schiff base ligands 1–7 were synthetized and characterized by different analytical and spectroscopic techniques. The tetragonally elongated octahedral geometry for the manganese coordination sphere and the global µ-aquo dimeric structure were revealed by X-ray diffraction (XRD) studies for 1, Mn2L12(H2O)2(N(CN)2)2 (N(CN)2 = dicyanamide). Complexes 1–4 behave as more efficient peroxidase mimics as compared to 5–7. Electrochemical oxidation of the lignin model veratrylalcohol (VA) to veratrylaldehyde (VAH) is efficiently catalyzed by a type of dimanganese(III) complexes in a chlorine-free medium. The electrocatalytic reaction proceeds through the oxidation of chloride into hypochlorite at alkaline pH along with the formation of hydrogen from water as a subproduct.

16O/18O oxygen isotope exchange kinetics in MnO 4 − as probed by 55Mn NMR

The effect of the 16O ↔ 18O substitution in the coordination sphere of permanganate anion MnO4− on the chemical shift of 55Mn nuclei have been studied by 17O and 55Mn NMR. Time constants τn,k of oxygen exchange in the water–permanganate anion system have been estimated. In nearly neutral solutions (pH ≈ 6.8–7.2), the oxygen exchange time is on the order of tens of hours. Bubbling gaseous HCl through this solution for a few seconds leads to the equilibrium distribution of oxygen isotopes in the manganese coordination sphere. The observed temperature dependences of isotope-induced 55Mn NMR shifts in Mn16 O44-n18On– (n = 0–4) have been treated as a result of rovibrational averaging of Mn–O bond lengths. The change in the Mn—O bond length in caused by the 16O → 18O isotope substitution is on the order of 10–4 A.

Synthesis, structural characterization, theoretical calculations and catecholase mimetic activity of manganese-Schiff base complexes

Potentially hexadentate N2O4 donor Schiff base ligands N,N′-bis(3-ethoxysalicylidenimino)-1,2-diaminoethane (L1H2) and N,N′-bis(3-methoxysalicylidenimino)-1,3-diaminopropane (L2H2) have been used to synthesize two manganese(III) compounds [MnL1(SCN)(H2O)].[MnL1(H2O)2](ClO4)·CH3CN (1) and [MnL2(SCN)(H2O)] (2). All the ligands and the complexes have been characterized by microanalytical and spectroscopic techniques. The X-ray structural characterization of all the complexes shows a tetragonally elongated octahedral geometry for the manganese coordination sphere. The octahedron entities are further connected in pairs through intermolecular hydrogen bonding to form μ-aquo dimers, which are further stabilized by π–π interactions occurring between the aryl rings of the Schiff base ligands. These noncovalent interactions have been investigated in terms of energies and geometries using theoretical calculations. The catalytic activity of compounds 1 and 2 on the oxidation of 3,5-di-tert-butylcatechol was determined spectrophotometrically by monitoring the increase of the 3,5-di-tert-butyl-o-benzoquinone characteristic absorption band at about 400nm over time in methanol saturated with O2. The complexes were able to oxidize 3,5-di-tert-butylcatechol to the corresponding o-quinone with distinct catalytic activity.

Six-coordinate manganese(3+) in catalysis by yeast manganese superoxide dismutase

Reduction of superoxide (O2-) by manganese-containing superoxide dismutase occurs through either a "prompt protonation" pathway, or an "inner-sphere" pathway, with the latter leading to formation of an observable Mn-peroxo complex. We recently reported that wild-type (WT) manganese superoxide dismutases (MnSODs) from Saccharomyces cerevisiae and Candida albicans are more gated toward the "prompt protonation" pathway than human and bacterial MnSODs and suggested that this could result from small structural changes in the second coordination sphere of manganese. We report here that substitution of a second-sphere residue, Tyr34, by phenylalanine (Y34F) causes the MnSOD from S. cerevisiae to react exclusively through the "inner-sphere" pathway. At neutral pH, we have a surprising observation that protonation of the Mn-peroxo complex in the mutant yeast enzyme occurs through a fast pathway, leading to a putative six-coordinate Mn(3+) species, which actively oxidizes O2- in the catalytic cycle. Upon increasing pH, the fast pathway is gradually replaced by a slow proton-transfer pathway, leading to the well-characterized five-coordinate Mn(3+). We here propose and compare two hypothetical mechanisms for the mutant yeast enzyme, differing in the structure of the Mn-peroxo complex yet both involving formation of the active six-coordinate Mn(3+) and proton transfer from a second-sphere water molecule, which has substituted for the -OH of Tyr34, to the Mn-peroxo complex. Because WT and the mutant yeast MnSOD both rest in the 2+ state and become six-coordinate when oxidized up from Mn(2+), six-coordinate Mn(3+) species could also actively function in the mechanism of WT yeast MnSODs.

