Mohamed S. El-Ezaby

Complexes of vitamin B6. XVII: Crystal structure and molecular orbital calculations of the dichloro-bis-pyridoxol palladium (II) complex

Abstract The structure of dichloro-bis-pyridoxol palladium(II) complex [PdCl2(C8H11O3N)2] has been determined from three dimensional X-ray data collection. The complex crystallizes in the monoclinic space group P21/c with Z=2 and cell dimensions a=5.265(3), b=17.250(6), c=10.253(6) A, β= 95.40(2)°. The structure was refined to a final R factor of 0.060 for 1813 reflections with F ⩾ 3σ(F). The palladium atom lies in a symmetry center of inversion in a square plane coordinated to two chlorine atoms and two pyridine nitrogen atoms. Charge distributions and bond order matrix calculated from ARCANA-MO are given.

Complexes of vitamin B6—I Copper (II) and nickel (II) complexes of pyridoxol

Abstract The reaction of pyridoxol with copper (II) and nickel (II) ions were examined by spectrophotometric methods. Stability constants, measured at 27°C and 0·5 M NaCl ionic strength, have been determined using traditional as well as computerized least square procedures. By a consideration of the stability constants of the complexes of related ligands with copper (II) and nickel (II) ions, it is concluded that pyridoxol acts as a monodentate ligand, the heterocyclic ring nitrogen atom being the ligand atom.

Equilibrium studies of some divalent metal ions complexes with pyridoxol, pyridoxal and pyridoxamine

Abstract The interaction of vitamin B6 compounds, pyridoxol, pyridoxal and pyridoxamine with metal ions Mn2+, Co2+, Ni2+ and Hg2+ are studied. Values for stability constants are evaluated and the reaction stiochiometries are investigated in aqueous 0.50 M KCl at 25°C. It is shown that only pyridoxamine possesses a fairly good affinity to interact with metal ions. A linear relation holds between the stability constants of the first complex and the ionization potential of the divalent metal ions.

Equilibrium and kinetic studies of the Fe(III) complex formation with glycinehydroxamic acid

Abstract Spectrophotometric methods were utilized for stability constant determinations of the Fe(III) interaction with glycinehydroxamic acid (GX) at I = 0.15 M NACl and T = 25°C. Program SQUAD II was used to assess the absorbance data in the wavelength range 300–520 nm. Four constants were determined for 1:1:1, 1:1:0, 2:1:1 or 3:1:3 and 2:1:0 complex species in the pH range 1.0–7.5. The kinetics of the interactions of Fe(III) with GX were also studied in the pH range 1.0–3.0 by the stopped flow method. The observed rate constant at a given pH was k obs = A + BT GX. The parameters A and B are functions of pH in the range 1.7–3.0 and only A is a function of pH in the range 1.0–1.7. The mechanism of complex formation was discussed in the light of the experimental results and the equilibrium study. It has been concluded that FeOH 2+ is the reactive species in the complex formation of FeGXH 3+ species while Fe(OH) 2 + is the reactive species in the complex formation of FeGX 2+ species.

Complexes of hydroxamates. II: Mechanistic studies involved in the nickel−glycinehydroxamate−pyridoxal system

Abstract The formation constants of the binary complexes of Ni(II) with glycinehydroxamic acid (GX) were determined by pH-metric titration at 25°C and I = 0.15 M NaCl. In addition, the mechanism of complex formation was studied under the same experimental conditions as the equilibrium study using the stopped-flow technique. It has been concluded that Ni 2+ and NiOH + were the active species in the complex formation reactions. The interaction of GX with pyridoxal (PL) in the absence of metal ions was proved to occur at pH ≥ 5 by polarographic and spectrophotometric techniques, and was found to be fast enough not to be followed by the stopped-flow technique. The formation constants were also determined by pH-metric titration. However, the reaction of PL wth Ni(II)-GX was investigated at pH > 7.0. Ternary-complex formation was proved to exist prior to the possible formation of the Schiff base metal complex. Furthermore, deprotonation of the hydroxamic NH group was found compatible with the proposed reaction mechanism.

