Recovery of Cr(III) from Tannery Spent Chrome Liquor for Reuse

This paper embodies details on the extraction behavior of Cr(III) along with Al(III), Fe(III), Mg(II), Mn(II), Co(II), Ni(II), and Cu(II) from hydrochloric acid media employing the Cyanex 301-toluene system. All of these metals, except Cr(III), Mg(II), and Mn(II), are extracted into the organic phase. This property of the extractant has been used to separate Cr(III) from the binary mixtures. The partition data have been extended onto spent chrome liquor, and this waste has been treated in such a manner so that it becomes suitable for use in trivalent plating baths. The hydrolytic stability and recycling capacity has been reported. Because the concentration of Cr(III) in the waste is much lower than that required for chromium depositions in Cr(III) plating baths, a concentration step using MgO as a precipitating agent has been appended. To summarize, this paper envisages a new approach to tannery waste management that focuses on treating spent chrome liquors using a solvent extraction technique in such a manner that the waste becomes suitable for use in trivalent plating baths. This would not only help abate pollution but also recover the metal in a pure form.     

The rare case of a intrapulmonary hamartochondrosarcoma (author''s transl)

The specific inhibitory effect of benzhydroxamic acid on the cyanide-insensitive respiration could be reversed in whole cells of the yeast Saccharomycopsis lipolytica, by addition of Fe(III), in a way suggesting a competition between the added iron and an enzyme-bound metallic ion, both central atoms for the ligand benzhydroxamic acid. The possibility that added metal ions modify the penetration of BHAM into the cells was ruled out. Co(II), Cu(II) and Al(III) could substitute for Fe(III). A linear relation between the concentration in added Fe(III) and the reversed respiration rate was observed. At a given cell concentration, the reversion by added Fe(III) of the inhibitory effect of benzhydroxamic acid on the alternative respiration appeared more related to the degree of inhibition rather than to the concentration in added inhibitor. Increasing cell concentrations required increasing amounts of Fe(III) to reach the same level of reversion. No reversal occurred at concentrations in added Fe(III) lower than 0.1 mM, whatever the benzhydroxamic concentration, the cell concentration or the yeast batch.     

The effect of impurities on the extraction of copper from sulfate medium using LIX®984N in kerosene

In this research, the effect of impurities such as Zn(II), Mn(II), Fe(III) and Fe(II) on the extraction of copper using LIX®984N in kerosene has been evaluated. Hence, a series of extraction tests have been conducted to determine the effects of each foreign ion on copper extraction. It was found that the extraction of copper from sulfate media by the exractant LIX®984N in kerosene is completely pH dependent to pHs around 2.18 vol % of LIX®984N was found to be optimum for the maximum extraction efficiency of copper and 93.9% extraction was achieved at the end of 600 seconds. The order of extraction of different ions in pHs around 2 by the extractant LIX®984N was found to be as: Cu(II) > F(III) > Mn(II) > Fe(II) > Zn(II). For pHs greater than 2.5, the order of extraction will be changed as Cu(II) > Fe(III) > Mn(II) > Zn(II) > Fe(II) and this order for the pHs ≥ 4.5 was as Cu(II) > Fe(III) > Zn(II) > Fe(II) > Mn(II) which offers a good separation of ions using LIX®984N.     