Complexes with 3-(pyridin-2-yl)imidazo[1,5-a]pyridine ligands by spontaneous dimerization of pyridine-2-carboxaldehyde within the coordination sphere of manganese(ii) in a one-pot reaction

Condensation of two molecules of pyridine-2-carboxaldehyde (pyca), in the presence of MnCl(2) and ammonium diethyldithiophosphate, produced, in a one-pot reaction, Mn(II) complexes containing 3-(pyridin-2-yl)imidazo[1,5-a]pyridines as κ(2)(N,N) chelate ligands which could be easily separated from the metal, this being a convenient way to pyridyl-aza-indolizine heterocycles.

Crystal Structure and Magnetic Properties of a New Chiral Manganese(II) Three-Dimensional Framework: Na3[Mn3(HCOO)9

A structural and magnetic characterization of a trinuclear chiral Mn(II) formate three-dimensional framework exhibiting a triangular arrangement is presented. Compound Na(3)[Mn(3)(HCOO)(9)] was obtained by solvothermal synthesis and crystallizes in the chiral cubic space group P2(1)3 and is well described by a Delta conformation. The structure displays triangular Mn(3) building blocks, in which the metal centers are bonded by formate ligands in a syn-anti mode (Mn-Mn 5.697(1) A). The coordination sphere of manganese(II) is completed by six oxygen atoms from six formate ligands, resulting in an octahedral geometry. Magnetic susceptibility measurements showed antiferromagnetic interactions at high temperature and a strongly field dependent magnetic behavior below 40 K. At fields higher than 1.0 kOe only the antiferromagnetic interactions can be observed. At applied fields lower than 1.0 kOe magnetic susceptibility becomes irreversible with maxima observed at 22 and 34 K. These maxima suggest a weak ferromagnetic behavior because of spin canting, allowed by the presence of the noncentrosymmetric syn-anti HCOO bridges linking the Mn sites. This non-collinear antiferromagnetism and irreversible behavior can be due to the existence of a high degree of frustration in this unique lattice composed of linked triangular arrangements of interacting magnetic centers.

Self‐Assembly of Dimeric MnIII–Schiff‐Base Complexes Tuned by Perchlorate Anions

AbstractThe synthesis and structural and spectroscopic characterisation of 12 manganese(III)–Schiff‐base complexes of the formula [MnLn(H2O/CH3OH)2](ClO4)(mH2O) (m = 0, 1) are described. The multidentate H2Ln and H4Ln Schiff‐base ligands were obtained by condensation of different diamines (1,2‐diaminoethane, 1,2‐diamino‐2‐methylethane, 1,2‐diamino‐2,2‐dimethylethane, 1,3‐diamino‐2,2‐dimethylpropane) and 2,3‐dihydroxybenzaldehyde, 3‐methoxy‐2‐hydroxybenzaldehyde or 3‐ethoxy‐2‐hydroxybenzaldehyde. The crystal structures of H2L3 and H4L10 ligands were solved by X‐ray crystallography, revealing their ability to bind metal centres. Recrystallisation of complexes 2 and 8 from methanol yielded single crystals of [MnL2(H2O)2](ClO4) and [MnL8(H2O)2](ClO4). Their X‐ray characterisation shows a tetragonally elongated octahedral geometry for the manganese coordination sphere, formed by the chelation of the N2O2 donor set of the inner compartment of the Schiff base to the equatorial plane of the octahedron and by the binding of two water molecules in the axial positions. The octahedron entities are linked in pairs by μ‐aquo bridges between neighbouring axial water molecules and also by π–π stacking interactions, establishing dimeric structures. Perchlorate anions are accommodated in the cavities of the framework and form hydrogen bonds with the aqua ligands, leading to infinite spirals of the complexes generated by a screw axis along a. Moreover, the peroxidase activity of the crystallographically solved complexes 2 and 8, studied in the presence of the water‐soluble trap ABTS, agrees with the predicted rate according to the activity/rhombicity correlation.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Manganese(II) tri- tert-butoxysilanethiolate complexes with imidazole-based coligands: A neutral complex with four independent ligands and MNOS 2 core (M = Mn) related to the liver alcohol dehydrogenase catalytic center (M = Zn)

