J.J.R.Fraústo da Silva

Metal complexes of cyclic tetra-azatetra-acetic acids

The cyclic tetra-aza complexones cDOTA ([12]ane N(4).4ac), cTRITA ([13]ane N(4).4ac) and cTETA ([14]ane N(4).4ac) have been synthesized and characterized by elemental analysis, titration, melting-point determination and NMR (and infrared) spectroscopy. The ionization constants and the stability constants of the MH(2)L, MHL and ML complexes formed with alkali, alkaline-earth and some transition metals were determined at 25.0 +/- 0.1 degrees and ionic strength 0.10M [KNO(3) and (CH(3))(4)NNO(3)]. It was confirmed that cDOTA forms the most stable Ca(2+) and Sr(2+) complexes but the reported inversion of the order of stability of the complexes of these two ions with cTRITA was not confirmed. Also, the values of the stability constants determined in this work differ substantially from those previously reported for ML species. cDOTA is an interesting alternative to classical non-cyclic complex-ones for the complexometric determination of Ca(2+) and Mg(2+) but neither this ligand nor the other two offer advantages over EDTA or DCTA for the complexometric titration of transition metals.

The distribution of elements in cells

The selective uptake of elements by proteins can be based on several routes: (a) the equilibrium binding of different elements by different protein ligands using charge, size, electron affinity and stereochemical preferences; (2) kinetic insertion of an element into such a coordination site of a protein; (3) removal of the element to a special compartment by pumping followed by (1) or (2). A cellular system also limits the amount of each type of metal-binding apoprotein by genetic regulation of its symbiosis with element uptake. Such a limitation generates much greater selectivity. Finally we consider how the observed selection of elements by proteins has changed in evolution through changes of availability of elements and their combinations in the environment1.

The stability of the metal complexes of cyclic tetra-aza tetra-acetic acids

The stability constants of the complexes formed by three tetra-aza macrocyclic complexones (DOTA, TRITA and TETA) with Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+) and Pb(2+) were determined with an automated titration instrument with data acquisition and the calculations were performed with the Superquad program, confirming and extending the range of values previously available. Both 1:1 and 2:1 metal-to-ligand complexes were now considered including their protonated species. The results show that DOTA is a powerful but unselective ligand whereas TETA, although not so powerful as DOTA, is an interesting selective ligand for pairs of metal ions, e.g., Cd(2+) and Pb(2+).

Vanadium haloperoxidases from brown algae of the Laminariaceae family.

Vanadium haloperoxidases were extracted, purified and characterized from three different species of Laminariaceae--Laminaria saccharina (Linné) Lamouroux, Laminaria hyperborea (Gunner) Foslie and Laminaria ochroleuca de la Pylaie. Two different forms of the vanadium haloperoxidases were purified from L. saccharina and L. hyperborea and one form from L. ochroleuca species. Reconstitution experiments in the presence of several metal ions showed that only vanadium(V) completely restored the enzymes activity. The stability of some enzymes in mixtures of buffer solution and several organic solvents such as acetone, ethanol, methanol and 1-propanol was noteworthy; for instance, after 30 days at least 40% of the initial activity for some isoforms remained in mixtures of 3:1 buffer solution/organic solvent. The enzymes were also moderately thermostable, keeping full activity up to 40 degrees C. Some preliminary steady-state kinetic studies were performed and apparent Michaelis-Menten kinetic parameters were determined for the substrates iodide and hydrogen peroxide. Histochemical studies were also performed in fresh tissue sections from stipe and blade of L. hyperborea and L. saccharina, showing that haloperoxidase activity was concentrated in the external cortex near the cuticle, although some activity was also observed in the inner cortical region.

Dissociation constants of Br∅nsted acids in D2O and H2O: studies on polyaza and polyoxa-polyaza macrocycles and a general correlation

Abstract Using data on a series of polyaza and polyoxa-polyaza macrocyclic compounds, a satisfactory linear correlation was established between the dissociation constants of their acid forms obtained in deuterium oxide (p K D ) and in water (p K H ). The isotopic effect, Δp K =p K D - p K H , increases with increasing p K , being larger for the weaker acids. The p K D vs. p K H correlation may be used to determine p K D or p K H values when one of the constants is known; the statistical errors of the estimates are not significatively higher than those of the experimental determinations. By including data available in the literature for a series of different acids (mineral and carboxylic acids, phenols, protonated amines, amino acids, etc.; a total of 138 pairs of data for 70 different compounds), a good general p K D vs. p K H correlation was also obtained (correlation coefficient 0.998), showing that the dependence of the isotopic effect on the nature of the acid is not as important as previous studies, based on limited and not always satisfactory Δp k vs. p K H correlations, suggested.