Oxygen equilibrium properties of chromium (III)-iron (II) hybrid hemoglobins

Cr(III)-Fe(II) hybrid hemoglobins, alpha 2(Cr) beta 2(Fe) and alpha 2(Fe) beta 2(Cr), in which hemes in either the alpha- or beta-subunits were substituted with chromium(III) protoporphyrin IX (Cr(III)(PPIX), were prepared and characterized by oxygen equilibrium measurements. Because Cr(III)PPIX binds neither oxygen molecules nor carbon monoxide, the oxygen equilibrium properties of Fe(II) subunits within these hybrids can be analyzed by a two-step oxygen equilibrium scheme. The oxygen equilibrium constants for both hybrids at the second oxygenation step agree with those for human adult hemoglobin at the last oxygenation step (at pH 6.5-8.4 with an without inositol hexaphosphate at 25 degrees C). The similarity between the effects of the Cr(III)PPIX and each subunits' oxygeme on the oxygen equilibrium properties of the counterpart Fe(II) subunits within hemoglobin indicate the utility of Cr(III)PPIX as a model for a permanently oxygenated heme within the hemoglobin molecule. We found that Cr(III)-Fe(II) hybrid hemoglobins have several advantages over cyanomet valency hybrid hemoglobins, which have been frequently used as a model system for partially oxygenated hemoglobins. In contrast to cyanomet heme, Cr(III)PPIX within hemoglobin is not subject to reduction with dithionite or enzymatic reduction systems. Therefore, we could obtain more accurate and reasonable oxygen equilibrium curves of Cr(III)-Fe(II) hybrids in the presence of an enzymatic reduction system, and we could obtain single crystals of deoxy-alpha 2(Cr) beta 2(Fe) when grown in low salt solution in the presence of polyethylene glycol 1000 and 50 mM dithionite.     

Zn-exchange and M?ssbauer studies on the [Fe-Fe] derivatives of the purple acid Fe(III)-Zn(II)-phosphatase from kidney beans

In order to perform M?ssbauer studies, Zn(II) in the Fe(III)-Zn(II) purple acid phosphatase of the red kidney bean has been exchanged by incubating the semiapoenzyme with 57Fe(II). The resulting Fe(III)-57Fe(II) enzyme has 125% activity, compared with that of the Zn(II) enzyme. It can be oxidized by H2O2 or peroxydisulfate to the Fe(III)-57Fe(III) species with a 30-times lower activity. Incubation of the metal-free apoenzyme with 57Fe(II) in the presence of O2 leads to the 57Fe(III)-57Fe(II) species which is stable in dilute solutions, but partially oxidized during the concentration procedure to the 57Fe(III)-57Fe(III) enzyme. Limited reduction of the oxidized enzyme with ascorbate delivers a mixture of the Fe(II)-Fe(II)/Fe(III)-Fe(III) species, but not the mixed valent Fe(III)-Fe(II) species, indicating that after the transfer of the first electron the second electron of the ascorbate radical is immediately transferred to the second Fe(III). The M?ssbauer spectra of the oxidized species show at 4.2 K two quadrupole doublets with delta of 0.51 mm/s and 0.53 mm/s and delta E of 1.46 and 0.96 mm/s indicating high spin Fe(III) in two different binding sites, obviously with a higher asymmetry in the chromophoric Fe(III) site. The values are too low for a mu-oxo bridge. The mixed-valent Fe(III)-Fe(II) species shows two quadrupole doublets with delta values of 0.55 mm/s and 1.14 mm/s and delta E values of 1.43 mm/s and 3.01 mm/s at 70 K for high spin Fe(II) and Fe(III), but the signal of the Fe(II) component shows magnetic patterns at 4.2 K indicating a half-integer spin system with antiferromagnetic coupling. The Fe(II)-Fe(II) system exhibits two quadrupole doublets with delta values of 1.18 mm/s and 1.22 mm/s and with delta E values of 3.69 mm/s and 2.68 mm/s again indicating a higher asymmetry in the originally chromophoric Fe(III)-binding site. Addition of phosphate shows only minor differences in the oxidized enzyme and in the mixed valent Fe(III)-Fe(II) system. Interaction with O2 is discussed.     

