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Several molecular mechanisms for cleavage of the oxalate carbon-carbon bond by manganese-dependent oxalate decarboxylase have recently been proposed involving high oxidation states of manganese. We have examined the oxalate decarboxylase from Bacillus subtilis by electron paramagnetic resonance in perpendicular and parallel polarization configurations to test for the presence of such species in the resting state and during enzymatic turnover. Simulation and the position of the half-field Mn(II) line suggest a nearly octahedral metal geometry in the resting state. No spectroscopic signature for Mn(III) or Mn(IV) is seen in parallel mode EPR for samples frozen during turnover, consistent either with a large zero-field splitting in the oxidized metal center or undetectable levels of these putative high-valent intermediates in the steady state. A narrow, featureless g ‫؍‬ 2.0 species was also observed in perpendicular mode in the presence of substrate, enzyme, and dioxygen. Additional splittings in the signal envelope became apparent when spectra were taken at higher temperatures. Isotopic editing resulted in an altered line shape only when tyrosine residues of the enzyme were specifically deuterated. Spectral processing confirmed multiple splittings with isotopically neutral enzyme that collapsed to a single prominent splitting in the deuterated enzyme. These results are consistent with formation of an enzyme-based tyrosyl radical upon oxalate exposure. Modestly enhanced relaxation relative to abiological tyrosyl radicals was observed, but site-directed mutagenesis indicated that conserved tyrosine residues in the active site do not host the unpaired spin. Potential roles for manganese and a peripheral tyrosyl radical during steady-state turnover are discussed.The oxalate decarboxylase (OxDC, 1 or YvrK based on the corresponding open reading frame) from Bacillus subtilis has recently garnered attention because of its mechanistically intriguing reaction (1-4) (Scheme 1). Purified OxDC crystallizes as a hexamer of 43-kDa subunits, each of which is a member of the "bicupin" structural family (5), and contains two mononuclear manganese centers with amino acid ligands that are functionally similar to those of manganese superoxide dismutase (MnSOD) (6); namely three histidine residues and one carboxylic acid residue. Oxalate 13 C and 18 O isotope effects on V max /K m of YvrK suggest the presence of a slow, partially rate-determining step prior to the irreversible C-C bond cleavage (4), which was proposed to be a proton-coupled electron transfer.The requirement for and substoichiometric consumption of dioxygen in the YvrK catalytic process, involvement of open shell manganese (2), and chemical precedent for oxalate decarboxylation in the Kolbe electrolysis (7,8) and the BorodinHunsdiecker reaction (9 -11) have led to various mechanistic proposals involving radical intermediates, and enzymic manganese complexes with formal oxidation states of either Mn(III) (2, 4) or Mn(IV) (3). A previously published electron paramagneti...

Formylbiliverdin and related chlorophyll-derived molecules are possible products of heme catabolism and other biologically important oxidative processes and are likely to be initially formed as metal complexes. To explore the properties of the formylbiliverdin moiety bound to transition metal ions, complexes of octaethylformylbiliverdin (H2OEFB) with Cu(II), Ni(II), and Co(II) have been prepared, since attempts to prepare an iron complex have produced only an unstable material. Transmetalation of MgII(OEFB), made by photooxidation of MgII(octaethyl-porphyrin), with a metal(II) acetate yields the low-spin complexes:  CuII(OEFB), NiII(OEFB), and CoII(OEFB). Single-crystal X-ray diffraction of CuII(OEFB) reveals that it consists of a four-coordinate copper(II) center which is bound to the four nitrogen atoms in distorted planar coordination. The tetrapyrrole ligand has a helical geometry. The structure of the solid is complicated by the existence of three molecules in the asymmetric unit and C−H···O hydrogen bonding between pairs of these in the tab/slot arrangement seen in complexes of octaethylbiliverdin. Both CuII(OEFB) and CoII(OEFB) can be converted to the verdoheme analogues, [CuII(OEOP)]+ and [CoII(OEOP)]+, where OEOP is the anion of octaethyl-5-oxaporphyrin, by the addition of hydrogen peroxide. Additionally, [CuII(OEOP)]+ can be produced by heating a toluene solution CuII(OEFB) in the presence of trifluoroacetic acid under dioxygen. Carbon monoxide is produced when CuII(OEFB) is converted to [CuII(OEOP)]+ by either method. [CuII(OEOP)](PF6) has been characterized by single-crystal X-ray diffraction which shows that the cation has a planar, porphyrin-like structure. The room-temperature EPR spectrum of this complex shows that the copper is four-coordinate with four nitrogen based ligands, but frozen solutions of [CuII(OEOP)]+ show a triplet EPR spectrum indicative of a dimeric species much like that in the X-ray crystal structure. The 1H NMR spectrum of diamagnetic NiII(OEFB) has been shown to be consistent with the helical structure through the use of lanthanide and chiral lanthanide shift reagents. The EPR spectra of CoII(OEFB) show that it forms a low-spin adduct with pyridine and that this adduct acts as a reversible dioxygen carrier. The geometric and electronic structural properties of these complexes of formylbiliverdin are compared to those of analogous compounds of biliverdin and of porphyrins.