The reaction of [ Mn { SSi ( OBu t ) 3 } 2 ( MeOH ) 4 ] with imidazole and its two methyl substituted derivatives leads to different types of heteroleptic manganese(II) thiolate complexes. Reaction with 1-methylimidazole gives the silanethiolate devoid of methanol but with two nitrogen ligands and thus central MnN 2S 2 core. The reaction with imidazole leads to the methanol solvated complex with only one nitrogen ligand but manganese coordination sphere enlarged to MnO 2NS 2 due to an O, S-chelation by tri- tert-butoxysilanethiolate ligand. Molecules of this compound interact through a set of N–H⋯(Me)O–H⋯S hydrogen bonds with methanol hydroxyl group being simultaneously acceptor and donor. With 2-methylimidazole the product is an assembly of two different neutral complexes joined again by hydrogen bonds, however, this time of N–H⋯S type. One of these complexes has the previously mentioned MnO 2NS 2 core. The second neutral complex exhibits four donor atoms (MnNOS 2core) derived from four independent ligands, i.e., two silanethiolate rests, one N-heterocyclic base and one alcohol. This structure presents similarities with a zinc-based alcohol dehydrogenase active site that have never been obtained before, including with other metals (Zn, Co). It may, therefore, be considered the first neutral structural model of liver alcohol dehydrogenase (LADH).

Manganese-modified hexagonal mesoporous aluminophosphate MnHMA: Synthesis and characterization

Manganese-modified hexagonal mesoporous aluminophosphate (MnHMA) was synthesized hydrothermally in a microwave oven in the presence of cetyltrimethylammonium chloride (CTACl) as a template. The mesoporous structure was elucidated by HRTEM. Local environment of aluminum and phosphorus forming the mesoporous aluminophosphate framework was studied by MAS NMR. The local environment of manganese incorporated in HMA was investigated by X-ray absorption spectroscopy. XANES studies show that manganese in the as-synthesized and template-free MnHMA is present in the form of Mn 2+ and Mn 3+ ions in the ratio 40–60%. EXAFS analyses revealed four oxygens in the first coordination sphere of manganese in the as-synthesized and template-free MnHMA at the distances 2.142(5) Å and 2.15(1) Å, respectively. EPR and ESEEM measurements revealed that Mn 2+ in both as-synthesized and template-free products was located within the mesoporous aluminophosphate framework.

Novel peroxidase mimics: μ-Aqua manganese–Schiff base dimers

The interaction of manganese(II) carboxylate salts [Mn(O 2CR) 2, where R = ethane, pentane] with H 2L 1 [ N, N′-bis(3-methoxy-2-hydroxybenzaldehyde)-1,2-phenylenediamine] and H 2L 2 [ N, N′-bis(3-ethoxy-2-hydroxybenzaldehyde)-1,2-phenylenediamine] was studied. MnL 1(O 2CEt)(H 2O) ( 1), MnL 1(O 2CPe n )(H 2O) ( 2), MnL 2(O 2CEt)(H 2O) 2 ( 3) and MnL 2(O 2CPe n )(H 2O) 2 ( 4) were isolated and thoroughly characterised by elemental analysis, FAB mass spectrometry, infrared and 1H NMR spectroscopy, magnetic susceptibility measurements, molar conductivities, and cyclic and normal pulse voltammetry. Compounds 1 and 2 were crystallographically characterised revealing a tetragonally elongated octahedral geometry for the manganese coordination sphere and also a dimeric nature through μ-aqua bridges. Complexes 1– 4 behave as efficient peroxidase mimics in the presence of the water-soluble trap ABTS, probably due to their ease to coordinate the substrate molecule. A correlation between rhombicity of the complexes and peroxidase activity has also been established.