Resonance Raman spectra of iron(III)-, copper(II)-, cobalt(III)-, and manganese(III)-transferrins and of bis(2,4,6-trichlorophenolato)diimidazolecopper(II) monohydrate, a possible model for copper(II) binding to transferrins

Fe(III), Cu(II), Co(III), and Mn(III) complexes of ovo- and human serum transferrins show resonance enhanced Raman bands near 1600, 1500, 1270, and 1170 cm-1 upon excitation with laser frequencies which fall within the visible absorption bands of those metalloproteins. Comparison of the visible absorption and resonance Raman spectra of the Cu(II)-transferrin complexes with those for the Cu(II) model compound, bis(2,4,6-trichlorophenolato)diimidazolecopper(II) monohydrate, indicates that the resonance Raman bands are due to enhancement of phenolic vibrational modes. For the model (Cu(II) compound, a normal coordinate analysis was used to aid our assignment of the observed resonance bands at 1562, 1463, 1311, and 1122 cm-1 to A1 vibrational modes of the 2,4,6-trichlorophenolato moiety. These assignments are consistent with those made for Cu(II)-transferrins. The latter assignments were based upon calculated A1 frequencies for p-methylphenol (Cummings, D.L., and Wood, J.L. (1974), J. Mol. Struct. 20, 1). The wavelength shifts in the resonance bands for the model compound from those for Cu(II)-transferrins are due to the influence of the chloro substituents on the planar vibrations of phenol. These results clearly identify tyrosine as a ligand in copper binding to transferrins.     

The effect of Fe(II) and Fe(III) on the efficiency of copper electrowinning from dilute acid Cu(II) sulphate solutions with the chemelec cell: Part I. Cathodic and anodic polarisation studies

The results are presented from an investigation into the effect of the Fe(III) and Fe(II) concentration on the current efficiency for copper electrowinning using the Chemelec cell. The results are from polarisation studies of acidic sulphate electrolytes containing Cu(II), Fe(II) and Fe(III). The mass transfer coefficients for these cationic species are determined, and the effect of using different anode materials on the overvoltage for the oxidation of Fe(II) and oxygen evolution is also examined.     

厌氧环境下邻菲罗啉分光光度法测定纤维水镁石中Fe(II)与Fe(III)

纤维水镁石中同时存在有不同价态的Fe即Fe(II)与Fe(III),这对开展水镁石相关研究具有重要意义。而不同价态Fe含量测定过程中存在Fe(II)在空气中易氧化成Fe(III)而导致测定不准确的问题。针对该问题,实验采用厌氧培养箱作为操作环境,对纤维水镁石的处理均在厌氧条件下进行。利用邻菲罗啉与经盐酸羟胺还原产生的总Fe(II)显色反应原理测定纤维水镁石中的总Fe,之后测定过程中不加盐酸羟胺与邻菲罗啉反应测定溶液中的Fe(II),利用总Fe与Fe(II)的差值测定Fe(III),从而实现了邻菲罗啉分光光度法对纤维水镁石中Fe(II)与Fe(III) 的准确测定。试验发现在波长为510nm,显色体系pH值为2~5,显色时间为10min条件下,Fe(II)质量浓度在0.02~5.0mg/L范围内与其吸光度符合比尔定律,校准曲线的相关系数为0.9999。采用实验方法对两个纤维水镁石样品分别进行6次平行测定,结果显示总Fe、Fe(II)及Fe(III)的相对标准偏差(RSD,n=6)均小于1%。采用电感耦合等离子体原子发射光谱法(ICP-AES)进行全Fe测定结果对照,结果表明两种方法的测定结果保持一致。     

Heme binding by a bacterial repressor protein, the gene product of the ferric uptake regulation (fur) gene of Escherichia coli