The triplet state properties of 7-azatryptophan (7AW), 5-hydroxytryptophan (5HW), and the 5HW anion (pH ) 12.3) have been investigated by optically detected magnetic resonance, ODMR, in zero applied field. Also the triplet state properties of tryptophan (Trp) have been reexamined. Analysis of "slow-passage" ODMR responses during phosphorescence decay (delayed ODMR) that contain the effects of fast passage is introduced using Marquardt-Levenberg nonlinear least-squares fitting. The parameters determined by the analysis of delayed ODMR are the band center frequency (ν 0 ) and bandwidth (ν 1/2 ) of the inhomogeneously broadened ODMR band, the apparent decay constants of the triplet sublevels, and their relative radiative rate constants. The values of ν 0 and ν 1/2 are obtained with greater accuracy than is possible with analysis that does not account for fast passage effects on the band shape. The kinetic parameters are influenced somewhat by residual spin-lattice relaxation (SLR) effects, however. The zero-field magnetic resonance frequencies and band widths have been determined for Trp, 7AW, 5HW, and 5HW anion, along with the apparent kinetic parameters for Trp and 7AW. Microwave-induced delayed phosphorescence transient responses of 5HW and its anion are analyzed globally to give kinetic parameters free of residual effects. The ODMR band widths of 5HW and its anion greatly exceed those of Trp, while those of 7AW are significantly narrower. The zero-field-splitting (ZFS) parameter E is reduced in 5HW and its anion to about half its value in Trp, while the E-parameter of 7AW increases by about 10%. Sublevel decay constants relative to Trp are hardly changed in 5HW, but increase significantly in the anion. Decay constants of Trp increase moderately with 7-aza substitution, as expected from introduction of spin-orbit coupling at the N atom. The pattern of ODMR bandwidths of Trp and 7AW is consistent with a positive correlation of solvent induced shifts in the ZFS Dand E-parameters.

The binding of p7 nucleocapsid protein of type 1 human immunodeficiency virus (HIV-1) to various oligonucleotides and polynucleotides has been investigated by phosphorescence and optically detected magnetic resonance (ODMR) spectroscopy. The intrinsic spectroscopic probe used in these studies is the photoexcited triplet state of Trp37, which is associated with the C-terminal zinc finger of p7 and is its only tryptophan residue. Complex formation produces a red-shift of the phosphorescence 0, 0-band (DeltaE0,0) of Trp37 as well as a reduction of the zero field splitting (zfs) D parameter. Increases of -DeltaE0,0 (A < C < U < G