Tert-Butylimido compounds of manganese-(VII), -(VI), -(V) and -(II); nitrido, amido, alkyl, zinc and aluminium compounds

The interaction of Mn(NBut)3Cl with Li(NHBut) under prescribed conditions leads to the manganese(VI) dimer, [Mn(NBut)2(µ-NBut)]21 and lithium salts 2, 3 of the manganese(VII) nitrido anion [Mn(N)(NBut)3]2–, but the interaction with Li(NHC6H3Me2-2,6) gives the paramagnetic, tetrahedral, spiro manganese(II) complex [graphic omitted]H]24. The interaction of Mn(NBut)3Cl with methyllithium or dimethylzinc gives the dimeric methyl compound [MnMe(NBut)(µ-NBut)]25, while interaction with ZnR2(R = Et, CH2But, CH2CMe2Ph, CH2SiMe3 or CH2Ph) gives similar alkyls 6–10 respectively. The interaction of 1 with ZnR2 produces Mn2(NBut)2(µ-NBut)4ZnR (R = Me 11 or CH2But12), and with Al2Me6, Mn2(NBut)2(µ-NBut)4AIMe213. The interaction of [Li(dme)]2[Mn(NBut)4](dme = 1,2-dimethoxyethane) and Al2Me6 gives Mn[(µ-NBut)2AlMe2]214. The structures of compounds 1, 2, 3, 4, 7, 10 and 11 have been determined by X-ray crystallography. Compound 1 has been shown to be isostructural with its rhenium analogue, although twinning or disorder problems precluded accurate analysis. The nitrido compounds 2 and 3 have a common tetrahedral co-ordination sphere of manganese bonded to one nitride and three imido groups. The lithium cations are bonded to one nitride and three imido groups. The lithium cations are bonded to the imido nitrogen atoms, chloride ions and ether oxygen atoms. Compound 4 has a molecular structure which comprises a four-co-ordinate tetrahedral ‘spiro’ manganese atom chelated by two N(But)C(H)C6H3(Me)NH– ligands. The six-membered chelate ring formed is approximately planar and the spiro rings (one generated from the other by the two-fold axis through Mn) are almost perpendicular, with a dihedral angle of 91.5° at the MnII centre. The two formally single Mn–N bonds are slightly different at 2.036 and 2.135 A. The alkyl dimers 7 and 10 have closely analogous symmetrical structures, comprising two tetrahedral manganese atoms bridged by two imido groups and each bonded to one terminal imido and one alkyl ligand. With respect to the central Mn2N2 unit the two alkyls and the two imides adopt cis or Z arrangements. The geometry of the planar Mn2N2 unit in each structure has acute Mn–N–Mn (average 83.6°) and obtuse N–Mn–N (average 96.3°) angles consistent with the presence of direct Mn–Mn interaction, which formally has double bond character. Compound 11 has a molecular structure which contains a two-fold axis passing through the methyl and the Zn and Mn atoms. There is an unusual trigonal-planar ZnII atom bonded to a methyl group and two bridging imido groups; Mn(1), in oxidation state V, is bonded to four bridging imido ligands, and Mn(2), oxidation state VI, is bonded to two bridging and two terminal imido ligands.

Manganese complexes of 1,3-bis(3-nitrosalicylideneamino)-propan-2-ol. Structure and magnetic and electrochemical properties

