Manganese Coordination Research Articles

A Pillared Three-Dimensional Manganese(II) Coordination Network Containing Rectangular Channels: Synthesis, X-Ray Structure, and Magnetic Properties

An unusual pillared 3-D manganese(II) coordination network based on carboxylate-bridged trimanganese cores [Mn6(isonicotinate)10(H2O)2](ClO4)2(EtOH)2(H2O)3, 1, was synthesized by treating manganese(II) perchlorate and 4-pyridinecarboxyaldehyde under hydro(solvo)thermal conditions. A single crystal X-ray diffraction study revealed that 1 contains rectangular channels that are occupied by perchlorate counterions and disordered ethanol and water guest molecules. The included ethanol and water guest molecules as well as the coordinated water molecules can be removed under vacuum at room temperature to afford a nanoporous solid that maintains the framework structure of 1. Magnetic measurements indicate antiferromagnetic interactions among the high-spin Mn(II) centers in 1 with coupling constants of −3.08 K and −4.96 K for the roughly isosceles triangular trimanganese cluster. Crystal data for 1: C64H62Cl2 Mn6N10O35, monoclinic space group P21/n, a=18.7892(3) Å, b=23.2307(1) Å, c=21.1355(4) Å, B=95.085(1)°, Z=4, R1=0.0719, wR2=0.261, and GooF=1.02.

Synthesis and crystal structure of an infinite one-dimensional chain containing a poly-metallocage of MnII with 4,4′-bis(imidazol-1-ylmethyl)biphenyl

A new manganese(II) co-ordination polymer, [Mn(bimb)3][ClO4]2·2H2O [bimb = 4,4′-bis(imidazol-1-ylmethyl)biphenyl], was synthesized and characterized by X-ray crystallography, magnetic susceptibility and luminescent measurements. The cation structure of the complex is an infinite one-dimensional chain containing a poly-metallocage. Each MnII co-ordinates with six nitrogen atoms of six different bimb ligands with a distorted octahedral environment and each bimb ligand in turn binds to two MnII. The perchlorate counter ions and water molecules are incorporated between two adjacent chains through four C–H⋯O hydrogen bonds.

Nitrogen ligation to the manganese cluster of Photosystem II in the absence of the extrinsic proteins and as a function of pH.

Three extrinsic proteins (PsbO, PsbP and PsbQ), with apparent molecular weights of 33, 23 and 17 kDa, bind to the lumenal side of Photosystem II (PS II) and stabilize the manganese, calcium and chloride cofactors of the oxygen evolving complex (OEC). The effect of these proteins on the structure of the tetramanganese cluster, especially their possible involvement in manganese ligation, is investigated in this study by measuring the reported histidine-manganese coupling [Tang et al. (1994) Proc Natl Acad Sci USA 91: 704-708] of PS II membranes depleted of none, two or three of these proteins using ESEEM (electron spin echo envelope modulation) spectroscopy. The results show that neither of the three proteins influence the histidine ligation of manganese. From this, the conserved histidine of the 23 kDa protein can be ruled out as a manganese ligand. Whereas the 33 and 17 kDa proteins lack conserved histidines, the existence of a 33 kDa protein-derived carboxylate ligand has been posited; our results show no evidence for a change of the manganese co-ordination upon removal of this protein. Studies of the pH-dependence of the histidine-manganese coupling show that the histidine ligation is present in PS II centers showing the S(2) multiline EPR signal in the pH-range 4.2-9.5.

Manganese (II) Coordination Complexes Involving Nitronyl Nitroxide Radicals

Three new complexes involving nitronyl nitroxide and PhCOO- or N3- ligands have been synthesized and structurally and magnetically characterized. These compounds are formulated as Mn(L)4(X)2·nH2O, L = 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PNN) or 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (PN) and X = PhCOO- or N3-. Compound A [Mn(PNN)2(PhCOO)2(H2O)2] presents the following structural parameters: triclinic, space group P1 (No. 2), a = 6.859(1) A, b = 11.271(2) A, c = 13.978(6) A, α = 88.88(2)°, β = 89.38(2)°, γ = 78.28(2)°, Z = 1. The complex has a centrosymmetric distorted octahedral geometry in which the manganese ion is bound to two radical ligands through the nitrogen atom of the pyridine rings, two benzoate groups, and two water molecules. The two compounds B [Mn(PNN)4(N3)2] and C [Mn(PN)4(N3)2] are isostructural with the following structural parameters: B; triclinic, space group P1 (No. 2), a = 7.177(4) A, b = 13.767(3) A, c = 13.928(4) A, α = 90.20(2)°, β = 102.94...