Two crystalline forms of bis(2,2,-bi-2-thiazoline)bis(thiocyanato)iron(II), Fe(C6H8N2S2)2(NCS)2, were obtained. While one of them (polymorph A) shows the high-spin-low-spin transitions with a hysteresis over the temperature range ca. 170-180 K, in agreement with the literature data, the other one (polymorph B) is paramagnetic over the temperature range 77-300 K. The X-ray data are (data in parentheses refer to the polymorph B): crystal system triclinic (triclinic), space group Pi (Pi), a = 8.521 (8) A (10.846 (3) A), b = 11.063 (9) A (10.847 (3) k),c = 12.498 (6) A (12.526 (4) A), a = 96.83 (4)°(115.72 (2)°), ß = 91.81 (6)°(93.92 (2)°), 7 = 106.24 (6)°(119.97 (2)°), V= 1120 (1) A3 (1070 (1) A3), Z = 2 (2), and R = 0.0481 (0.0490). The molecules building up both crystal lattices have the cis configuration and are very similar. The single crystals of both kinds, doped with Mn(II), were investigated by electron paramagnetic resonance spectroscopy. The magnitudes of the zero-field-splitting parameters for the Mn(II) ion and the orientation of the ZFS tensor found both above and below the "crossover" temperature suggest that the spin transition does not cause major changes of the molecular symmetry. A considerable slowing down of the spin transition rate in the large single crystals with respect to the powder samples was detected by EPR. The line widths observed for the low-spin resonance observed during the spin transition in for this transition by earlier studies on powders.

The condensation of two molecules of 3,s-di-tert-butylcatechol (dbc) with ammonia under oxidizing conditions is believed to proceed via a keto-imine radical. Further condensation, and subsequent reaction with tin(I1) or lead(I1) halides, yields complexes of identical composition, M [dbqdi],, where dbqdi is 3,5-di-tertbutyl-1,2-quinone-1-(2-hydroxy-3,5-di-tert-butylphenyl)imine anion; this name is used without prejudice as to the actual structure of the ligand. The two compounds differ in crystallographic and magnetic properties, and ESR measurements show that the tin compound has a triplet ground state. Both involve six-coordination of the metal in a distorted environment; tin is in the +IV state, but lead is +II, and there are corresponding differences in the form of the ligand. For snIv(dbqdi),, the crystal parameters are orthorhombic, space group C222,, a = 19.597(4), b = 24.375(4), c = 23.755(6) A, Z = 8, and R = 0.073 for 1405 unique reflections. For pbU(dbqdi),, monoclinic, space group P2,ln, a = 14.56(1), b = 25.967(9), c = 15.13(1) A, P = 99.81(7)', Z = 4, R = 0.065 for 491 1 unique reflections. The different complexation behaviour is shown to be a consequence of the redox properties of the two metallic elements.Key words: tin, lead, ESR, coordination chemistry, electron transfer.Resume : On croit gtntralement que la condensation de deux moltcules de 3,s-di-tert-butylcattchol (dbc) avec de I'ammoniac, dans des conditions oxydantes, se produit par le biais d'un radical ctto-imine. Une condensation substquente, avec rtaction ulterieure avec des halogenures d1Ctain(II) ou de plomb(II), fournit des complexes de composition identique, M(dbqdi),, dans lesquels dbqdi est l'anion 3,s-di-tert-butyl-1.2-quinone-1-(2-hydroxy-3,s-di-tert-butylphtny1)imine; on utilise ce nom sans tgard B la structure rtelle du coordinat. Les proprietts cristallographiques et magnetiques des deux composts sont differentes et les mesures de RPE indiquent que le derive de l'ttain possbde une ttat fondamental triplet. Les deux impliquent un niveau de coordination de six pour le metal, avec un environnement dtformt; l'ttain est dans un &tat +IV alors que pour le plomb il est de +I1 et on peut observer des differences correspondantes dans le forme du coordinat. Pour le snIv(dbqdi),, les cristaux sont orthorhombiques, groupe d'espace C222,, avec a = 19,597(4), b = 24,375(4) et c The reactions of substituted orthobenzoquinones with main group elements, or the low oxidation state derivatives of such elements, have been the subject of a number of recent papers from this laboratory. In general, the products of these processes are the appropriate catecholate or semiquinone derivatives, which have been characterized by spectroscopic and crystallographic methods. An important feature of these results is that the redox reactions in question have been shown to be one-electron transfer processes (1).We have now extended this research to an investigation of the structure of the compounds formed when tin or lead in the +I1 state react with a ...

EPR and NMR Study of the Spin-Crossover Transition in Fe(4,4/-bi-l,2,4-triazole)2(SCN)2'H20 and Fe(4,4/-bi-l,2,4-triazole)2(SeCN)2.H20. X-ray Structure Determination of Fe(4,4/-bi-l,2,4-triazole)2(SeCN)2*H20