Four complexes [MnII(3NO2-Hsalpro)]1, [MnII2(3NO2-salpro)(O2CMe)]·4MeOH 2, [MnIII2(3NO2-salpro)2(H2O)]·H2O 3 and [{MnIII(3NO2-salpro)}2]·2dmf 4, [3NO2-H3salpro = 1,3-bis(3-nitrosalicylideneamino)propan-2-ol, dmf = dimethylformamide] have been synthesized and studied. The structure of 4 was solved by direct methods and refined to conventional agreement indices R=R′= 0.041. The molecular structure consists of discrete [{MnIII(3NO2-salpro)}2] units and lattice-held dmf molecules. The co-ordination octahedron around each MnIII in cludes three donors from each of the two pentadentate trianionic ligands. Both ligands asymmetrically bridge the Mn atoms affording an intra-dinuclear Mn ⋯ Mn′ separation of 3.223(1)A. The symmetry-related manganese centres are in a distorted-octahedral environment. Infrared, ESR spectroscopic, magnetic susceptibility and electrochemical studies of 1–4 evidence a variety of structural types and nuclearities. Complex 1, resulting from the reaction of manganese(II) acetate with 3NO2-H3salpro, is characterized by the MnL stoichiometry while 2, resulting from the reaction of manganese(II) acetate with Na3(3NO2-salpro), is characterized by the Mn2L stoichiometry. Two structurally different dinuclear manganese(III) complexes characterized by the same overall formulation [{MnIII(3NO2-salpro)(solvent)}2] were obtained from the reaction of 1 with O2. In non-dehydrated methanol, the asymmetric dinuclear species 3 is formed. When this complex is dissolved in dry dmf the water molecule is driven out of the manganese co-ordination sphere and a subsequent ligand environment reorganization affords 4. Variable-temperature magnetic susceptibility studies established the presence of isotropic magnetic exchange interactions between the manganese(III) centres of the dimeric species 3 and 4(J=–1.9 and –1.6 cm–1, respectively) and crystalline field anisotropy of MnIII(D=–2.6 and –2.5 cm–1, respectively). Electrochemical studies indicated that the MnII2, MnIIMnIII, MnIII2 and MnIIIMnIV oxidation states are accessible for 4 in dmf–water.

NMR paramagnetic relaxation enhancements due to manganese in the S0 and S2 states of Photosystem II-enriched membrane fragments and in the detergent-solubilized Photosystem II complex

The NMR paramagnetic relaxation enhancement (NMR-PRE) produced in the solvent proton resonance by manganese in the S0 and S2 states of the oxygen evolving center (OEC) has been recorded for three Photosystem II (PS II)-enriched preparations: (1) PS II-enriched thylakoid membrane fragments (TMF-2 particles); (2) salt-washed (2M NaCl) TMF-2 particles; and (3) the octylglucopyranoside (OGP)-solubilized PS II complex. The second and third preparations, but not the first, are depleted of the peripheral 17 and 23 kD polypeptides associated with the OEC. It has been proposed that depletion of these polypeptides increases the exposure of OEC manganese to the aqueous phase. The NMR-PRE response measures the quantity (T1m+τm)(-1), where T1m is the spin relaxation time and τm is the mean residence time with respect to chemical exchange reactions of solvent protons in the manganese coordination sphere, and, thus, the NMR-PRE provides a direct measure of the solvent proton chemical exchange rate constant τm (-1). This study tested whether the 17 and 23 kD polypeptides shield the OEC from the solvent phase and whether their depletion enhances the S2 and S0 NMR-PRE signals by removing a kinetic barrier to the solvent proton chemical exchange reaction. The amplitude of the S2 NMR-PRE signal, measured in its chemical exchange-limited regime (τm>T1m), is slightly decreased, rather than increased, in preparations (2) and (3) relative to (1), indicating that removal of the 17 and 23 kD polypeptides slightly slows, rather than accelerates, the rate-limiting steps of the solvent proton chemical exchange reactions. In addition, the lifetime of the S2 state was shortened several-fold in the solubilized PS II complex and in salt-washed TMF-2 membranes relative to untreated TMF-2 control samples. The S0 NMR-PRE signal, which is present in TMF-2 suspensions, was not detected in suspensions of the solubilized PS II complex, even though these samples contained high concentrations of active manganese centers (approximately double those of the TMF-2 control) and exhibited an S2 NMR-PRE signal of comparable amplitude to that of the TMF-2 preparation. These results suggest that the 17 and 23 kD extrinsic polypeptides do not shield the NMR-visible water binding site in the OEC from the aqueous phase, although their removal substantially alters the proton relaxation efficiency by shortening T1m.

?-? Rearrangement of vinylphosphonium ligand in the coordination sphere of manganese, the crystal and molecular structure of [?5-C5H5(CO)2Mn(?2-PhHc=CHP+Ph3]Cl?

1. We observed rearrangement of the σ-vinylphosphonium ligand to a π-olefin phosphonium ligand, occurring in the coordination sphere of the manganese under the action of protic acids. 2. By the x-ray diffraction analysis method we established the structure of [Cp(CO)2Mn (π-PhHC=CHP+Ph3)]Cl−.

