Bioinorganic chemistry最新文献

Equilibria between hemin and imidazole in various organic and aqueous organic solvents have been investigated by uv-visible spectrophotometry at 25°C, and the following types of equilibria have been established:

Electron paramagnetic resonance spectra of polycrystalline copper complexes of butanedioic, pentanedioic, hexanedioic, heptanedioic, and decanedioic acids are presented, together with 77 K electronic spectra. The complexes are formulated as dimeric copper carboxylate units linked into infinite chains. Monomer impurities are also present and increase in quantity with the length of the diacid. The monomer and dimer signals occur at very different field strengths, but the g values calculated from the S = $\text{1}\text{2}$ spectra are similar to those calculated from the S = 1 spectra. The EPR method can thus be used to locate copper ions in possible biological frameworks and to study the geometry around the metal sites. The distortion from axial symmetry around the copper increases with the length of the diacid, as shown by the observed zero-field splitting parameters. Gaussian analysis of the optical absorptions yields information used with EPR data to calculate covalency and Fermi contact terms. Sodium, potassium, and lithium salts transform the dimeric polymers into monomeric polymers. The presence of magnetic exchange interactions in copper dicarboxylates is discussed and thereby shown to be of interest in the study of copper ions in molecules of biological importance containing carboxylate groups.

The structural basis for intarmolecular electron transfer in xanthine oxidase (EC 1.2.3.2) has been probed using temperature-jump perturbation and optical spectroscopic methods. Redox equilibria were found to be temperature-insensitive; hence it is argued that electron transfer is not accompanied by any extensive macromolecular conformational changes. No evidence for absorption phenomena ascribable to optical electron transfer could be found throughout the course of reductive titration of the biological particle. The combined results suggest that long-range electron transfer in the xanthine oxidase can best be described as occurring between only weakly interacting redox sites embedded in a rigid protein matrix.

Several thiosemicarbazone-metal complexes inhibit the RNA dependent DNA polymerase and the transforming ability of Rous sarcoma virus. Some complexes are equally as active as the free ligand whereas the activity of others is greatly enhanced. The 2-formyl pyridine thiosemicarbazone copper (II) complex is the most potent compound of this class that we tested. Some copper complexes of salicylaldehyde derivatives are very active also, particularly N-n-butyl, N-n-hexyl and N-benzylsalicylaldimine; no nickel complex of any salicylaldehyde compound is active. In addition, other metal ligands, such as dithizone, diacetyl bis (mercaptoethylimine), N-butyl thiocarbamate, 0,0′ dimethyl dithiophosphate, potassium dithiooxalate, and cis-Pt^II(NH₃)₂Cl₂ were tested with varying results.

A new tyrosine-containing sulfur-peptide, N-mercaptoacetyl-l-tyrosine (MAGT), was synthesized and its tyrosine effect on complex formation with Cu(II) investigated by electronic, circular dichroism (CD), fluorescence, and electron spin resonance (esr) spectra. The 1:1 MAGT-Cu(II) complex showed the following spectroscopic data: 545 nm (absorption maximum), 310 nm, 340 nm, and 580 nm (CD extrema), 280 nm and 308 nm (fluorescence peaks), and g_∥ = 2.17, g_⊥ = 2.05, A_∥ = 199.2 G, and A_N = 14.3 G (esr parameters). The tyrosine residue is near to but not directly bound to the Cu(II) in the MAGt complex. The present result has been discussed with respect to the role of tyrosine residues in blue copper proteins.