Thiophene-2,5-dicarboxylic acid incorporated self-assembly of one-, two- and three-dimensional coordination polymers

Six thiophene-2,5-dicarboxylic acid incorporated and self-assembled zinc(II), cobalt(II) and manganese(II) coordination polymers [Zn(Tda)(py)]n (1), [Zn(Tda)(bipy)(H2O)·1.5H2O]n (2), [Zn(Tda)(phen)(H2O)]n (3), [Co(Tda)(phen)(H2O)]n (4), [Mn(Tda)(phen)]n (5) and [Mn(Tda)(H2O)2]n (6) have been synthesised and structurally characterised. Complex 1 is characterised as a two-dimensional parallelogram with a cavity of about 10.4×10.4 A, while complexes 2 and 3 (4) are one-dimensional linear and zig-zag coordination polymers with mainly hydrogen-bonding and stacking interactions contributing to their crystal packing, respectively. Complex 5 is a two-dimensional porous sheet with alternating 16- and 8-membered rings, while complex 6 is a three-dimensional porous coordination polymer. The diverse coordination properties of thiophene-2,5-dicarboxylate make it a good building block for the construction of coordination polymers of different architectures, which are dependent on both the end-capping ligand and the coordination geometry of the metal ions. Physical and thermal properties of these complexes have also been studied.

Synthesis, ab initio structure determination, and characterization of manganese(III) phenyl phosphonates

Two new manganese(III) phenyl phosphonates, Mn(HO 3PC 6H 5)(O 3PC 6H 5)·H 2O and Mn(HO 3PC 6H 5)(O 3PC 6H 5), were synthesized. The crystal structure of Mn(HO 3PC 6H 5)(O 3PC 6H 5) was solved ab initio from powder X-ray diffraction data. This compound crystallizes in the triclinic system, a = 10.2149(5), b = 15.0028(8), c = 5.0324(3) Å, α = 95.563(4)°, β = 116.858(4)°, γ = 95.684(5)°, V = 675.87(6) Å 3, space group P 1 , and Z = 2. The final agreement factors were R WP = 12.8%, R P = 9.1%, and R F = 3.2%. There are 22 non-hydrogen atoms in the asymmetric part of the unit cell, and the positional parameters were refined with the help of soft constraints. The octahedral manganese coordination spheres are distorted due to the Jahn-Teller effect. The structure of this organic–inorganic compound is layered. The thermal behavior of Mn(HO 3PC 6H 5)(O 3PC 6H 5)·H 2O was studied and its thermal decomposition product was identified.

Probing the mechanism of an Mn2+-dependent ribozyme by means of platinum complexes.

The smallest ribozyme system known is the pentanucleotide GAAACp, which is specifically cleaved by Mn2+, in the presence of poly(U), generating guanosine 2',3'-cyclic phosphate and AAACp. A plausible mechanism has been proposed, involving the participation of two Mn2+ with structural and catalytic roles, the first one cross-linking the two N7 atoms of G1 and A4, and the other binding to the N7 atom of A2 and activating the 2'-OH group of G1 [Kazakov, S. & Altman, S. (1992) Proc. Natl Acad. Sci. USA 89, 7939-7943]. In the present work, we have utilized the high affinity of Pt(II) complexes for N7 atoms of purines in an attempt to form a stable active ribozyme by replacing the structural Mn2+ by Pt2+. We thus replaced the proposed kinetically labile G1N7-Mn2+-A4N7 cross-link by an inert N7-trans-Pt(NH3)(2)(2+)-N7 cross-link. In a complementary investigation, the N7 atoms of the individual purines of GAAACp were selectively blocked by a Pt(NH3)(3)(2+) residue to determine which of the N7 sites participate in the Mn2+-mediated cleavage. Other N7-Pt(II)-N7 crosslinks were also investigated. Accordingly, we have prepared four monoadducts, each bearing the Pt(NH3)(3)(2+) residue on one of the purines and a series of chelates of trans-Pt(NH3)(2)(2+) and cis-Pt(NH3)(2)(2+) and have tested them for Mn2+-induced cleavage. Binding of Pt(NH3)(3)(2+) to G1 or A4 did not alter the efficiency of the specific cleavage between G1 and A2 catalyzed by Mn2+/poly(U), whereas cross-linking of G1 and A4 by trans-Pt(NH3)(2)(2+) inhibited it completely. Hence, a cross-link between G1 and A4 is not required for the site-specific cleavage. Binding of Pt(NH3)(3)(2+) to A2 or A3 strongly inhibits the G1/A2 cleavage, suggesting that these bases are likely to be involved in manganese coordination in the cleaving complex. A site-specific Mn2+-dependent cleavage between A4 and C5 was observed for the G1-A4 and G1-A3 adducts cross-linked by trans-Pt(NH3)(2)(2+), the G1-A2 adduct cross-linked by cis-Pt(NH3)(2)(2+), and the three monoadducts bearing the Pt(NH3)(3)(2+) residue on G1, A2 or A3; poly(U) did not exert any influence on this cleavage.