The fur gene product, Fur, of Escherichia coli is a repressor when it binds Fe(II). Since heme and iron metabolism are closely linked and Fur is rich in histidine, a ligand for heme, the binding of heme to Fur was investigated. The oxidized Fur-heme complex is stable and low spin with a Soret maximum at 404 nm and no 620-nm band. CO coordinates with the reduced heme-Fur complex, causing a shift from 412 nm to 410 nm, and stabilizes it, increasing the half-life from 5 to 15 min. Circular dichroism (CD) spectra in the Soret region show heme bound in an asymmetric environment in Fur, both in the oxidized and reduced-CO forms. Quenching of tyrosine fluorescence by heme revealed rapid, tight binding (Kd < 1 microM) with an unusual stoichiometry of 1 heme:1 Fur dimer. Fur binds Mn(II), a model ligand for the endogenous Fe(II), much more weakly (Kd > 80 microM). Far-ultraviolet CD spectroscopy showed that the alpha-helix content of apo-Fur decreases slightly with heme binding, but increases with Mn(II) binding. Competition experiments indicated that heme interacts with Fur dimers at the same site as Mn(II) and can displace the metal. In contrast to Mn(II), Zn(II) did not quench the tyrosine fluoroescence of Fur, affected the CD spectrum less than Mn(II), but did bind in a manner which prevented heme from binding. In sum, Fur not only binds heme and Zn(II) with sufficient affinity to be biologically relevant, but the interactions that occur between these ligands and their effects on Mn(II) binding need to be taken into account when addressing the biological function of Fur.     

Effect of manganese(II) and boric acid on the electrowinning of cobalt from acidic sulfate solutions

The effect of Mn(II) and boric acid on the electrowinning of cobalt from acidic sulfate electrolyte was investigated by electrochemical methods. The results indicated that the presence of Mn(II) or boric acid alone increased the current efficiency by 2 to 3 pct but that their combination had no additional effect. The preferred crystal orientation pattern changed from (110) (100) (101) to (100) (110) (101) in the presence of 0.1 to 10 g dm⁻³ Mn(II); an additional change, to (110) as the only preferred orientation, occurred at a very high concentration of Mn(II) (100 g dm⁻³). Boric acid had no effect on the crystal pattern of the cobalt deposits. In general, the presence of boric acid increased the brightness of all the deposits, irrespective of the presence of Mn(II) in the electrolyte, and had no effect on the deposit morphology. The cathode polarization behavior of the cobalt electrocrystallization was affected only marginally by the presence of Mn(II) in the electrolyte, but the presence of boric acid with or without Mn(II) had a significant effect. Boric acid was found to suppress the rate of electroreduction of cobalt on stainless steel by an order of magnitude, irrespective of the presence of Mn(II).     

Removal of iron and formation of copper(i) from solutions containing iron(ii) and copper(ii) by addition of chloride ion or acetonitrile

Solutions of Cu(II) and Fe(II) establish the redox equilibrium

$\text{Cu(II) + Fe(II)}\text{?}\text{K}\text{Cu(I) + Fe(III)}$

which is displaced to the right by addition of either Cl^? or acetonitrile (AN). Log K varies from ?10.5 in water to about ?2.5 in 4 M NaCl or AN, allowing iron to be removed selectively from copper (II) solutions either by solvent extraction with Versatic acid or by precipitation as goethite or j jarosite. To establish the required conditions Eh-pH diagrams have been developed for the CuH₂OCl and CuH₂OANSO₄²-systems at 25°C and 90°C. It is demonstrated that the catalytic effect of Cu(II) on the oxidation of Fe(II) to Fe(III) by O₂ is dependent on the concentration of Cl^? or AN and on the position of this redox equilibrium. Applications to removing iron from hydrometallurgical solutions are discussed and tested.     

The dioxygenation rate in lipoxygenase catalysis is determined by the amount of iron (III) lipoxygenase in solution

