Using high-frequency EPR spectroscopy we have found that a cluster comprising eight iron(III) ions, Fe8, which is essentially flat, has a ground S = 10 state and an Ising-type anisotropy. For the first time both ac susceptibility and Mössbauer spectroscopy could be used in order to monitor the relaxation time of the magnetization, which was found to follow a thermally activated behavior, as in a superparamagnet, with τ 0 = 1.9 × 10 −7 s and an energy barrier of 22.2 K. The set of data allowed us to conclude that the origin of the anisotropy in nanosize molecular clusters is associated with the single ion contributions and not with the shape of the clusters.

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The synthesis and characterization of isocyanide complexes of (porphyrinato)iron(III) species, [(Porph)-Fe(t-BuNC) 2 ]ClO 4 , Porph ) OEP, TPP, are reported. The crystal structures of [(TPP)Fe(t-BuNC) 2 ]ClO 4 and [(OEP)-Fe(t-BuNC) 2 ]ClO 4 have been determined. Consistent with the expected effect from the strong π-acceptor character of the axial tert-butyl isocyanide ligands, the X-ray structure of the complex shows that the porphyrinate ring is strongly ruffled. The spectroscopic properties of these complexes suggest the possibility of "blurring" of the definitions of the electron configurations of low-spin Fe(III) macrocycles having (d xy ) 1 electronic ground states, with the extreme possibilities being low-spin Fe(III)-(macrocycle) 2-, with the unpaired electron localized in the d xy orbital of the metal, and low-spin Fe(II)-(macrocycle) 1-• , with the unpaired electron localized on the macrocycle. EPR spectroscopy of the TPP and OEP complexes shows that the g-values (g ⊥ ) 2.20-2.28, g | ) 1.94-1.83) are consistent with an electron configuration that is (d xz ,d yz ) 4 (d xy ) 1 , the purest (d xy ) 1 ground state system with the most complete quenching of orbital angular momentum discovered thus far (∑g 2 as small as 13.5). Proton NMR spectra of [OEPFe(t-BuNC) 2 ]-ClO 4 in CD 2 Cl 2 , recorded over the temperature range -100 to +37°C, also support the (d xy ) 1 ground state, where ruffling of the porphyrinate ring makes it possible for unpaired electron spin delocalization to the 3a 2u (π) orbital of the porphyrinate ring. This orbital has very large electron density coefficients at the meso positions and hence explains the very large negative contact shift of the meso-H; its size indicates considerable (∼19%) spin delocalization from low-spin Fe(III) to the 3a 2u (π) orbital by porphyrin f Fe π donation. Mössbauer and IR spectral data are also consistent with the (d xy ) 1 ground state.

The green primary compound of chloroperoxidase was prepared by freeze-quenching the enzyme after rapid mixing with a 5-fold excess of peracetic acid. The electron paramagnetic resonance (EPR) spectra of these preparations consisted of at least three distinct signals that could be assigned to native enzyme, a free radical, and the green compound I as reported earlier. The absorption spectrum of compound I was obtained through subtraction of EPR signals measured under passage conditions. The signal is well approximated by an effective spin Seff = 1/2 model with g = 1.64, 1.73, 2.00 and a highly anisotropic line width. Mössbauer difference spectra of compound I samples minus native enzyme showed well-resolved magnetic splitting at 4.2 K, an isomer shift delta Fe = 0.15 mm/s, and quadrupole splitting delta EQ = 1.02 mm/s. All data are consistent with the model of an exchange-coupled spin S = 1 ferryl iron and a spin S' = 1/2 porphyrin radical. As a result of the large zero field splitting, D, of the ferryl iron and of intermediate antiferromagnetic exchange, S.J.S'.J approximately 1.02 D, the system consists of three Kramers doublets that are widely separated in energy. The model relates the EPR and Mössbauer spectra of the ground doublet to the intrinsic parameters of the ferryl iron, D/k = 52 K, E/D congruent to 0.035, and A perpendicular (gn beta n) = 20 T, and the porphyrin radical.(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative interpretation is presented for EPR spectra from integer-spin metal centers having large zero-field splittings. Integer-spin, or non-Kramers, centers are common in metalloproteins and many give EPR signals, but a quantitative understanding has been lacking until now. Heterogeneity of the metal's local environment will result in a significant spread in zero-field splittings and in broadened EPR signals. Using the spin Hamiltonian Hs = S.D.S + beta S.g.B and some simple assumptions about the nature of the zero-field parameter distributions, a lineshape model was devised which allows accurate simulation of single crystal and frozen solution spectra. The model was tested on single crystals of magnetically dilute ferrous fluosilicate. Data and analyses from proteins and active-site models are presented with the microwave field B1 either parallel or perpendicular to B. Quantitative agreement of observed and predicted signal intensities is found for the two B1 orientations. Methods of spin quantitation are given and are shown to predict an unknown concentration relative to a standard with known concentration. The fact that the standard may be either a non-Kramers or a Kramers center is further proof of the model's validity. The magnitude of the splitting in zero magnetic field is of critical importance; it affects not only the chance of signal observation, but also the quantitation accuracy. Experiments taken at microwave frequencies of 9 and 35 GHz demonstrate the need for high-frequency data as only a fraction of the molecules give signals at 9 GHz.

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We report Mössbauer and EPR measurements on horseradish peroxidase in the native state and the reaction intermediates with peroxide and chlorite. A detailed analysis of the electronic state of the heme iron is given, and comparisons are drawn with related systems. The native enzyme is high-spin ferric and thus has three Kramers doublets. The unusual magnetic properties of the ground doublet and the large energy of the second, (E2-E1)/k approximately equal to 41 K, and third doublet, (E3-E1)/k greater than or equal to 170 K, can be modeled with a quartet admixture of approximately 11% to the spin sextet. All evidence suggests a ferryl, OFeIV, state of the heme iron in compounds I and II and related complexes. The small isomer shift, delta Fe approximately equal to 0.06 mm/s, the (positive) quadrupole splitting, delta EQ approximately equal to 1.4 mm/s, the spin S = 1, and the large positive zero field splitting, D/k approximately equal to 35 K, are all characteristic of the ferryl state. In the green compound I the iron weakly couples to a porphyrin radical with spin S' = 1/2. A phenomenological model with a weak exchange interaction S . J . S', magnitude of less than or equal to 0.1 D, reproduces all Mössbauer and EPR data of compound I, but the structural origin of the exchange and its apparent distribution require further study. Reaction of horseradish peroxidase with chlorite leads to compound X with delta Fe = 0.07 mm/s and delta EQ = 1.53 mm/s, values that are closest to those of compound II. The diamagnetism of compound III and its Mössbauer parameters delta Fe = 0.23 mm/s and delta EQ = -2.31 mm/s at 4.2 K clearly identify it as an oxyheme adduct.

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