An EXAFS Study of bis(µ-oxo) Bridged Dinuclear Manganese Coordination Compounds as Models for the Manganese Complex of Photosystem II

A complex containing four manganese centres is thought to be the catalytic site of photosynthetic water oxidation. The structure and mechanism of action of this complex are largely unknown. Mn-coordination complexes can be used as models for the manganese complex of Photosystem II (PS II). We report here an X-ray Absorption Spectroscopic study of seven different manganese coordination compounds, six of which are binuclear and contain the structural his (μ-oxo) unit. In all the compounds, manganese is ligated by 3,3'-17-crown-6-SALPN, providing one crown ether moiety per Mn centre. Alkali or alkaline-earth cations were introduced into this crown-ether moiety in binuclear complexes, bringing the cations (Na + , K + , Ca 2+ and Ba 2+ ) into the vicinity (< 4A) of the manganese. The results are discussed in terms of their relevance to the PS II manganese complex.

Synthesis, structure and polarized optical spectroscopy of two new fluoromanganese(III) complexes †

The syntheses and crystal structures of two new fluoromanganese(III) complexes are reported; [MnF3(H2O)(2,2′-bipy)] 1 and 4,4′-bipyH2[MnF4(H2O)2]·2H2O 2, where 2,2′-bipy and 4,4′-bipy are 2,2′-bipyridyl and 4,4′-bipyridyl, respectively. Compound 1: monoclinic, space group P21/n, a = 1973.7(4), b = 749.0(2), c = 903.1(3) pm, β = 95.22(3)°, Z = 4, R1 = 0.051. Compound 2: monoclinic, space group P21, a = 516.4(2), b = 1851.9(4), c = 986.3(3) pm, β = 99.07(2)°, Z = 2, R1 = 0.028. The manganese co-ordination environment was found to be octahedral in both compounds, but strongly distorted by the Jahn–Teller effect as a result of the high-spin d4 configuration of Mn3+. A very extensive intermolecular hydrogen-bond framework is present in both compounds. For compound 1 the octahedra are linked through hydrogen bridges resulting in octahedral manganese chains. For compound 2, the octahedra [MnF4(H2O)2]– are associated via hydrogen bonds into chains, which in turn are connected by interchain hydrogen bridges. The polarized optical spectra of single crystals are presented and explained in terms of intraconfigurational d4 transitions split by ligand fields of nearly Cs and D4h symmetries for compounds 1 and 2, respectively. The results are compared with those available for other MnIII fluorides.

An extended X-ray absorption fine structure study of bis(μ-oxo) bridged dinuclear manganese co-ordination compounds containing alkali- and alkaline-earth-metal cations in a crown ether moiety †

An X-ray absorption spectroscopic study was made of seven different manganese co-ordination compounds, six of which are dinuclear and contain the structural bis(µ-oxo) unit. In all the compounds each manganese is ligated by one crown ether Schiff-base moiety. Alkali- or alkaline-earth-metal cations were introduced into this crown ether moiety in dinuclear complexes, bringing the cations (Na+, K+, Ca2+ and Ba2+) into the vicinity (<4 Å) of the manganese. The position of the X-ray absorption edge follows the order expected for the different oxidation states of manganese: 6551.6 eV for a manganese(III) monomer, 6552.1 eV for a MnIIIMnIV(µ-O)2 dimer and 6553.5 eV for a manganese(IV) dimer. Analysis of the extended X-ray absorption fine structure of the compounds yielded information about the immediate ligation to manganese. The first shell consists of light atoms (O/N) and is composed of two sub-shells, with average distances of 1.9 Å and 2.3 Å from the Mn. The dinuclear compounds clearly show the characteristic 2.7 Å Mn–Mn distance. In the cation-containing compounds a manganese–metal distance of approximately 3.6 Å is found. The introduction of Ba2+ in the crown ether moiety induces substantial changes in the ligation pattern of the light elements. The properties of the compounds provide a model for discussion of the properties of the photosynthetic manganese complex, and the role and position of the Ca2+ cofactor.

