Vasilios Lepentsiotis

Friedrich-Alexander-University of Erlangen-Nürnberg, Erlangen, Bavaria, Germany

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Publications (8)30.01 Total impact

  • Vasilios Lepentsiotis · Rudi van Eldik · Frans F. Prinsloo · Jakobus J. Pienaar
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    ABSTRACT: The reactions of the water soluble complex FeIII(TPPS) [TPPS = 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrinate] with peroxomonosulfate, hydrogen peroxide and sulfite/oxygen have been investigated kinetically as a function of reactant concentration and pH. The spectral changes recorded for the reactions between the FeIII(TPPS) dimer and peroxomonosulfate and hydrogen peroxide can be interpreted in terms of a redox cycle between (TPPS)FeIIIOFeIII(TPPS) and (TPPS)FeIIIOFeIV(O)(TPPS+), and in terms of multiple redox cycles also involving FeII(TPPS) for the FeIII(TPPS)–sulfite–oxygen system. In the case of peroxomonosulfate and hydrogen peroxide a slow redox cycle (1000 s) between iron-(III) and -(IV) complexes is observed at low [SO52–] and [H2O2]. In the case of sulfite–oxygen the kinetic traces are quite different; the FeIII/FeIV redox cycle is very fast (a few seconds) and is only observed after what appears to be an induction period. Furthermore, it also depends significantly on the selected experimental conditions (pH, sulfite and oxygen concentration). Rapid-scan techniques were used to study these redox cycles. Reaction mechanisms for the redox cycling of the FeIII(TPPS)–SO52– system, and for the multiple redox cycling of the FeIII(TPPS)–sulfite–oxygen system, are proposed. They are based on reactions that participate in the suggested mechanism for the iron-catalysed autoxidation of sulfite. In contrast to the FeIII(TPPS)–HSO5– system, which is insensitive to oxygen, oxygen plays an essential role in the multiple redox cycles of the FeIII(TPPS)–sulfite–oxygen system, which is accounted for in the proposed mechanism. Computer simulations based on the proposed reaction mechanisms are in good agreement with the observed experimental kinetic traces and indicate that for the FeIII(TPPS)–sulfite–oxygen system the formation of the SO5˙– radical is the main oxygen-consuming step during the overall redox process.
    No preview · Article · Aug 1999 · Journal of the Chemical Society Dalton Transactions
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    ABSTRACT: The reactions of the water-soluble complexes [NiCR](2+) (where CR = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene) and [NiKGH-CONH(2)](+) (where KGH-CONH(2) = lysylglycylhistidinecarboxamide) with sulfite/O(2) and peroxymonosulfate have been investigated using spectrophotometric and rapid-scan techniques. In most cases, the spectral changes suggest the formation of an intermediate Ni(III) species, followed by decomposition reactions which involve a back-reaction to Ni(II). Only in the case of the [NiCR](2+)-S(IV)-O(2) system is the formed Ni(III) species stable in solution. When sulfite and oxygen are used to oxidize Ni(II) to Ni(III), the reaction is oxygen dependent and an induction period could be observed, whereas the use of the strong oxidizing agent peroxymonosulfate resulted in no induction period and no oxygen dependence. In addition, the oxidation of Ni(II) to Ni(III) was faster if peroxymonosulfate was used instead of sulfite/O(2). The [NiKGH-CONH(2)](+) complex reacts much faster with sulfite/O(2) and peroxymonosulfate than the [NiCR](2+) does. Rate constants for the oxidation process and possible reaction mechanisms, based on available literature data, that can account for the observed kinetic observations in a qualitative way are presented, and the results are correlated with previously obtained data on DNA modification using these systems.
    No preview · Article · Aug 1999 · Inorganic Chemistry
  • V. Lepentsiotis · R. van Eldik · F.F. Prinsloo · J.J Pienaar

