Mn Ligand Research Articles

A multinuclear nmr relaxation study of the interaction of divalent metal ions with l-aspartic acid

Carbon-13 spin-lattice relaxation times, T 1, have been measured for aqueous solutions of L-aspartic acid, L-alanine, O-phospho-L-serine, and 2-mercapto-L-succinic acid in the presence of the paramagnetic metal ions, Cu 2+ and Mn 2+, and Mg 2+ as a diamagnetic control, at ambient temperature and neutral pH. Nitrogen-15, oxygen-17 and proton relaxation times were also obtained for L-aspartic acid and phosphorus-31 relaxation times for O-phospho-L-serine under similar conditions. The structures of these complexes in solution were determined from the various metal ion-nuclei distances calculated from the paramagaetically-induced relaxation. These results indicate that the Cu 2+ interaction with L-aspartic acid is through α-amino and β-carboxyl groups while Mn 2+ coordinates most strongly through α-and β-carboxyl groups, with the possibility of a weak interaction through the amino group. An examination of the coordination of these divalent metal ions to an analog of L-aspartic acid in which the β-carboxyl group is replaced by a phosphate group ( O-phospho-L-serine) indicated that Cu 2+ coordination is now probably through the α-amino and phosphate groups, while this analog is a monodentate ligand for Mn 2+ coordinating through the phosphate group. Removal of the β-carboxyl group (L-alanine) also results in Cu 2+ coordination through the α-carboxyl and α-amino groups, and the same ligand interactions are observed with Mn 2+. Replacement of the α-amino group of L-aspartic acid with an - SH group (2-mercapto-L-succinate) is sufficient to eliminate any specific coordination with either Cu 2+ or Mn 2+.

Electron spin resonance studies of ribulose bisphosphate carboxylase: identification of activator cation ligands

Ribulosebisphosphate carboxylase (RuBP carboxylase)forms a stable model complex containing stoichiometric amounts of enzyme sites, activator C0(2), divalent activator cation, and the transition-state analogue carboxyarabinitol bisphosphate (CABP). Incorporation of Mn(2+) in the model complex permits investigation of the environment of the activator cation by electron spin resonance (ESR)techniques. Measurements at 9 GHz on the Mn(2+)-containing complex prepared by using dimeric Rhodospirillum rubrum enzyme produce a spectrum which indicates that the cation is bound in an anisotropic environment. Measurements at 9 GHz on the spinach enzyme model complex produce a spectrum in which several of the fine structure transitions are obvious. In contrast, the spectrum produced from Mn(2+) bound to R. rubrum enzyme exhibits an intense powder pattern for the central fine structure transition; the other four fine structure transitions produce powder patterns that are in homogeneously broadened and therefore are not as apparent.Low-temperature measurements at high field (35 GHz) result in substantially simplified spectra. The spectrum of Mn(2+) bound to the R. rubrum enzyme shows less fine structure than the spectrum of Mn(2+) bound in the octameric spinach enzyme complex, where substantial hyperfine splitting is resolved in three of the five fine structure transitions. Measurements at 35 GHz on Mn (2+) bound in the dimeric R. rubrum enzyme complex produce spectra in which only the central fine structure transition produces a prominent signal. However, these samples are characterized by several narrow spectral features which permit investigation of the identity of Mn(2+)ligands by 170 perturbation techniques. Preparation of the R. rubrum RuBP carboxylase model complex in (17)O-enriched water results in a sample which exhibits an obviously broadened 35-GHz Mn(2+) spectrum in comparison to unenriched samples. Removal of H(2)(17)O by gel filtration abolished the spectral broadening, indicating that the Mn(2+)-coordinated water molecules can slowly exchange. No spectral broadening was detectable due to (17)O in the carbamate oxygens derived from activator C(17)O(2). NMR relaxation rate,measurement sat 24.3 MHz demonstrate that stoichiometric amounts of carboxyarabinitol bisphosphate eliminate enhancement of the proton relaxation rate observed in ternary enzyme-C0(2)-Mn(2+) complexes prepared by using dimeric R. rubrum enzyme. This observation, coupled with results of the H(2)(17)O ESR experiments,is compatible with the suggestion that the water molecules which coordinate directly to bound Mn(2+) are nonexchangeable on an NMR time scale but can be displaced by solvent water within 1-2 h. Carboxyarabinitol bisphosphate was selectively enriched with (17)O in the carboxyl group or in the oxygen on C-2. Mn(2+)-containing complexes prepared with either of the (17)O-enriched analogues produced spectra which were broadened in comparison to matched (l6)O controls. Thus,Mn(2+) coordinates directly to CABP, arguing for the participation of cation in the catalytic process.

