Hayat M. Marafie

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.

Complexes of vitamin B6, part VII: Ternary complexes of cobalt(II), nickel(II), and copper(II) involving pyridoxamine and some amino acids☆

Abstract The stability constants of the ternary Cu(II), Ni(II), and Co(II) complexes containing pyridoxamine (PM) and as a second ligand (L) glycine, DL-alanine, DL-valine, and β-phenylalnine were determined by pH-metric titration in 0.50 M KNO3 at 30°C. The corresponding constants of the equilibrium, log X, are greater than would be expected for purely statistical reasons (log X = 0.6), except for few complex cases of Co(II). It has been also concluded that amino acids compete more than pyridoxamine for Ni(II) and Co(II) through the formation of 1:2:1:0 species rather than 2:1:1:0 of PM−:L−:M2+:H+.

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 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.

Complexes of hydroxamates V: equilibrium and kinetics of the formation of the binary and ternary complexes involved in the nickel(II)-histidinehydroxamic acid-pyridoxal system

Equilibrium-based computer models utilizing SUPERQUAD program were made to determine the formation constants of the binary complexes of Ni(II) with histidine-hydroxamic acid (HX) from pH-metric titration data at 25 degrees C and I = 0.15 M NaCl. The species were monomeric in the pH range 3.0-8.0. The mechanism of their complex formation was determined using the stopped flow technique under the same experimental conditions of the equilibrium study. It has been concluded that Ni2+ and NiOH- were the active species in the complex formation reactions. Moreover, the reaction of HX with pyridoxal (PL) was studied in the absence of metal-ions by polarographic and spectrophotometric techniques at pH greater than or equal to 5.0. No rates were observed by using the stopped-flow methods. The formation constants for the binary system (HX-PL) and the ternary system (Ni(II)-HX-PL) were also determined by the same program applied on data obtained from pH metric titration at 25 degrees C and I = 0.15 M NaCl. Ternary complex formation involving PL and the species of the Ni(II)-HX system was also investigated kinetically in the pH range of 6.5-10.5. Several rate steps have been observed which have been interpreted qualitatively in terms of sequence of processes involving the condensation of aldehydic form of PL with amino moiety of HX. A comparison with other pertinent systems is also discussed.

Palladium complexes with biological ligands-I. Equilibrium and reaction mechanism of Pd(II) complexes with ethionine

Abstract An equilibrium study has been carried out on the interaction of ethionine(eth) with Pd(II) in aqueous solution at I = 0.16 M (Cl− and 25°C using potentiometic methods. It has been concluded that five complex species exist in the pH range 2.8–4.8. these species are: PdCl3(Eth0H02, PdCl2(Eth)−, PdClOH(Eth)−, Pd(Eth)2(H)2+2 and Pd(Eth)02. In addition, the stopped-flow method has been used to study the reaction kinetics of Pd(II) with Eth. Three kinetic steps were observed in the pH range 1–5.5. These steps are dependent on the total concentration of Eth (TEth) as well as the pH of the medium. The observed pseudo-first order rate constants for the three reaction kinetic steps at constant pH are expressed empirically by kiobs = mi + m′iTEth. The parameters mi and m′i are pH-dependent. It has been concluded that PdCl2−4 and PdCl2OH2− species play an important role in the complex formation reactions with Eth. The data were interpreted in terms of the complex species obtained from the equilibrium study. cis-trans substitution reactions have been suggested to account for some kinetic steps.

Mixed ligand complexes involved in the system pyridoxamine-glycylglycine-imidazole copper(II)

Abstract Ternary as well as quaternary complexes involved in the system pyridoxamine-glycylglycine-imidazole-copper(II) have been studied by pH-metric titration at 37°C and I = 0.15 M (NaNO3). The data were assessed by program MINIQUAD 75. The formation constants were compared with those obtained from the system of pyridoxamine-glycine-imidazole with some bivalent metal ions previously reported. They are ∼ 2.5 log units less. In addition, the enhancement of quaternary complex formation has been described in terms of binary and ternary complex formation reactions. It has been also found that deprotonation of the peptidic proton of glycylglycine took place in the presence of more than one different ligand.

Palladium complexes with biological ligandsIII. Kinetics and reaction mechanism of the interaction of Pd(II) with methionine and S-methyl-l-cysteine☆

Abstract Two rate steps have been observed when excess Pd(II) was reacted with methionine (Meth) and S-methyl- l -cysteine (SMC) in the pH range 1–3.5. The fast step in both systems was attributed to the formation of a mononuclear chelated complex, and the slow step may be due to the effect of hydrolysis and/or polymerization processes. The mechanism of the fast reaction was discussed in terms of the species PdCl42−, [PdCl3(OH)]2−, H2L+ and HL (L = Meth or SMC). It was concluded that [PdCl3(OH)]2− was the more reactive species in the complex formation reaction. It was also found that substitution reactions involving Meth were more labile than those involving SMC.

Complexes of hydroxamates. III: Equilibrium and kinetic studies on the reactions of iron(III) with monohydroxamic acids

Spectral analysis of iron(III) complexes with acetohydroxamate (AX) and histidinehydroxamate (HX) in the UV-visible region revealed that many species may exist in pH range 1.0–7.5. The solution spectra were unstable in pH range ~2.7–4.0. Different species were obtained from fresh solutions and overnight solutions. The difference was rationalized due to hydrolysis and/or polymerization of complexes in solution, especially in pH range 2.7–4.0. The kinetics of the reactions of Fe(III) with AX and HX were accomplished, and mechanisms were suggested for both systems. In both cases, Fe3+ and FeOH2+ species were found to be the active species in the complex formation of 1:1 complex.