Metal Complexes with Macrocyclic Ligands. Part XLII. Tetraazamacrocyclic nickel(II) complexes with a methylthio or a methoxy pendant chain as model for cofactor F430

The 14-membered macrocyclic Ni²⁺ complexes of 1 and 2 , with a methylthio pendant chain, and those of 3 and 4 , with a methoxy pendant chain, have been synthesized and their chemistry has been studied. Solution spectra in H₂O, MeCN, and DMF indicate no participation of the side-chain donor group in metal coordination. This is also the case in the solid state as shown by the X-ray structures of the Ni²⁺ complexes with 1 and 2 , in which a tetrahedrally distorted square-planar geometry around the Ni²⁺ results by the coordination of the four N-atoms of the macrocycle. Cyclic voltammetry of these complexes in MeCN reveals that Ni²⁺ is reversibly reduced to Ni⁺ between ?0.7 and ?0.8 V vs. SCE. For the complexes with 1 and 2 , the thioether bond is cleaved at more negative potentials, whereby a thiol group is formed. This thiol group is then oxidized at ca. +0.7 V vs. SCE, when a glassy C electrode is used, or at ca. 0 V vs. SCE at a dropping Hg electrode. No cleavage of the ether bond in the complexes with 3 and 4 is observed under similar conditions. Reduction of the Ni²⁺ complexes of 1 and 2 with Na-amalgam in DMF produces small amounts of methane only in the case of 1 , indicating the importance of the proximity between the Ni^I centre and the MeS group.     

Comparative Spectroelectrochemical Studies of Lyophilized and Nonlyophilized Laccases from Cerrena unicolor Basidiomycete

The electrochemical, spectroelectrochemical, and kinetic investigations of two preparations of Cerrena unicolor laccase, lyophilized (LLAC) and nonlyophilized frozen enzymes (FLAC), were performed. It was found that the value of the redox potential of the T1 site of C. unicolor laccase is ca. 750 vs. NHE. It was also shown that one of the redox potentials of the T2/T3 cluster of C. unicolor laccase is close to 400 mV, as was previously confirmed for other blue multicopper oxidases, such as trees and fungal laccases, ascorbate oxidase, and bilirubin oxidase. Furthermore, the poor stability of both preparations, but especially of LLAC, in their reduced state was confirmed using mediated and mediatorless spectroelectrochemical studies. DET‐based biocatalytic reduction of O₂ by C. unicolor laccase was only obtained, when FLAC was directly adsorbed on a spectrographic graphite electrode. Moreover, only low values of the steady‐state potentials of gold and graphite electrodes modified by C. unicolor laccase were also found. Heterogeneity of the 3‐D structures of laccase molecules, conformational changes, and partial denaturation of the enzyme, which appeared after enzyme isolation, purification, and especially lyophilization, were found to be the reasons for the low bioelectrocatalytic current, the high K_M‐value towards O₂, and the unusual electrochemical behavior of C. unicolor laccase used in the present study. In spite of the comparable specific activity and long‐term stability of both preparations in homogeneous solution, the stability of immobilized LLAC was found to be inadmissibly low for both fundamental studies and possible electrochemical applications. Indeed, FLAC is a much better source of enzyme than its lyophilized counterpart.     

A novel polynuclear donor complex based on helical peptides with aligned electroactive moieties

Two newly synthesised 21-amino acid peptides substituted with ferrocene groups preserve the helical structure of the peptide and behave as multi-centre donor systems with six electroactive reversible redox centres per molecule. The formal potential of the hexaferrocene compound is slightly less positive than that of its single centre analogue.     

Electrochemistry of the system Cu(II)Cu(I)Cu(0) in acidified thiocyanate solutions

The system Cu(II)Cu(I)Cu(0) in acidified thiocyanate medium was investigated at carbon, mercury, and copper amalgam electrodes using cyclic voltammetry, normal, differential and reverse pulse voltammetry, double potential step chronocoulometry, and exhaustive coulometry. Reduction of Cu(II) to Cu(I) on carbon electrodes proceeds quasireversibly. At moderate concentrations of Cu(II) and SCN^? the reduction of Cu(II) leads to three-dimensional precipitation of CuSCN which can be deposited at the electrode surface. At high concentration of SCN^? complexation dominates over precipitation and only soluble species are formed. At mercury and copper amalgam electrodes the situation is more complicated. The three- dimensional precipitation is preceded by strong thiocyanate-induced adsorption of Cu(I) which results in formation of a mono layer at potential well-separated from those where diffusing product is formed.     