The dioxygenation rate in reactions catalyzed by lipoxygenase-1 from soybeans has been measured as a function of the enzyme present in the Fe(III) form with rapid kinetic techniques. The experiments were carried out at pH 10, 25 degree C. The product concentration and the fraction of iron (III) lipoxygenase were monitored by measuring the absorbance at 243 nm and the tryptophan fluorescence at 330 nm (excitation at 287 nm), respectively. In reactions started with 1.3 microM iron (II) lipoxygenase and 9 microM linoleate, the initial rate, r(init) (estimated from the increase in absorbance over the initial 0.02 s of the reaction), is very small (4 s-1). In contrast, when the reactions are started with 1.3 microM (III) lipoxygenase, r(init) is large (150 s-1). In reactions started with mixtures of iron(II) and iron(III) lipoxygenase, r(init) is linearly related to the initial concentration of the Fe (III) enzyme form. Redistributions of the Fe(II) and Fe(III) enzyme forms during the reaction with 12 nM enzyme and 10, 50, or 100 microM linoleate appear to be directly reflected in changes in the dioxygenation rate. The observations provide solid evidence for the hypothesis that only iron (III) lipoxygenase can catalyze the hydrogen abstraction step in the dioxygenation reaction, and thus can be regarded as the active enzyme species. The observed dynamics are accurately predicted by a nonallosteric, two-step model for lipoxygenase catalysis [Schilstra et al. (1992) Biochemistry 31, 7692-7699].     

As(III), As(V), Hg, and Pb Removal by Fe-Oxide Impregnated Activated Carbon

The removal of As(III), As(V), Hg(II), and Pb(II) by virgin and Fe(III) impregnated activated carbons (FeAC) was investigated. Iron-oxide impregnation increased the pHzpc of the carbon from 7.5 to about 8.2–8.7 but did not alter the surface area or the pore volume. The amount of acid or base needed to reach a given pH increased following impregnation, indicating that the FeAC was more active than its non-impregnated counterpart. Metal removal was a function of pH with removal increasing with pH for Hg(II) and Pb(II) and decreasing with pH for As(V). As(III) removal was not a strong function of pH below pH = 5, increased to a maximum at pH ≈ 7, then decreased with increasing pH. As(III) and As(V) removals were about one and two orders of magnitude higher, respectively, for the FeAC compared with the non-impregnated carbon, while Hg(II) and Pb(II) removals were only slightly higher. Fe-impregnation appears to be most effective for the anionic contaminants.     

磷酸盐沉淀法除铬热力学研究

除铬是含铬电镀污泥湿法冶金过程重要步骤.针对磷酸盐沉淀法从溶液中净化除铬过程进行热力学分析,绘制了25 ℃时Me-P-H₂O(Me: Cr(III), Zn(II), Cu(II), Fe(II), Fe(III), Ni(II))系组浓度对数-pH图,利用热力学平衡图对磷酸盐沉淀法从含铁等金属元素中净化除铁和磷酸铬碱分解过程进行热力学分析.结果表明,pH值为1.0 ~ 5.0磷酸盐形成由易至难依次为Cr(III)>Fe(III) >Fe(II)>Ni(II)>Cu(II)>Zn(II); 磷酸盐沉淀法难以有效地将Cr(III)与Fe(III)分离,而可分离Cr(III)和Fe(II),且较优pH约为2;整个pH值范围Me-P-H₂O系可以分为难溶磷酸盐稳定区、Me(OH)_n稳定区; 高pH区磷酸盐中的Me转变为稳定的Me(OH)_n,实现磷酸盐碱分解.验证实验表明,加入1.1倍理论量的磷酸钠,控制沉淀pH值为2.0,铬、铁、锌、铜、镍沉淀率分别为94.12 %、5.51 %、0.33 %、0.22 %、0.34 %; 氢氧化钠分解磷酸铬时,磷、铬浸出率分别为90.63 %、5.10 %,实现磷铬有效分离.实验与理论基本相符.      

