Irma Crivelli

Copper(II) complexes of a hydroxamic acid from maize

Abstract The thermodynamics of formation for DIMBOA-Cu(II) complexes (where DIMBOA = 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-4H-one, a hydroxamic acid from maize) has been investigated in aqueous solutions by a potentiometric method. DIMBOA forms 1:1 and 1:2 chelates with Cu(II) at ionic strength 0.05 M (NaCl04). The stability constants measured were about 105 and 104 for the 1:1 and 1:2 complexes respectively, determined at 10, 20 and 30°. The contribution of ΔH and ΔS to the stability of complexes is examined and the pK values are compared with other ligands found in maize. Although DIMBOA has similar or higher constants to form copper complexes than other plant ligands, its possible role as a transport agent in maize remains to be established.

Redox chemistry of copper acetate binuclear complexes in acetic acid-methanol mixture as solvent

Abstract The redox chemistry of the photochemically generated bioinorganic model (CH 3 COO) 2 Cu II Cu I (OOCCH 3 ) 2 and the parent species (CH 3 COO) 2 Cu II Cu II (OOCCH 3 ) 2 (complex I ) are reported. The purpose is to achieve a more complete physicochemical characterization of the acetate binuclear copper complex and consequently a closer evaluation of its behaviour as a model of type III Cu sites in cuproproteins. Complex I may be sequentially reduced (broad not resolved peaks around −0.2 and −0.4 V) and reoxidized (peaks at 0.16 and 0.36 V), but in competition with a thermal disproportionation to Cu II and Cu 0 . Controlled potential electrolysis (CPE) at −0.4 V measures quantitatively the overall reduction process including disproportionation and involves four electrons per mole of initial complex I . Structural changes seem to play a role in the oxidation-reduction processes as suggested by ip/√v vs v studies (Nicholson and Shains criteria), and they make the processes irreversible. The mixed-valence Cu II -Cu I species could not be produced by CPE at any voltage before the second reduction peak; however, its electrochemical generation by pulses at −0.4 V render direct experimental evidence that the reduction process is sequential. The voltammograms of the Cu II -Cu I species generated photochemically are more complicated: the reduction and oxidation peaks are centered at −0.2 and 0.0 V, respectively. The species Cu II -Cu II originated from the oxidation of this mixed valence complex, called as complex II , shows similarities and differences with complex I . The results are discussed in relation to the structure stability and to the copper interactions in the binuclear species considered as models for the copper protein centers.

The 2-Mercaptobenzothiazole and Copper(II) Reaction

Abstract A systematic study of different synthetic routes for the reaction of 2-mercaptobenzothiazole with Cu(II) was pursued. The products were carefully analyzed in regard to the oxidation state of copper and its stoichiometry. Conclusions were drawn in relation to the nature of the products obtained and the conditions of the synthesis.

Experimental evidence of the disproportionation equilibrium in copper mixed-valence complexes

Abstract Experimental evidence for a dismutation equilibrium in the mixed-valence (MV) [M n + –M ( n +1)+ ] system type has been elusive and its existence can be established only when the oxidation–reduction processes involved are reversible. Previous research in the field of binuclear Cu(II)Cu(II), Cu(I)Cu(I) and the related MV Cu(II)Cu(I) complexes allowed us to obtain electrochemical evidence for the disproportionation equilibrium in some of these systems. In this communication we report and discuss experiments with [(RCOO) 2 Cu(II)Cu(I)(OOCR) 2 ] − (R=CH 3 , Ph) type MV complexes that give direct non-electrochemical experimental evidence for the presence in solution of the disproportion equilibrium: 2[( RCOO ) 2 Cu ( II ) Cu ( I )( OOCR ) 2 ] − ⇔( RCOO ) 4 Cu 2 ( II )+[( RCOO ) 4 Cu ( I ) 2 ] 2 − It was possible to isolate the different components of the disproportionation equilibrium by varying temperature and solvent conditions. To our knowledge, this is the first non-electrochemical experimental evidence of this equilibrium for copper MV complexes in solution. Furthermore, as is discussed in the text, these results may be the basis for giving an alternative point of view to that given in some studies already reported in the literature, which relate to solvent and temperature effects on the intensity and energy of the intervalence transfer bands and also to changes in the EPR spectra of MV species.