Reactions of diazoles with manganese and iron carbonyls

The compounds of two types were isolated in the reactions of dimanganese decacarbonyl with diazoles. The compounds with an unaltered oxidation state of the metal were formed as a result of nucleophilic substitution of a carbonyl ligand of Mn 2(CO) 10. The compounds with an altered oxidation state of the central atom were produced in redox-transformations of Mn 2(CO) 10. The substances were characterized by the data of mass-, IR-, 1H-NMR-spectra. The EPR-data were obtained for paramagnetic Mn 2+ salts. The reactions of Mn 2(CO) 10 with diazoles are compared with those of iron carbonyls.

Manganese and calcium binding sites of concanavalin A

The atomic co-ordinates of concanavalin A, a lectin which specifically binds saccharides containing mannosyl and glucosyl residues, have been refined with the fast Fourier least-squares method of Agarwal (1978), with X-ray crystallographic data to 1.75 Å. This metalloprotein contains six co-ordinated Mn 2+ and seven co-ordinated Ca 2+ which are 4.25 Å apart and are close to the carbohydrate binding site. Both ions are pseudo-octahedral. This is possible for calcium since the average position of two oxygens (from the same carboxyl group) forms one apex of the octahedron. The refinement has improved this region considerably. The ligands for Mn 2+ are five oxygens, three from carboxyl side-chains and two from water molecules, and one nitrogen from a histidyl side-chain. The co-ordination sphere for Ca 2+ contains seven oxygens, three from two carboxyl side-chains, one from a peptide carbonyl, one from an amide side-chain and, as does Mn 2+, two from water molecules. The averages for the five Mn 2+O and the seven Ca 2+O bonds are 2.28 ± 0.04 (standard deviation) and 2.45 ± 0.09, respectively. For Mn 2+, this average is 0.10 Å higher than that found for the Mn(II)hexa-aquoion and is toward the high end of the range usually found for Mn 2+O complexes. This is of particular significance since Mn 2+ is used as a probe for nuclear magnetic resonance spectroscopic studies of native Mn 2+- and Mn 2+-substituted metalloproteins. Also, refinement confirms the non-proline cis peptide bond, which is immediately adjacent to the Ca 2+ and the carbohydrate binding sites. For the final model, the bond lengths and angles for backbone atoms have root-mean-square deviations from standard values of 0.030 Å and 5.2 deg., and side-chain atoms have slightly lower values. The crystallographic residual, R factor, for this model is 0.167 for 22,042 structure factors.

Electron paramagnetic resonance studies of MN(II) complexes with myosin subfragment 1 and oxygen 17-labeled ligands.

Ligands in the first coordination sphere of Mn(II) in the complex of MnADP with myosin subfragment 1 from rabbit skeletal muscle have been investigated. EPR spectroscopy was used to detect superhyperfine coupling between unpaired electrons of the metal ion and the nuclei of oxygen atoms specifically labeled with oxygen 17. The results show that ADP is a beta-monodentate ligand for Mn(II) and that there are probably two water oxygens directly bound to Mn(II). The inhibitory complex of vanadate with subfragment 1 . MnADP was also investigated. Vanadate-dependent changes in the EPR spectra for enzyme-bound Mn(II) indicate that the coordination sphere of MN(II) changes upon binding of vanadate. ADP remains a beta-monodenate ligand in the complex and experiments with 17O-labeled water indicate that two oxygen atoms originally in water are ligands in the complex. However, the oxygens of vanadate equilibrate with those of water during sample preparation so that one of these ligands may be a vanadate oxygen. Three additional ligands, probably from the protein, are required to complete the sextet of ligands to Mn(II) in both complexes studied.

Synthesis and Characterisation of Manganese(II) Complexes of Some Semicarbazones and Thiosemicarbazones

Abstract Manganese(II) complexes of the general composition Mn(ligand)2X2, where X = cl, Br, I and the ligands are R1R2C = NNHCOHH and R1R2C = NNHCSNH2 (R1 =R2 = cycloheptane and cyclopantane) have been prepared and characterised by elemental analysis, magnetic moments, infrared, electronic spectra and electron spin resonance spectral studies. All these complexes are of octahedral geometry.

EPR studies of the Mn(II) complex with elongation factor Tu and GDP Identification of oxygen ligands to Mn(II) by observation of 17O superhyperfine coupling.

The coordination sphere of Mn(II) in the complex with GDP and elongation factor Tu from Escherichia coli has been probed by EPR spectroscopy with 17O-labeled ligands. Inhomogeneous broadening in the EPR signals for Mn(II) due to unresolved superhyperfine coupling to the 17O nucleus was used to identify directly bound oxygen ligands. Results with GDP selectively enriched with 17O either in the alpha-phosphate or in the beta-phosphate revealed that GDP was a beta-monodentate ligand for Mn(II) in the complex with the protein. Results with 17O-enriched water showed that two water molecules are coordinated to the Mn(II). The EPR spectrum for the complex is characteristic of octahedral coordination for Mn(II). Hence, three ligands from the protein are required to complete the sextet of ligands.

