The proton NMR spectra of unligated ferrous porphyrins, proposed to exist in the intermediate S = 1 spin state, have been recorded and analyzed. The dominant dipolar shifts for the phenyl protons in tetraphenylporphinatoiron(I1) result in a quantitative measure of the axial magnetic anisotropy, which, together with the average moment, yield p l = 4.9 pcg and pi 1 = 3.2 peg. The resultant contact shifts for pyrrole substituents reflect extensive a spin transfer, which originates solely from porphyriniron change transfer. The contact shift pattern indicates that dxzyz is unoccupied, confirming the spin-triplet ground state. The large a contact shifts require unpaired spins in dxr,dyr, and strongly support the ground state configuration, (d,)2(dzz)2(dxz,dyz)2r which has been previously shown to be the one favored by the average moment and Massbauer data.

k = 1.62 X 10-2 sec-1 at 25", log A = 9.51 =t 0.79, and E, = 15.4 & 1.0 kcal/mol. 6+ Fe is Fe02-and all oxygens are kinetically equivalent. celerated by dilute acid. HzO, HzOZ, and SO?-, and the results are compared with other strong oxidizing agents. and S03*-are quantitative and show simple stoichiometry. in S032-, but oxygen transfer from Fe042-to the product (02 or S042-) occurs only in the latter case.In basic media the rate of 1 8 0 exchange between Fe042-and water follows the law R = k[FeO4*-1, whereIn aqueous media the ion of Both water exchange and water oxidation are ac-Stoichiometric, kinetic, and 1 8 0 studies have been applied to the reaction of Fe02-withThe reaction with HZOZ The reactions are first order in Hz02 and second order ethods for preparing and analyzing potassium M ferrate, K2Fe04, of high purity (97-9973 have existed for some time, but little is known about its aqueous chemistry. K2Fe04(s) is usually prepared by strongly basic media and isolated from saturated KOH ~o l u t i o n .~-~ Electrolytic means may also be used.5Analysis consists of the oxidation of Cr(lI1) in basic media followed by titration of the Cr(V1) formed with Oc1-oxidation of freshly prepared iron hydroxide in (2) H.

1find Q, = -7.6 X hartree, Qy = -2.0 X lo4 hartree, and Q, = 2.7 X hartree. Then eq, = -0.44 au, eq, = -1.1 au, and eq, = 1.6 au. For 63Cu, Q, = -7.1 X hartree, Qy = -2.2 X lo4 hartree, Q, = 2.9 X lo4 hartree, eq, = -0.38 au, eq, = -1.2 au, and eq, = 1.6 au. This compound has nuclear quadrupole coupling constants consistent with the few other 63Cu(II) oxygen-coordinated compounds of this high symmetry, mostly octahedral, that have been examined (8)(9)(10) and 14 and Table 111). This system is a slightly elongated octahedron. Its QD value is at the low end of the range of values previously found for octahedral enviroiments but above values found for more elongated octahedra.14 The inclusion of the Q E parameter here could account for the observed reduction in the QD value. The large QD (or eq,) value indicates an elongated distribution of charge, mainly attributable to the d, , orbital vacancy in the Cu(I1) valence shell, with some reduction-perhaps 25%-owing to covalent charge ~p r e a d i n g .~J~J~.~~ Quantitative discussions of the relationship between QD, QE, and electronic structure are given elsewhere (e.g., ref 5 , 9, 10, and 23, and work to be published). The Q E value indicates some distortion from a completely axial charge distribution. The moderate rhombic component of that distribution (eq, -eq, = 0.8 au) must be related to a slight difference between two pairs of equatorial metal-oxygen bond distances revealed by the structural report on the host ~rysta1.l~ Finally, the utility of EPR powder spectra to determine nuclear quadrupole coupling constants has been confirmed in this study. Our predictions as well as the values initially derived from computer simulations of powder spectra were borne out by the subsequent detailed single-crystal analyses. The close agreement between the sets of EPR parameters obtained by the two methods demonstrates the adequacy of powder-spectrum simulation for the determination of nuclear quadrupole coupling constants. Acknowledgment.A variety of synthetic iron(II1) porphyrin complexes (FeII'PorX, X = S03CF3-, C104-, and C(CN),-) were examined with multinuclear NMR spectroscopy ('H, 13C, I9F, and j5Cl). Deviations from NMR Curie law behavior, diminished magnetic moments, and characteristic ESR g = 4 values support previous evidence for the quantum-mechanical admixture of S = 5/2 and S = 3/2 states. NMR studies of titrations with the corresponding tetrabutylammonium salts and dipolar shift calculations show the ligands are coordinated rather than ion paired in solution. Solvent studies indicate more S = 5/2 character is present in aromatic solvents than in chlorinated solvents. Although the tricyanomethanide complex is thought to exhibit a "pure" S = 3/2 state in crystalline form, solution measurements are consistent with spin admixture.

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The mammalian heme peroxidases are distinguished from their plant and fungal counterparts by the fact that the heme group is covalently bound to the protein through ester links from glutamate and aspartate residues to the heme 1- and 5-methyl groups and, in the case of myeloperoxidase, through an additional sulfonium link from the Cbeta of the 2-vinyl group to a methionine residue. To duplicate the sulfonium link in myeloperoxidase and to obtain information on its mechanism of formation, we have engineered a methionine residue close to the 2-vinyl group in recombinant pea cytosolic ascorbate peroxidase (rpAPX) by replacement of Ser160 by Met (S160M variant). The S160M variant is isolated from Escherichia coli as apo-protein. Reconstitution of apo-S160M with exogenous heme gives a red protein (S160M(R)) which has UV-visible (lambda(max)/nm = 407, 511, 633) and steady-state kinetic (kcat = 156 +/- 7 s(-1), KM = 102 +/- 15 microM) properties that are analogous to those of rpAPX. The reaction of S160M(R) with H2O2 gives a green protein (S160M(G)). Electronic spectroscopy, mass spectrometry, and HPLC analyses are consistent with the formation of a covalent linkage between the methionine residue and the heme vinyl group in S160M(G). Single-wavelength and photodiode array stopped-flow kinetic analyses identify a transient Compound I species as a reaction intermediate. The results provide the first direct evidence that covalent heme linkage formation occurs as an H2O2-dependent process that involves Compound I formation. A mechanism that is consistent with the data is presented.

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