    No preview · Article · Jan 1999 · Journal of the Chemical Society Dalton Transactions
  • Vasilios Lepentsiotis · Rudi van Eldik · Dimitri M. Stulov · Sergei V. Makarov
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    ABSTRACT: Stability studies have been made on aminoiminomethanesulfinic acid [aimsa, thiourea dioxide, (NH2)2CSO2] and the reactions of aimsa and its decomposition product (dithionite, S2O42–) with iron(III) 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrinate [FeIII(tpps)] in the presence of oxygen have been investigated. Application of NMR and stopped-flow spectrophotometry revealed direct evidence for the existence of two different forms of aimsa in aqueous solution. The slow formation of one of these forms, NH2NHCSO2H, is shown to determine the strong dependence of the reactivity of aimsa on the age of the stock solutions. Both aimsa and dithionite react in a similar way with FeIII(tpps) in alkaline solutions. The SO2˙– radical plays a key role in the redox reactions. The ratio of the oxygen and radical concentration determines which kind of reaction (oxidation, reduction or decomposition) will dominate. In general a high oxygen concentration and a low radical concentration favour the oxidation and decomposition of the metalloporphyrin, whereas a high radical concentration and a low oxygen concentration favour the reduction. In strongly basic solutions ([NaOH] = 0.5 M) a redox cycle between FeIII(tpps) and FeII(tpps) is observed at low aimsa and dithionite concentration. Possible mechanisms for the decomposition of aimsa in alkaline solutions, as well as for the reactions between FeIII(tpps) and aimsa or dithionite, are proposed.
    No preview · Article · Sep 1998 · Journal of the Chemical Society Dalton Transactions
  • Vasilios Lepentsiotis · Rudi van Eldik
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    ABSTRACT: The reaction of the water-soluble complex FeIII(tpps) [tpps = 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrinate] with ascorbic acid has been investigated in aqueous solution by a combination of rapid-scan and stopped-flow techniques. The spectral changes recorded for the reaction between the FeIII(tpps) monomer and ascorbic acid in a weakly acidic medium suggest the formation of the FeIII(tpps˙+) radical cation and the partial formation of FeII(tpps), which depend on the concentration ratio of ascorbic acid and oxygen. In a basic medium the FeIII(tpps) dimer is reduced to FeII(tpps), showing an induction period which depends on the oxygen concentration of the solution. Measurements with an oxygen-sensitive electrode clearly demonstrated that during the induction period oxygen is used up by ascorbate and that the decrease in oxygen concentration is accelerated by the presence of FeIII(tpps), thus demonstrating the catalytic effect of the metalloporphyrin on the autoxidation of ascorbate. This suggests that in the presence of FeIII(tpps) a redox cycle takes place [FeII/III(tpps)] which consumes oxygen, with the result that no spectral changes are observed during the induction period.
    No preview · Article · Mar 1998 · Journal of the Chemical Society Dalton Transactions
  • F. F. Prinsloo · Ch. Brandt · V. Lepentsiotis · J. J. Pienaar · R. van Eldik
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    ABSTRACT: The complex-formation reactions between aquated Fe(III) and S(IV) oxides were investigated using rapid-scan techniques. The spectra clearly show the formation of different Fe(III)−S(IV) complexes, which depend on the concentration ratio and pH employed.
    No preview · Article · Dec 1996 · Inorganic Chemistry
  • Vasilios Lepentsiotis · Frans F. Prinsloo · Rudi van Eldik · Heinz Gutberlet
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    ABSTRACT: The influence of mixed sulfur–nitrogen oxides on the redox chemistry of iron-(III) and -(II) species in aqueous solution was studied kinetically as a function of various concentrations. Two oxides, viz. HONH(SO3)– and HON(SO3)22–, were found to reduce FeIII to FeII, whereby these oxides undergo an iron(III)-induced hydrolysis reaction. The hydrolysis products were analysed by ion chromatography. These redox reactions can in principle account for the inhibiting effect of sulfur–nitrogen oxides on the metal-catalysed autoxidation of sulfur(IV) oxides. Possible reaction mechanisms are suggested.
    No preview · Article · May 1996 · Journal of the Chemical Society Dalton Transactions
  • Vasilios Lepentsiotis
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    ABSTRACT: Diss. - Bibliogr. na konci kapitol
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