Manganese Coordination Compounds: Classification and Analysis of Crystallographic and Structural Data

Manganese Coordination Compounds: Classification and Analysis of Crystallographic and Structural Data

Induction of luminol chemiluminescence by the manganese cluster of the photosystem II water-oxidizing complex to the S0, S1, S2, and S3 states.

Luminol chemiluminescence (CL) (lambda(max) = 425 nm) induced by the manganese cluster of photosystem II (PSII) (t(max) = 1-5 min) along with CL induced by mono-, di-, or tetranuclear manganese coordination complexes (t(max) = 2-40 s) is observed when either 200 mM sodium phosphate or 1 M Tris-HCl + 200 microM EDTA are present in the reaction medium (pH 8.5) containing peroxidase. This light emission is not observed when Tris is used in the reaction medium without EDTA. The yield of a given CL without peroxidase is 5%-20% of that with peroxidase. The peroxidase-dependent CL is inhibited by catalase (I50 = 0.14-0.17 mu M), while the peroxidase-independent CL is not inhibited by 100 mM ethanol, 1 mM NaN3, 20 mu M Cyt c, or 0.6 mu M catalase. The CL induced by the Mn cluster of the water-oxidizing complex (WOC) in the S3 or S2 state exceeds that of lower S states by 15-20-fold. The magnitude of CL induced by Mn complexes is dependent on the ligand type of the complex. The ligand types of the Mn complexes and the WOC in different S states are ranked according to the magnitude of the induced CL: 1,10-phenanthroline > 2.2'-bipyridine > WOC (S3 or S2) > hydrotris(pirazolyl) borate > WOC (S0-2 or S0, S1) > 1,4,7-triazacyclononane. It is concluded that CL is caused by H2O2 formed as a result of the oxidation of luminol by triplet molecular oxygen. The Mn cluster of WOC and manganese coordination complexes, acting as catalysts of this reaction, show oxidase activity. Upon S2-S3 or S1-S2 transition, changes occur in the ligand environment of the Mn cluster of WOC influencing the induction of luminol CL.

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.

Preparation, crystal structure and spectroscopic study of azido(N,O-quinaldato)triaquamanganese(II) monohydrate andthiocyanato(N,O-quinaldato)-(O-quinaldic acid)diaquamanganese(II), [Mn(NC 9H 6COO) (N 3(H 2O) 3]·H 2O and [Mn(NC 9H 6COO)(NC 9H 6COOH)(NCS)(H 2O) 2

Two mixed ligand complexes of quinaldic acid and manganese(II) azide or thiocyanate, namely azido(N,O-quinaldato)triaquamanganese(II) monohydrate ( 1) and thiocyanato(N,O-quinaldato)(O-quinaldic acid)diaquamanganese(II) ( 2), have been prepared and characterized by single-crystal X-ray methods. The coordination environment of the manganese(II) atom in complex 1 is a distorted octahedron. Each metal atom is chelated by a quinaldate anion [via its nitrogen atom, MnN = 2.292(3) Å, and a carboxylate oxygen atom, MnO = 2.147(3) Å] and links three aqua molecules [MnO distances from 2.189(3) to 2.217(3) Å] and a terminal azido ligand [MnN = 2.181(3) Å]. The structure of complex 2 features distorted octahedral manganese(II) coordination geometry, chelated quinaldate anion [MnN = 2.271(7) Å and MnO = 2.158(5) Å], O-bonded neutral quinaldic acid molecule [MnO = 2.228(5) Å], two aqua molecules [MnO = 2.180(5) and 2.203(6) Å] and a terminal N-thiocyanato group at an MnN distance of 2.119(7) Å. The IR spectra of these two complexes are presented and discussed in relation to their structures.

New Insights into the Reactivity of Mn(II) Coordination Complexes with Dioxygen

Abstract As a clearer picture of the oxygen activation processes by manganese(II) co-ordination complexes begins to emerge, we are more likely to understand the biological significance of the Mn-O2 interaction in various enzymes and to use the activated oxygen molecule for the catalytic oxidation of organic substrates. This Comment describes some of the recent advancements in the area of dioxygen activation by manganese(II) complexes. Details regarding the binding of the O2 moiety to the metal center and the reaction pathway that is taken during the four electron reduction of the O-O double bond are presented. Particular attention is given to new information on Mn(II) Schiff-base complexes as these results provide strong evidence for a more straightforward and simpler oxygen activation process than previously thought. Also described in this article are examples of unique intermetal oxygen atom transfer reactions that the oxo bridged dimers of Mn Schiff-base complexes undergo with electrophilic metal compo...