Tumor cell cytotoxicity of a novel metal chelator

We have synthesized a novel six-coordinate metal chelator from the triamine cis-1,3,5-triaminocyclohexane by the addition of a 2-pyridylmethyl pendant arm on each nitrogen, which we term tachpyr. The experiments described here were designed to explore whether this compound exhibits potential antitumor activity. When added to MBT2 or T24 cultured bladder cancer cells, tachpyr was profoundly cytotoxic, with an IC50 of approximately 4.6 micromol/L compared with 70 micromol/L for desferioxamine. To explore the mode of action of tachpyr, several metal complexes were prepared, including Fe(II), Ca(II), Mn(II), Mg(II), Cu(II), and Zn(II) tachpyr complexes. Of these, the Zn(II), Cu(II), and Fe(II) complexes were without toxic effect, whereas the Ca(II), Mn(II), and Mg(II) complexes remained cytotoxic. To further probe the role of Zn(II) and Cu(II) chelation in the cytotoxicity of tachpyr, sterically hindered tachpyr derivatives were prepared through N-alkylation of tachpyr. These derivatives were unable to strongly bind Fe(III) or Fe(II) but were able to bind Zn(II) and Cu(II). When added to cells, these sterically hindered tachpyr derivatives were nontoxic, consistent with a role of iron depletion in the cytotoxic mechanism of tachpyr. Further, the addition of tachpyr to proliferating cultures resulted in an early and selective inhibition of ferritin synthesis, an iron storage protein whose translation is critically dependent on intracellular iron pools. Taken together, these experiments suggest that tachpyr is a cytotoxic metal chelator that targets intracellular iron, and that the use of tachpyr in cancer therapy deserves further exploration.     

Selective adsorption of chromium(VI) from zinc(II) and other metal ions using persimmon waste gel

An adsorption gel was prepared from persimmon waste, a cellulosic material, rich in polyphenolic compounds, which exhibits a high affinity for chromium(VI). It was prepared by cross-linking persimmon waste with concentrated sulphuric acid. Adsorption tests for different metal ions such as Cr(VI), Cr(III), Fe(III), Zn(II), Cd(II) and Pb(II) from aqueous solution at pH values ranging from (pH 1 to 5) found Cr(VI) to be selectively adsorbed on the cross-linked gel over the other metal ions studied. The adsorption isotherm of Cr(VI) followed the Langmuir type of adsorption and exhibited a maximum loading capacity of 7.18 mol kg^− 1 at pH 1. Selective removal of Cr(VI) from Zn(II) was successfully demonstrated by using a column packed with the persimmon waste gel.     

The role of Mn(II)-peroxidase activity of mycobacterial catalase-peroxidase in activation of the antibiotic isoniazid

The catalase-peroxidase of Mycobacteria smegmatis exhibits Mn(II)-peroxidase activity characterized by a low Km for Mn(II) (5 microM) and a high Km for t-butyl hydroperoxide (100 mM). This activity, monitored by the formation of Mn(III)-malate or -malonate, is inhibited by Co(II) but not by superoxide dismutase. Optical evidence for binding of Mn(II) to the resting (ferric) enzyme is found in a change in intensity of the Soret peak upon titration with Mn(II). A potential role for Mn(III) in the antimycobacterial action of the antibiotic isoniazid is suggested by the rapid reduction of Mn(III)-malonate by this drug. The stoichiometry of the reaction is consistent with two single electron transfer steps per mole of isoniazid.     

Protein phosphatase inhibitors induce the release of serotonin from rat basophilic leukemia cells (RBL-2H3)

Oxidation capacities of laccase, manganese peroxidase (MnP) and lignin peroxidase (LiP) from Phlebia radiata were compared using non-phenolic (veratryl alcohol and ABTS) and phenolic (syringaldazine, vanillalacetone and Phenol red) compounds as reducing substrates. The effect of Mn(II) on enzyme reactions was also studied. Highest specific activities were recorded with laccase in the oxidation of phenolic compounds or ABTS and irrespective of Mn(II) concentration. LiP and MnP oxidized all these substrates but only the catalysis of MnP was dependent upon Mn(II). Only LiP clearly oxidized veratryl alcohol. However, Mn(II) interfered with this reaction by repressing veratraldehyde formation. These results point to multiple participation of manganese ions, either as a reducing (Mn(II)) or oxidizing (Mn(III)) agent in the enzymatic reactions.     