Trichlorodiethylenetriaminecobalt(III), [CoCl₃dien], which is provided with three good leaving ligands and, hence, capable of binding ATP in a characteristic mode, accelerated effectively and specifically hydrolysis of ATP to ADP and Pi. A kinetic study of the reaction indicated that the rate of hydrolysis was first order with respect to the concentration of ATP in the presence of an excess of [CoCl₃dien]. The rate constant was calculated to be 1.05 × 10⁻² min⁻¹ at pH 4.0 (50°C), corresponding to a catalysis of the hydrolysis of ATP by a factor of 150. The complex possessing one good

The binding of lasalocid A (X537A) to Pr(III) in methanol has been studied by lasalocid fluorescence, circular dichtoism, and ¹H and ¹³C NMR spectroscopy. It is clear that in addition to a mono complex, bis and tris complexes are also formed. Values of the binding constants and spectral properties of the various complexes have been determined by computer fitting of the binding isotherms. The Pr(III) ion binds only at the salicylic “head” of the lasalocid A, in stark contrast with other known structures. The lasalocid A appears to have an “open” conformation in these complexes. The relevance of these results to the structure and conformation of the Ca(II) complex is discussed. The first order rate constant for the dissociation of the tris complex has also been determined.

Apparent Mg²⁺ and H⁺ stability constants of 5-phosphoribosyl α-1-pyrophosphate (ligand, L) complexes were determined from pH titration data at 25°C with an average of 0.17 M NaCl or KCl and 0.20 M ionic strength. The logarithms of calculated macroscopic overall stability constants are: 3.2 (MgL^3-), 4.8 (Mg₂L^-), 6.5 (HL^4-), 12.4 (H₂L^3-), 9.4 (MgHL^2-), and 11.0 (MgH₂L). Comparison of the stepwise Mg²⁺ stability constants (log k = 3.2 and 1.6) with those of MgADP^- and MgAMP or Mg-hexose-1-P suggests that the first and second Mg²⁺ bind to the 1-PP and 5-P groups of the ligand, respectively. Reasonable assumptions about relative microscopic constants indicate that several of the microscopic isomers do not achieve significant concentrations over a large range of conditions. Judging from other data on organophosphate complexes, it is likely that the constants of this study may be extrapolated with little error to other conditions of ionic strength 0.1–0.2 M) and temperature (e.g., 15–35°C), and widely different monovalent ion concentrations.

The substitution reactions of tetrakis-(4-N-methylpyridyl)porphinecobalt (III) (Co^IIITMpyP) with azide and with 1,1,3,3-tetramethyl-2-thiourea (TMTU) have been studied as a function of pH at 25° and an ionic strength of 0.5 M. The mechanistic pathway proposed for thiocyanate [1] and pyridine [2] is applicable to these ligands as well once allowance is made for two attacking forms of azide, N₃^— and HN₃. A TMTU axial substituent has about the same influence on the rate of further ligand substitution as does SCN^— and a much larger influence than does azide. Similar behavior between bound SCN^— and bound TMTU is also shown in electron-transfer reactions with Ru(NH₃)₆²⁺ - Whereas both sulfur-containing ligands enhance the rate relative to the diaquo complex, the azide complex undergoes reduction an order of magnitude more slowly than does the diaquo complex.

The magnetic circular dichroism spectra were observed for various iron-sulfur complexes with and without inorganic sulfur as models for rubredoxin and 2-Fe ferredoxin. The MCD band shapes ascribed the bands around 390 and 490 nm to Faraday A terms for mononuclear iron sulfur complexes. These bands are probably assigned to the charge-transfer transitions from the thiol sulfur orbital to the iron t₂ and e 3d-orbitals, respectively. The MCD magnitudes decreased by more than one-half for binuclear iron-sulfur complexes with inorganic sulfur in comparison with those for the mononuclear complexes. The low MCD magnitude as well as the possible core symmetry as low as D_2d attributed the MCD bands to Faraday B terms. Incorporation of inorganic sulfur produced new MCD bands, some of which can be assigned to the charge-transfer transitions from the inorganic sulfur orbital to the iron t₂ and e 3d-orbitals. Among complexes studied here, the bis(o-xylyldithiolato) ferrate(III) monoanion gave the MCD spectrum which resembles that of a rubredoxin. This implies that the MCD spectroscopy also assessed the complex as a good rubredoxin model. However, the binuclear complex bis[o-xylyldithiolato-μ₂-sulfidoferrate(III)] dianion failed to offer the MCD spectrum similar to that of the spinach ferredoxin.