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Kinetic Studies and Mechanism of Hydrogen Peroxide Catalytic Decomposition by Cu(II) Complexes with Polyelectrolytes Derived from L-Alanine and Glycylglycine.

Skounas S, Methenitis C, Pneumatikakis G, Morcellet M - Bioinorg Chem Appl (2010)

Bottom Line: The catalytic decomposition of hydrogen peroxide by Cu(II) complexes with polymers bearing L-alanine (PAla) and glycylglycine (PGlygly) in their side chain was studied in alkaline aqueous media.The energies of activation for the reactions were determined at pH 8.8, in a temperature range of 293-308 K.The trend in catalytic efficiency is in the order PGlygly>PAla, due to differences in modes of complexation and in the conformation of the macromolecular ligands.

View Article: PubMed Central - PubMed

Affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Athens, Panepistimiopolis, 15771 Athens, Greece.

ABSTRACT
The catalytic decomposition of hydrogen peroxide by Cu(II) complexes with polymers bearing L-alanine (PAla) and glycylglycine (PGlygly) in their side chain was studied in alkaline aqueous media. The reactions were of pseudo-first order with respect to [H(2)O(2)] and [L-Cu(II)] (L stands for PAla or PGlygly) and the reaction rate was increased with pH increase. The energies of activation for the reactions were determined at pH 8.8, in a temperature range of 293-308 K. A suitable mechanism is proposed to account for the kinetic data, which involves the Cu(II)/Cu(I) redox pair, as has been demonstrated by ESR spectroscopy. The trend in catalytic efficiency is in the order PGlygly>PAla, due to differences in modes of complexation and in the conformation of the macromolecular ligands.

No MeSH data available.


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(a) EPR spectra of a frozen solution (77 K) of PAla-Cu(II) at different reaction times. [Cu(II)] = 1.0 × 10−3 M, R = [PAla]/[Cu(II)] = 4, pH = 8.7. (b) The variation of the intensity of Cu(II) signal, as (%) of the initial signal (t = 0 min), during the reaction time for the systems Pala-Cu(II) and PGlygly-Cu.
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fig5: (a) EPR spectra of a frozen solution (77 K) of PAla-Cu(II) at different reaction times. [Cu(II)] = 1.0 × 10−3 M, R = [PAla]/[Cu(II)] = 4, pH = 8.7. (b) The variation of the intensity of Cu(II) signal, as (%) of the initial signal (t = 0 min), during the reaction time for the systems Pala-Cu(II) and PGlygly-Cu.

Mentions: The two systems exhibited a completely different behavior. For the PAla-Cu(II) system we can detect three stages of the reaction (Figure 5). In the first, very short stage, a slight increase in the intensity of the signal is observed. In the second stage a rapid decrease in the intensity occurs until it reaches about 20% of the starting intensity without entire disappearance. In the final stage the signal starts building up again in a slow rate without ever reaching the original intensity. From our results it was not easy to assign any new EPR absorbing Cu(II) species, formed during the reaction. Furthermore, as any additional spectral lines in the area of g = 4 were not detected, any Cu(II)-Cu(II) strong antiferromagnetic coupling was precluded to be the reason for the decrease in the intensity of the signal [47, 48]. Finally, as the formation of a diamagnetic Cu(III) complex (d8 low-spin complex) can also be ruled out, since it requires ligands able to give strong fields as tetrapeptides or ligands able to give a four-nitrogen in plane coordination [49], and no evidence of ESR lines due to paramagnetic Cu(III) complexes could be found, the reduction of EPR-silent Cu(I) complex is strongly suggested. The small initial increase in the intensity of the signal could be explained as the result of the adoption from the negatively charged macromolecular catalyst of a more extended structure that enables the approach of the, also, negatively charged peroxo anion that comes from the dissociation of H2O2 that occurs at this pH. In this structure, the metallic centers are further apart from each other and any small spin exchange between them is diminished.