Cathodic and anodic stripping determination of traces of adenine and adenosine based on accumulation of copper(I) compounds at mercury or amalgam electrodes

In the presence of adenine and adenosine, the copper(II)/copper(Hg) couple splits to the copper(II)/copper(I) and copper(I)/copper(Hg) couples. Sparingly soluble complexes of copper(I) with adenine and adenosine can be accumulated on the electrode surface either by reduction of Cu(II) ions or by oxidation of the copper amalgam electrode. The copper(I)/adenine deposit can be stripped either cathodically or anodically with detection limits of 5×10^?9 and 2×10^?8 mol dm^?3, respectively. The copper(I)/ adenosine complex yields only the cathodic stripping peak with a detection limit of 9×10^?6 mol dm^?3. The stripping peaks obtained for the copper(I)/adenine and copper(I)/ adenosine complexes are better defined and appear over a wider range of pH than the peaks related to the corresponding mercury compounds. Adenosine cannot be determined in the presence of adenine bur adenine can be determined in the presence of moderate amounts of adenosine.     

Solvent extraction of groups 4 and 14 metal chelate complexes: Model experiments for chemical studies on coordination number of element 104

To verify the idea that the coordination number of element 104 could be influenced by relativistic effects the studies on complexation of possible homologs of element 104 in group 4 (Zr⁴⁺, Hf⁴⁺, Ti⁴⁺) and 14 (Sn⁴⁺, Pb⁴⁺) by thenoyltrifluoroacetone (TTA) and tropolone (T) were performed. Contrary to monodentate ligands, TTA forms octacoordinate complexes not only with Zr⁴⁺ and Hf⁴⁺ but also with Ti⁴⁺, which are extracted to organic phase. Tropolone forms octacoordinate neutral complexes also with group 14 metal cations Sn⁴⁺ and Pb⁴⁺. Since octacoordinate complexes of Ti⁴⁺, Sn⁴⁺ and Pb⁴⁺ with TTA and T are much weaker than the respective complexes of Zr⁴⁺ and Hf⁴⁺, extraction of the former complexes to organic phase requires higher values of pH. The behavior of 104⁴⁺ in experiments with TTA or T extractants should answer the question concerning the similarity of 104⁴⁺, in respect to CN to its lighter congeners Zr⁴⁺ and Hf⁴⁺ or Ti⁴⁺ and group 14 elements.     

Influence of relativistic effects on hydrolysis of Ra2+

Summary Using ²²⁴Ra radiotracer the first hydrolysis constant (pK_1h) of Ra²⁺ cations has been determined. The pK_1h value of Ra²⁺ was compared with the pK_1h values of other Group 2 cations. It has been shown that the electrostatic hydrolysis model based on assumption that pK_1h is a linear function of reciprocal ionic radii (1/r_i) does not describe well the hydrolysis of Group 2 metal cations. The reason of higher Ra²⁺ hydrolysis as expected is the influence of relativistic effects on bonding 7s and 7p_1/2 orbitals.     

Determination of traces of purine by cathodic stripping voltammetry at the hanging copper amalgam drop electrode

In the presence of purine, the copper(II)/copper(Hg) couple splits into copper(II)/copper(I) and copper(I)/copper(Hg) couples, which form two well-separated systems of peaks under voltammetric conditions. The copper(I)/purine complex adsorbs on the electrode surfacer and can be deposited on the electrode surface by electroreduction of copper(II) ions at the HMDE or by electro-oxidation of the hanging copper amalgam drop electrode (HCADE). The deposit can be stripped either cathodically or anodically over the pH range 2–9. The cathodic stripping variant at the HCADE, in solution with pH 2, offers the best results, with linear response for the range 5 × 10^?9–1.5 × 10^?7 mol dm^?3 purine after an accumulation time of 3 min. The detection limit found with the HMDE in the presence of copper(II) ions is higher.