A violet mixed-valence copper-mercaptoethylamine complex generated electrochemically or by reversible interaction with oxygen

Abstract The synthesis, characterization and the reversible interaction with oxygen of a Cu(I)-mercaptoethylamine complex (I) in aqueous solution and room temperature is reported. As a result of the interaction with oxygen a violet species II is formed which has a similar absorption spectrum (λmax = 500 nm) to a violet species III obtained electrochemically from a solution of the Cu-mercarptoethylamine complex in a nitrogen atmosphere. In the absence of dissolved oxygen both violet species are stable but in presence of air they decompose into a greenish-white precipitate which does not contain the mercaptoethylamine ligand in its structure. A mechanism for the complex I-oxygen interaction is proposed which involves a superoxo mixed-valence species as intermediate. The electronic assignation of the 500 nm absorption band is also discussed. The remarkable special characteristics, i.e. (i) the simplicity of the ligand containing a soft donor atom; (ii) the fact that the reversible uptake of oxygen occurs at room temperature and (iii) the spectroscopical similarity of species II and III described above, allow new insights into the interaction of metal complexes with oxygen. These results suggest that a mixed-valence species may be associated with the O2 binding or oxygen activation processes in copper proteins. Also, considering the results recently published by M. Saraste et al., the relevance of the mercaptide bridged mixed-valence species II and III as models for the CuA site in cytochrome oxidase is also discussed.

Hush theory in a copper mixed valence bioinorganic model generated photochemically

Abstract In the present study a copper mixed valence (MV) compound has been generated photochemically from the (CH 3 COO) 2 Cu(II)Cu(II)(OOCCH 3 ) 2 dimeric species in 95/5% (vol./vol.) methanol/acetic acid mixture as solvent. The MV species shows two intervalence (IT) bands at 19.6 and 10.4 kK and seven complex lines in the EPR spectra at room temperature. Although the two bands are IT bands in character, only the second (960 nm) could be related through the Hush and Hopfield theories to a thermal electron transfer process between the copper centers with kinetic parameters Δ G # =21.6 kJ mol −1 and k th =1.2×10 9 s −1 (Hush) or 4.0×10 11 s −1 (Hopfield) at 298 K and related to the change from (d x 2 − y 2 ) 2 Cu(I)(d x 2 − y 2 )Cu(II) to (d x 2 − y 2 ) 1 Cu(II)(d x 2 − y 2 ) 2 Cu(I) electronic configuration ( D 4 h symmetry for each copper center). The electronic assignation of the more energetic band is also discussed. The value obtained for k th is in the range measured by EPR spectroscopy for other Cu(II)Cu(I) dimeric systems and provides evidence for the validity of the essence of the theoretical approach in copper binuclear systems although the value for the electronic coupling parameter is relatively high. The complex studied in this work has some characteristics in common with those of the type III copper observed in copper oxidases in its MV state such as: dimeric structure with tetragonal environment for the coppers, with oxygen perhaps as bridging ligand, EPR signal related to one unpaired electron interacting with both coppers and with k th values which compare well with some biological systems; so we believe it is a good bioinorganic model for type III copper in multicopper oxidases.

Iron in the blood plasma of the ascidian Pyura chilensis

Abstract 1. 1. In order to study the iron-binding in the blood plasma of the ascidian Pyura chilensis Molina 1782, the plasma components were separated by Sephadex G-75 chromatography, gel electrophoresis and fractionated precipitation combined with Mossbauer spectroscopy iron detection. 2. 2. Iron-binding in the plasma was also investigated by fractionation of 59Fe-labeled plasma, using gel chromatography. 3. 3. Proteins and N-acetylamino sugars were found as plasma components. 4. 4. Iron was detected in protein component(s) by colorimetric methods and Mossbauer measurements. 5. 5. The results, which strongly suggest a high-iron molecular weight plasma protein association, are compared with other iron concentrating ascidians.