Chemical reactions of 1-propyn-3-ol and 1-butyn-3-ol within the coordination sphere of manganese

1. Acetylenic π-complexes Cp(OC)2Mn(π-HC≡CCH2OH) and Cp(OC)2Mn(π-TC≡CCH(CH3)OH) have been demonstrated to react with bases and acids and to undergo conversion to the π-olefinic complexes Cp(OC)2Mn(π-H2C=CHC(H)=O) and Cp(OC)2Mn(π-H2C=CHC(CH3)=O). 2. Treatment of acetylenic π-complexes Cp(OC)2Mn(π-HC≡CCH2OH) and Cp(OC)2Mn(π-HC≡CCH-(CH3)OH) with Et2NH gives aminocarbene and π-olefinic manganese complexes.

Metal complexes in acetic acid. Part 5. Proton magnetic resonance studies on acetic acid exchange at the first coordination sphere of manganese(II)

Metal complexes in acetic acid. Part 5. Proton magnetic resonance studies on acetic acid exchange at the first coordination sphere of manganese(II)

On carbon monoxide insertion reactions: A molecular orbital analysis of the reaction pathway

A theoretical study of the carbon monoxide insertion reaction in methylmanganese-pentacarbonyl utilizing an all-valence-electron self-consistent-field molecular orbital method in the CNDO approximation is presented. Two possible reaction pathways are analyzed. In one of them the alkyl migration is assumed to be a spontaneous process, while in the other one this migration is accompanied by the entrance of a new CO radical into the coordination sphere of manganese. This allows to evaluate the role played by the latter in promoting the cis-migration of the CH 3 group. The present study is in good rapport, from a quantum chemical point of view, with the conclusions previously derived from the exhaustive mechanistic studies for this process.

Magnetic resonance studies of the manganese guanosine di- and triphosphate complexes with elongation factor Tu.

Analysis of titration data of EF-Tu-GDP with Mn(II) where free and bound Mn(II) were determined by proton relaxation rate of water (PRR) yields one tight Mn(II) binding site and a value of 2 muM for the dissociation constant of Mn(II) from the EF-Tu-MnGDP complex, K'A. The dissociation constant of manganese nucleotide from the ternary EF-Tu-MnGDP complex, K2, 0.2 muM, was derived from the known value of Ks, the dissociation constant for the binary EF-Tu-GDP complex, and the titration data of the ternary complex with excess GDP as titrant. The apparent number, n, of rapidly exchanging water ligands coordinated to bound Mn(II) in the ternary complex EF-Tu-MnGDP is estimated from the frequency dependence of the PRR of the complex to be approximately 1. The value of n and the values of PRR enhancements, epsilont = 4.3 for EF-Tu-MnGDP at 21 degrees, 24.3 MHZ and epsilont = 4.1 for the ternary GTP complex, are unusually low for protein-Mn-nucleotide complexes. The antibiotic X5108 which induces GTPase activity in EF-Tu-MgGTP was shown to bind stoichiometrically to EF-Tu-MnGDP and thereby change the PRR enhancement of the complex from 4.3 to 7.4. The characteristic broad lines in the EPR spectra of Mn(II) nucleotides are strikingly narrowed upon binding of Mn(II) nucleotides to EF-Tu. The long electron spin relaxation times inferred from the EPR spectra indicate a limited access of solvent water to the first coordination sphere of Mn(II) in its EF-Tu-nucleotide complexes. The frequency dependence of the PRR indicates that the electron spin relaxation time, T1e, is the dominant process modulating the Mn(II)-H2O interaction of the EF-Tu-MnGDP complex and consequently determines the correlation time. The value of T1e, estimated from the PRR experiments to be 2.5 ns at 21 degrees, is consistent with the lower limit of T1e obtained from the line widths of the EPR spectrum of the complex. Upon binding of a stoichiometric quantity of the antibiotic X5108, the EPR spectrum of EF-Tu-MnGDP is severely broadened indicating greater access of solvent water to the manganese coordination sphere, i.e. an opening of the nucleotide binding site as already suggested by the increased PRR enhancement.

Nitrogen-14 nuclear magnetic resonance studies of acetonitrile exchange from the first coordination sphere of manganese(II)

Nitrogen-14 nuclear magnetic resonance studies of acetonitrile exchange from the first coordination sphere of manganese(II)