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Nickel(II) complexes of Abeta(1-16)Y10A and its smaller fragments including Abeta(1-4), Abeta(1-6), Ac-Abeta(1-6) and Ac-Abeta(8-16)Y10A have been studied by potentiometric, UV-Vis and circular dichroism spectroscopic measurements. The formation of mixed metal complexes and the distribution of metal ions among the possible coordination sites in the Cu(II)-Ni(II)-Abeta(1-16)Y10A and Cu(II)-Ni(II)-Zn(ii)-Abeta(1-16)Y10A systems have also been evaluated. It was found that the hexadecapeptide and its fragments are effective nickel(II) binding ligands and complex formation processes of nickel(II) ions are quite similar to those of copper(II). Formation of mono- and di-nuclear complexes was detected in the nickel(II)-Abeta(1-16)Y10A system suggesting the existence of two separated metal binding motifs: the N-terminus and internal histidyl residues. The preference for the coordination at the N-terminus was supported by the spectroscopic measurements but in equilibrium with the metal binding at the internal histidyl sites. Neither zinc(II) nor nickel(II) can, however, substitute copper(II) in the mixed metal complexes of Abeta(1-16)Y10A, but both metal ions are able to alter the distribution of copper(II) ions among the various binding sites. Both N-terminus (amino and His6) and internal histidyl residues (His13 and His14) can work as dinuclear binding motifs, preferably accommodating copper(II) and zinc(II), respectively, while nickel(II) can occupy the remaining free coordination sites.

An inexpensive photoreactor using LED light sources and a fibre-optic CCD spectrophotometer as a detector was built by designing a special cell holder for standard 1.000 cm cuvettes. The use of this device was demonstrated by studying the aqueous photochemical reaction of 2,5-dichloro-1,4-benzoquinone. The developed method combines the highly quantitative data collection of CCD spectrophotometers with the possibility of illuminating the sample independently of the detecting light beam, which is a substantial improvement of the method using diode array spectrophotometers as photoreactors.

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Table of contents entryThe kinetics of the reactions between 1,4-benzoquinones and hydrogen peroxide were studied.A systematic study of substituent effects revealed a Hammett-like correlation, where the rate of reaction is mainly determined by electronic effects. 3 AbstractThe kinetics and mechanisms of the redox reactions between hydrogen peroxide and 1,4-benzoquinone, 2-methyl-1,4-benzoquinone, 2,6-dimethyl-1,4-benzoquinone, 2-chloro-1,4-benzoquinone and 2,6-dichloro-1,4-benzoquinone were studied in aqueous media using spectrophotometric monitoring. The formation and decay of a hydroxylated 1,4-benzoquinone was detected. The formation of the intermediate was first order with respect to the parent 1,4-bezoquinone and hydrogen peroxide, whereas inverse first order dependence was revealed with respect to hydrogen ion. The decomposition reaction had two parallel pathways: one was first order with respect to the intermediate, while the other showed second-order dependence.The values of the rate constant measured for the formation step were successfully correlated with both the redox potentials of the substituted quinone/hydroquinone systems and the pK a values of the hydroxylated quinone derivatives. Therefore, electronic effects govern the reactivity of the quinones in this process. NMR and GC-MS measurements were carried out to identify the products in the system. Quantum mechanical calculations were also carried out in these systems.4