Kinetic Studies and Mechanism of Hydrogen Peroxide Catalytic Decomposition by Cu(II) Complexes with Polyelectrolytes Derived from L-Alanine and Glycylglycine.

Skounas S, Methenitis C, Pneumatikakis G, Morcellet M - Bioinorg Chem Appl (2010)

(a) EPR spectra of a frozen solution (77 K) of PAla-Cu(II) at different reaction times. [Cu(II)] = 1.0 × 10−3 M, R = [PAla]/[Cu(II)] = 4, pH = 8.7. (b) The variation of the intensity of Cu(II) signal, as (%) of the initial signal (t = 0 min), during the reaction time for the systems Pala-Cu(II) and PGlygly-Cu.
© Copyright Policy - open-access
Related In: Results  -  Collection

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fig5: (a) EPR spectra of a frozen solution (77 K) of PAla-Cu(II) at different reaction times. [Cu(II)] = 1.0 × 10−3 M, R = [PAla]/[Cu(II)] = 4, pH = 8.7. (b) The variation of the intensity of Cu(II) signal, as (%) of the initial signal (t = 0 min), during the reaction time for the systems Pala-Cu(II) and PGlygly-Cu.
Mentions: The two systems exhibited a completely different behavior. For the PAla-Cu(II) system we can detect three stages of the reaction (Figure 5). In the first, very short stage, a slight increase in the intensity of the signal is observed. In the second stage a rapid decrease in the intensity occurs until it reaches about 20% of the starting intensity without entire disappearance. In the final stage the signal starts building up again in a slow rate without ever reaching the original intensity. From our results it was not easy to assign any new EPR absorbing Cu(II) species, formed during the reaction. Furthermore, as any additional spectral lines in the area of g = 4 were not detected, any Cu(II)-Cu(II) strong antiferromagnetic coupling was precluded to be the reason for the decrease in the intensity of the signal [47, 48]. Finally, as the formation of a diamagnetic Cu(III) complex (d8 low-spin complex) can also be ruled out, since it requires ligands able to give strong fields as tetrapeptides or ligands able to give a four-nitrogen in plane coordination [49], and no evidence of ESR lines due to paramagnetic Cu(III) complexes could be found, the reduction of EPR-silent Cu(I) complex is strongly suggested. The small initial increase in the intensity of the signal could be explained as the result of the adoption from the negatively charged macromolecular catalyst of a more extended structure that enables the approach of the, also, negatively charged peroxo anion that comes from the dissociation of H2O2 that occurs at this pH. In this structure, the metallic centers are further apart from each other and any small spin exchange between them is diminished.

Bottom Line: The catalytic decomposition of hydrogen peroxide by Cu(II) complexes with polymers bearing L-alanine (PAla) and glycylglycine (PGlygly) in their side chain was studied in alkaline aqueous media.The energies of activation for the reactions were determined at pH 8.8, in a temperature range of 293-308 K.The trend in catalytic efficiency is in the order PGlygly>PAla, due to differences in modes of complexation and in the conformation of the macromolecular ligands.

View Article: PubMed Central - PubMed

Affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Athens, Panepistimiopolis, 15771 Athens, Greece.

ABSTRACT
The catalytic decomposition of hydrogen peroxide by Cu(II) complexes with polymers bearing L-alanine (PAla) and glycylglycine (PGlygly) in their side chain was studied in alkaline aqueous media. The reactions were of pseudo-first order with respect to [H(2)O(2)] and [L-Cu(II)] (L stands for PAla or PGlygly) and the reaction rate was increased with pH increase. The energies of activation for the reactions were determined at pH 8.8, in a temperature range of 293-308 K. A suitable mechanism is proposed to account for the kinetic data, which involves the Cu(II)/Cu(I) redox pair, as has been demonstrated by ESR spectroscopy. The trend in catalytic efficiency is in the order PGlygly>PAla, due to differences in modes of complexation and in the conformation of the macromolecular ligands.

No MeSH data available.


Related in: MedlinePlus