Electrochemical synthesis of adducts of 2-aminopyridine or methanol in metal chelates of a N, N, N-tridentate Schiff base ligand. X-ray crystal structures of the Ni(II) and Zn(II) derivatives

Abstract The electrochemical syntheses of series of novel adducts of 2-aminopyridine and methanol in Co(II), Ni(II), Cu(II) or Zn(II) chelates of the N,N,N-tridentate basic form of the Schiff base 2-N-tosylamino(2′-tosylaminobenzylidene)aniline (H2L), [M(L)L′] (M=Co, Ni, Zn, L′=2-aminopyridine; M=Cu, L′=CH3OH), were performed by using the corresponding metal as a sacrificial anode. The compounds were characterized by elemental analysis, IR spectroscopy, FAB mass spectrometry, 1H NMR and magnetic measurements. The crystal structures of the Zn and Ni derivatives as well as of the Schiff base have been determined by X-ray diffraction. In the 2-aminopyridine complexes this ligand is bound to the metal through the endocyclic nitrogen whereas the aminogroup is involved in intramolecular NH⋯O hydrogen bonds with one of the tosyl SO2 groups.

Saccorhiza polyschides (phaeophyceae; phyllariaceae) A new source for vanadium-dependent haloperoxidases

Abstract Vanadium-dependent iodoperoxidases from the brown seaweed Saccorhiza polyschides (Lightfoot) Batters, collected at three different locations along the Portuguese west coast, were extracted, purified and characterized. Several extraction procedures were tested, including two-phase aqueous systems. The purification of the iodoperoxidases was achieved using hydrophobic interaction chromatography followed by chromatofocusing. It was possible to isolate three different isoforms of the enzyme, which show mainly iodoperoxidase activity. The three native enzymes have a relative M r around 125 kDa, and two subunits of M r about 64 kDa. Reactivation studies of the apoenzymes with several metal ions revealed that vanadium(V) was essential for enzymatic activity. These enzymes are remarkably thermostable, maintaining their maximum activity up to 50°. The kinetic parameters for the enzyme catalysed iodoperoxidase reaction were obtained at pH 6.1. In the concentration range studied (0.2–8 mM) there was no inhibition by H 2 O 2 whereas iodide inhibition was already apparent at the top values of the concentration range studied (2–25 mM).

Energy transfer in complexes between a crown ether and various lanthanide salts

Abstract The spectroscopic properties of complexes between benzo-15-crown-5 and europium and terbium nitrates, thiocyanates and perchlorates were investigated. The ligand fluorescence quantum yield in all the complexes is less than that of the uncomplexed crown ether, and in the complexes with terbium salts this quenching is accompanied by a sensitized emission from the metal ion. An intramolecular energy transfer from the excited singlet of the ligand to the metal ion is suggested to explain the emission from the metal ion. It is proposed that the charge transfer states associated with the lanthanide salts compete with the metal ion for the intramolecular transfer of excitation, and that this is important for the nitrate and thiocyanate complexes.

STUDIES ON URANYL COMPLEXES. III. URANYL COMPLEXES OF EDTA.

The uranyl complexes of EDTA have been studied by potentiometry ; stability constants of the 1:1 and 2:1 (metal to ligand) chelates have been determined, as well as the respective hydrolysis and polymerization constants. Possible structures for these species are discussed. To account for the abnormally high stability of UO(2)(H(2)O)HL-, hydrogen bonding between a protonated nitrogen atom of the ligand and one oxygen atom of UO(2)(2+) is suggested.

Stability constants of chloro-complexes of cadmium(ii) in sea-water medium

Potentiometry with ion-selective electrodes and anodic stripping voltammetry were used to measure the stability constants for cadmium chloro-complexes at 20 +/- 0.1 degrees in synthetic sea-water and at an ionic strength of 0.700 (NaClO(4) + NaCl). The values of beta(ML) and beta(ML(2)) obtained by the two methods are in good agreement.