Interaction of Phosphate with Iron(III) in Acidic Medium, Equilibrium and Kinetic Studies

Equilibrium reactions of iron(III) with phosphate were studied spectrophotometrically by UV-Vis in the pH range of ∼ 1.0-2.20. The STAR-94 Program was used to determine the number of absorbing species as well as the stoichiometries and formation constants of the complex species. Some literature values were further confirmed and new values of different stoichiometries were obtained. The kinetics and mechanism of Fe(III) with phosphate were studied in acidic medium. The reactive phosphate species were found to be only H3PO4 and H2PO− 4 and for Fe(III) were only Fe3+, FeOH2+ and Fe(OH)+ 2. The observed rate constants were pH as well as Tphos (total concentration of phosphate) dependent, i.e. Kobs,i = A i + B i Tphos + C i T2 phos (at a given pH).

Complexes of vitamin B6—III: Polarographic determination of the stability constants of cadmium(II) and zinc(II) complexes with pyridoxamine☆

Abstract The reactions of pyridoxamine with cadmium(II) and zinc(II) were followed polarographically. Stability constants measured at 25°C and 0.1 M NaCl ionic strength, have been determined by graphical and numerical methods. A conclusion was drawn that 1:1 and 1:2 mononuclear complexes were formed in solution in both systems. Moreover, 1:3 and 1:4 mononuclear complexes were formed in case of zinc(II)-pyridoxamine system.

Complexes of hydroxamates VI: Binary and ternary complexes involved in the palladium(II) monohydroxamic acids—glycylglycine systems

Abstract Binary complexes of dl -serine-, l -histidine-, glycine-, aceto-, methionine- hydroxamic acids as well as glycylglycine (Glygly) with Pd(II) have been studied in solution. Only Pd(II) complexes with methionine hydroxamic (MX) acid were successfully studied potentiometrically because the other hydroxamic acids formed instant black precipitates. The potentiometric data for Pd(II)-MX were assessed by the SUPERQUAD program. The formation constants of only two species were determined in a limited pH range. The ternary complexes of Pd(II) with Glygly and MX were also studied, in which only two species were formed. It has been possible to determine only one species for the binary interaction of Pd(II) with Glygly, which is not usually simple to obtain in the absence of MX. The differential pulse polarographic technique was applied to the binary and ternary complexes involved in the system of Pd(II)-MX-Glygly in a wider pH range than that in the potentiometric study.

Complexes of hydroxamates. Part 1. Interaction of methioninehydroxamic acid with iron(III) in aqueous solution

Abstract The binding affinity of Fe(III) to methioninehydroxamate (MX) has been studied spectrophotometrically at I=0.15 M NaCl and T=25 °C. Equilibrium data have been assessed by the program SQUAD(II) in the wavelength range 400–550 nm and in the pH range 1.5–5.0. Five formation constants were determined for the species Fe(MX)(H)3+, Fe(MX)2+, Fe(MX)2(H)23+, Fe(MX)2(H)2+ and Fe2(MX)33+. The stopped-flow kinetic data studied at 470 nm and in the pH range 1.0–3.0 is collectively expressed by the following rate equation at a given pH Rate=(A + BTMX)TMX where TMX=the analytical connection of MX and the parameters A and B are both functions of pH in the range 1.7–3.0, but only A in the range 1.2– 1.7. A proposed mechanism was discussed, based on the equilibrium study, where the role of the chloro species of Fe(OH)2+ and Fe(OH)2+ in the complex formation of Fe(III) with MX has been emphasized. Correlation of the results with pertinent systems has also been discussed.

Binary, ternary and quaternary complexes invloved in the systems pyridoxamine-glycine-imidazole with some bivalent metal ions

Abstract Equilibrium-based computer models using MINIQUAD-75 program were utilized to determine the stoichiometry and formation constants involved in the systems pyridoxamine(Pm)-glycine (Gly)-imidazole (lmd) with CO(II), Ni(II), Cu(II), Zn(II) and Cd(II) metal ions. The data were obtained from potentiometric pH titration of the various binary and ternary quaternary systems under physiological-like conditions (0.15 M NaNO3-37°C). Various composition ratios of metal and ligands were used. The ligand concentrations did not exceed 4 times the concentration of metal ion in the binary systems and 4 times of the metal ions in ternary systems. In case of the quaternary systems only imidazole concentrations were two or four times the concentrations of metal ions keeping those of other ligands equal to that of metal ions. The stability constants of the quaternary species are discussed in terms of binary and ternary constants as are the effect of ring size on the stability of mixed ligand species. In addition, electrostatic as well as statistical effects also are mentioned and the biological implications of these model equilibria are described.