NO reversibly reduces the water-oxidizing complex of photosystem II through S0 and S-1 to the state characterized by the Mn(II)-Mn(III) multiline EPR signal

Incubation of photosystem II preparations with NO at -30 degreesC results in the slow formation of a unique state of the water-oxidizing complex (WOC), which was recently identified as a Mn(II)-Mn(III) dimer [Sarrou, J., Ioannidis, N., Deligiannakis, Y., and Petrouleas, V. (1998) Biochemistry 37, 3581-3587]. Evolution of the Mn(II)-Mn(III) EPR signal proceeds through one or more intermediates [Goussias, C., Ioannidis, N., and Petrouleas, V. (1997) Biochemistry 36, 9261-9266]. In an effort to identify these intermediates, we have examined the time course of the signal evolution in the presence and absence of methanol. An early step of the interaction of NO with the WOC is the reduction of S1 to the S0 state, characterized by the weak Mn-hyperfine structure recently reported for that state. At longer times S0 is further reduced to a state which has the properties of the S-1 state, in that single-turnover illumination restores the S0 signal. The Mn(II)-Mn(III) state forms after the S-1 state and is tentatively assigned to an (iso)S-2 state, although lower states or a modified S-1 state cannot be excluded at present. Following removal of NO 60-65% of the initial S2 multiline signal size or the O2-evolving activity can be restored. The data provide for the first time EPR information on a state lower than S0. Furthermore, the low-temperature NO treatment provides a simple means for the selective population of the S0, S-1 and the Mn(II)-Mn(III) states.     

A Mn(II)-Mn(III) EPR signal arises from the interaction of NO with the S1 state of the water-oxidizing complex of photosystem II

It was shown recently [Goussias, C., Ioannidis, N., and Petrouleas, V. (1997) Biochemistry 36, 9261-9266] that incubation of photosystem II preparations with NO at -30 degrees C in the dark results in the formation of a new intermediate of the water-oxidizing complex. This is characterized by an EPR signal centered at g = 2 with prominent manganese hyperfine structure. We have examined the detailed structure of the signal using difference EPR spectroscopy. This is facilitated by the observations that NO can be completely removed without decrease or modification of the signal, and illumination at 0 degree C eliminates the signal. The signal spans 1600 G and is characterized by sharp hyperfine structure. 14NO and 15NO cw EPR combined with pulsed ENDOR and ESEEM studies show no detectable contributions of the nitrogen nucleus to the spectrum. The spectrum bears similarities to the experimental spectrum of the Mn(II)-Mn(III) catalase [Zheng, M., Khangulov, S. V., Dismukes, G. C., and Barynin, V. V. (1994) Inorg. Chem. 33, 382-387]. Simulations allowing small variations in the catalase-tensor values result in an almost accurate reproduction of the NO-induced signal. This presents strong evidence for the assignment of the latter to a magnetically isolated Mn(II)-Mn(III) dimer. Since the starting oxidation states of Mn are higher than II, we deduce that NO acts effectively as a reductant, e.g., Mn(III)-Mn(III) + NO--> Mn(II)-Mn(III) + NO+. The temperature dependence of the nonsaturated EPR-signal intensity in the range 2-20 K indicates that the signal results from a ground state. The cw microwave power saturation data in the range 4-8 K can be interpreted assuming an Orbach relaxation mechanism with an excited state at delta = 42 K. Assuming antiferromagnetic coupling, -2JS1.S2, between the two manganese ions, J is estimated to be 10 cm-1. The finding that an EPR signal from the Mn cluster of PSII can be clearly assigned to a magnetically isolated Mn(II)-Mn(III) dimer bears important consequences in interpreting the structure of the Mn cluster. Although the signal is not currently assigned to a particular S state, it arises from a state lower than S1, possibly lower than S0, too.