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.

An optically detected magnetic resonance study of Mn 4+ in Cs 2GeF 6 and K 2GeF 6

The photoexcitation and photoluminescence spectra ( 2E ↔ 4A 2) of MnF 6 2− diluted in Cs 2GeF 6 and K 2GeF 6 host lattices have been recorded at liquid helium temperature. The manganese coordination geometry is O h in Cs 2GeF 6 and D 3d in K 2GeF 6. Optically detected magnetic resonance (ODMR) has been employed to determine the g value of the 2E state in both coordination geometries. The trigonal distortion has a pronounced effect on the g value and produces a measurable splitting in the optical spectrum, but exerts relatively little effect on the radiative lifetime. The g value in the O h site ( g( 2E) = 2.0) is as predicted theoretically; however, the value obtained for the D 3d site is unexpectedly large ( g( 2 E ) = 2.95). The data obtained are compared with theoretical predictions and the results of similar studies of d 3 ions in a variety of lattices. The differences are shown to arise, in part, from the relative magnitude of the spin-orbit coupling and the trigonal field splitting.

Crystal and molecular structure of Octacarbonylbis(diphenylarsino)-methanedimanganese(0)

The crystal structure of the complex Mn2(CO)8(dam) (dam = Ph2AsCH2AsPh2) has been determined by three-dimensional X-ray diffraction methods. The crystals are triclinic, space group P1, with a 11.191(1), b 16.498(5), c 9.455(1) �, a 93.64(2), β 109.08(2), γ 89.36(2)� and contain two discrete, binuclear molecules of Mn2(CO)8(dam) per unit cell. The structure, solved by direct and Fourier methods, was refined by a least-squares procedure to R and Rw of 0.065 and 0.082 respectively for 1907 independent, statistically significant reflections collected by counter methods. The feature of particular interest in this compound is the accommodation of the bridging bidentate dam ligand [As.. .As separation 3.242(2) �] across a shorter Mn�-Mn� bond [2.962(3) �] which constrains the molecule so that a much less staggered configuration of the two manganese coordination octahedra is observed relative to the parent compound Mn2(CO)10, the rotation of the two equatorial planes in the former being 30�.

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.

Nuclear magnetic relaxation studies of the conformation of adenosine 5'-triphosphate on pyruvate kinase from rabbit muscle.

The conformation of adenosine 5'-triphosphate in the manganese complex of pyruvate kinase from rabbit muscle was determined from six metal to nucleus distances derived by nuclear magnetic relaxation techniques. On the enzyme, no direct metal-ATP coordination exists. The phosphorous atoms of ATP are 4.9 to 5.1 A away from manganese, a distance which indicates either a predominantly (greater than or equal to 94%) second sphere complex or, less likely, a highly distorted inner sphere complex. Thus, water ligands or ligands from the protein might intervene between the ATP molecule and the divalent metal ion and facilitate their interaction. The metal-gammaP distance of 5 A for pyruvate kinase-bound ATP is equal to that found for the phosphorous atom of phosphoenolpyruvate and cobalt(II) on pyruvate kinase (Melamud, E., and Mildvan, A. S. (1975) J. Biol. Chem. 250, 8193-8201), which is consistent with the overlap in space of the P-enolpyruvate-phosphorus and the gammaP of ATP at the active site. This observation explains the competitive binding of these two substrates to the enzyme, as detected by NMR and by early kinetic studies. From the phosphorus data and from measurements of the relaxation rates of 3 protons of ATP in the pyruvate kinase-metal-ATP complex, the conformation of ATP was characterized as extended with distances of 6.0, 9.1, and 7.5 A from manganese to the H8, H2, and H'1 protons, respectively. The torsion angle about the glycosidic bond (chi) which defines the conformation of the enzyme-bound riboside and adenine rings was determined to be 30 degrees. In contrast, the conformation of the binary Mn(II)-ATP complex in solution is folded around the metal with direct manganese coordination of the alpha-, beta-, and gamma-phosphorus atoms, and with metal to proton distances of 4.5, 6.4, and 6.2 A for the H8, H2, and H'1 protons, suggesting a second sphere manganese-adenine interaction. The chi angle equals 90 degrees for the binary complex primarily because of the metal-base interaction. Thus, a profound change in the conformation and structure of Mn(II)-ATP from a folded chelate to an extended second sphere complex results when the nucleotide binds to pyruvate kinase.