Preparation and certain reactions of nitrosylcarbonyl complexes [?5-C3H5M(CO)(NO)L]?BF ? 4 and [?5-C9H7M(CO)(NO)L]?BF ? 4 (M=Mn, Re, L=CO, PPh3)

1. The authors have obtained new compounds η5-C9H7M(CO) (NO) (PPh3)⊕BF4 (M=Mn or Re) by nitrosation of η5-indenyldicarbonyltriphenylphosphine complexes of Mn and Re. 2. The reaction of [C9H7Mn(CO)(NO)PPh3]⊕BF4⊖ with KX (X=I, Br, or CN) takes place with detachment of the Π ligand and formation of Mn(NO)2(PPh3)2X. In the analogous reaction with KI or I2, [C5H5Re(CO)2(NO) ]⊕BF4⊖ forms a new Π-complex C5H5Re(CO) (NO) I.

The multidentate chemistry of manganese(II). Part VI. Tridentate Nitrogenous Ligands Complexes

A large number of manganese(II) complexes of formulation [Mn(ligand)X2] (where ligand = linear tridentate, quadridentate or quinquidentate nitrogenous ligand; X = Cl, Br, I or NCS) are described. The tridentate ligand compounds, in which X = Cl or NCS have been shown, in general, to be five coordinate on the basis of conductivity measurements, X-ray powder data and infrared spectral analysis; but in some cases the compounds may possess a polymeric octahedral structure. The quadridentate ligand compounds are octahedral, as are a series of complexes of formulation [Mn(NNN)2](ClO4)2.

The multidentate chemistry of Manganese(II).I. Hybrid tridentate ligand complexes

The complex compounds obtained from the interaction of manganese(II) salts and four new tridentate ligands, each possessing the donor set NNAs, and prepared by the Schiff-base condensation of σ-dimethyl or σ-diethylarsinoaniline, are described. Three types of complex were obtained: (i) [Mn ligand X 2] (where X = Cl. Br, or I); ii, [Mn(ligand) 2]Y 2 (where Y = ClO 4 or BPh 4); (iii) Mn Ligand (NCS) 2. Physical measurements have been used to demonstrate that in each case the ligands act as tridentates, and that type (i) complexes are five-coordinate, type (ii) complexes are octahedral, and type (iii) complexes are polymeric octahedral species. Attempts to obtain manganese(II) complexes of various other ligands with the donor sets NNAs and ONAs were unsuccessful.

Magnetic Resonance Studies of Manganese (II) Binding Sites of Pyruvate Kinase: TEMPERATURE EFFECTS AND FREQUENCY DEPENDENCE OF PROTON RELAXATION RATES OF WATER

The stoichiometry and dissociation constant of the binary rabbit muscle pyruvate kinase-manganese(II) complex have been investigated in the temperature range 5–37° by monitoring the amount of Mn(II) free in solution utilizing electron paramagnetic resonance spectrometry. The results have been interpreted in terms of an equilibrium mixture of two conformational forms of the enzyme, one which has four binding sites for Mn(II) (active) and another which does not bind Mn(II) (inactive). The equilibrium is affected by monovalent ions; potassium, an activator of the pyruvate kinase reaction, stabilizes the active form as compared to the enzymatically inert tetramethylammonium ion. Longitudinal proton relaxation rates of water in solutions of pyruvate kinase and Mn(II) have been measured by pulsed nuclear magnetic resonance spectrometry as a function of temperature (5–37°) and frequency (8 to 60.0 MHz) in order to gain some insight into the mechanism of relaxation and to determine the number of protein ligands to the metal. The proton relaxation rates due to bound Mn(II), highly enhanced relative to the Mn(II) aquo-ion, have been analyzed according to the Solomon-Bloembergen-Morgan scheme. The rationale and method of analysis are presented in detail. Unlike the Mn(II) aquo-ion for which the relevant correlation time for the interaction between Mn(II) and the water protons is the rotational time and consequently frequency-independent, it has been found that the relevant correlation time for proton relaxation of water in the vicinity of bound Mn(II) is, at least in part, frequency-dependent. The frequency-dependent component is identified with the longitudinal electron spin relaxation time of the bound Mn(II) and the other component with the mean residence time of a water molecule in the first hydration sphere, i.e. the reciprocal of the water ligand exchange rate. From the results the number of water molecules coordinated to Mn(II) bound to pyruvate kinase has been estimated to be 3. It may be concluded that the enzyme probably provides three ligands for Mn(II) and thus the conformational changes previously detected by protein difference spectroscopy due to the binding of a divalent metal ion may be rationalized. Although the binding constant of the binary Mn(II)-enzyme complex varies upon substitution of tetramethylammonium for potassium ion, the enhancement of the proton relaxation rate of water for the binary complex is invariant with monovalent ion.