Portugaliae
Electrochimica Acta

In regard to the understanding of copper at the active site of "blue" proteins a series of open chain and macrocyclic N^S2-ligands have been studied potentiometricalfy, spectrophotometrically and voltammetrically. Cu2+ forms the species CuL2+ and sometimes CuLOH+, whereas Cu+ gives Cu(an)Lf^2+ (an — CH3CN), CuLH2+ and CuL+. From the stabilities of the Cu2+ and Cu+ species the redox potentials have been calculated and compared to the values obtained from cyclic voltammetry. The redox potentials for the open chain complexes are between 250 • 280 mV against SHE. The complexes with the macrocyclic ligands span a range of 340 mV, going from 80 to 420 mV against SHE, or six order of magnitude in relative stability reflecting the importance of subtle differences in steric requirements. This is exemplified through the X-ray structure of the Cu+ complex with one of the 16-membered macrocycles, which reveals a tetrahedral coordination geometry typical for the cuprous ion. A comparison between open chain and cyclic ligands shows that a macrocyclic effect is found for Cu2+, but not for Cu+. The ligand field strength is very different for the two types of ligands and the redox potentials are nearly independent of the chain length of the open chain ligands, but strongly depend on the ring size of the macrocycles.

After the impact produced by the introduction of anodic (and cathodic) stripping voltammetry as a very convenient method for the analysis of ultratrace compounds, especially metal ions, alternative methods for the pre-concentration step were investigated to extend the field of application of the stripping techniques to non-electroactive species. In adsorptive stripping voltammetry, species with some tendency to be adsorbed at electrode surfaces can be pre-concentrated in this way and then stripped just as in anodic or cathodic stripping voltammetry. This lecture is started with a general view of the methods used in the pre-concentration step of stripping voltammetric methods. Afterwards, focus is moved to adsorptive stripping voltammetry: first, some simple theoretical aspects are considered, related with the steps of adsorption and stripping; then, a reference is made to instrumentation, methodology and elimination of interferences; finally, examples of the use of the technique are presented.

A new electrolyte prepared from commercial poly(ethylene oxide) and neodymium triflate, in which the salt ions are solubilised by complexation with the oxygen atoms in the polymer structure, has been studied using ac conductivity measurements. The results obtained indicate that the levels of ionic conductivity registered in this system are similar to other multivalent ion based electrolytes. The nature of the charge transporting species in the electrolyte (cations, anions, charged or uncharged ion clusters) cannot be identified with the use of ac techniques and consequently only the total ionic conductivity is reported. The dependence of the ionic conductivity of the electrolyte on the concentration of added salt can however be correlated with the phase behaviour of the polymer - salt system.

The first harmonic alternating current polarography, with a selected phase angle of n/2 rad, gives structural and analytical information about the denaturation process of insulin. This method allows detecting any alteration in the composition and in the aggregational or conformational state of the protein. The reduction of the disulphide bonds and free Zn(II) gives complementary information on the capacitive contribution around the zero charge potential, for controlling the quality of the insulin samples. Kalousek K3 and K4 methods provide fast information about the structural state of proteins in solution. Proteins with disulphide bonds (as albumin and lysozyme) yield catalytic waves in Co(H)/amonia buffer that denote the different electrochemical activity of -SH lipophilic and hydrophilic residues.