Divalent Metal Salts Research Articles

Transition Metal(II) Complexes with Cefotaxime-Derived Schiff Base: Synthesis, Characterization, and Antimicrobial Studies

New [ML2(H2O)2] complexes, where M = Co(II), Ni(II), Cu(II), and Zn(II) while L corresponds to the Schiff base ligand, were synthesized by condensation of cefotaxime with salicylaldehyde in situ in the presence of divalent metal salts in ethanolic medium. The complexes were characterized by elemental analyses, conductance, and magnetic measurements, as well as by IR and UV-Vis spectroscopy. The low values of the molar conductance indicate nonelectrolyte type of complexes. Based on spectral data and magnetic moments, an octahedral geometry may be proposed for Co(II), Ni(II), and Zn(II) complexes while a tetragonal geometry for Cu(II) complex. Molecular structure of the Schiff base ligand and its complexes were studied using programs dedicated to chemical modeling and quantomolecular calculation of chemical properties. All the synthesized complexes were tested for in vitro antibacterial activity against some pathogenic bacterial strains, namely Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Bacillus subtilis, and Staphylococcus aureus. The MIC values shown by the complexes against these bacterial strains revealed that the metal complexes possess superior antibacterial activity than the Schiff base.

Synthesis and electrochemistry of metallophthalocyanines bearing {4-[(2E)-3-(3,4,5-trimethoxyphenyl)prop-2-enoyl]phenoxy} groups

In this study, metallophthalocyanines 4, 5, 6 was carried out by the cyclotetramerization of 4-{3-[(2E)-3-(3,4,5-trimethoxyphenyl)prop-2-enoyl]phenoxy}phthalonitrile 3. The synthesis of metallo derivatives [Zn(II), Co(II), Cu(II)] of phthalocyanines obtained from corresponding phthalonitrile derivative and in the presence of the anhydrous divalent metal salts (Zn(CH3COO)2, CoCl2 and CuCl2) was described. These phthalocyanines showed good solubility in organic solvents such as chloroform, tetrahydrofuran, DMSO and DMF. The newly synthesized Pcs were verified by IR, 1H NMR, 13C NMR, UV–vis and MS spectral data. The electrochemical properties of the zinc(II), copper(II) and cobalt(II) phthalocyanines were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods. The cobalt complex showed a metal based reduction process, while zinc phthalocyanines were giving ligand based electron transfer processes.

Biocatalytic Ketone Reduction - A Study on Screening and Effect of Culture Conditions on the Reduction of Selected Ketones

Microbial conversions are gaining importance in the synthesis of important drug metabolites and their intermediates as they are good alternative to chemical synthesis since they are enantio-selective and regio-selective and even can be carried out at ambient temperature and atmospheric pressure. Till date, biocatalytic reduction of acetophenone and its derivatives has been widely reported. In the present study, we have made an attempt to carry out the microbial bioreduction of o-hydroxyacetophenone by screening some of the selected microorganisms which were obtained from culture collection centre as well as those which are isolated in our Microbiology lab. The selected microorganisms include Aspergillus ochraceous, Aspergillus flavus, Aspergillus tubingenesis, Aspergillus niger, Rhizopus stolanifer MTCC 162, Rhizopus stolanifer MTCC 2591 and Baker’s yeast.Among the seven microorganisms screened for the bioreduction of o-hydroxyacetophenone, Baker’s yeast and Aspergillus tubingenesis showed significant bioconversion where as Aspergillus ochraceous exhibited the least bioconversion.In our earlier study it was found that Aspergillus flavus has the required bioreductase enzyme, which showed the maximum conversion of p-chloroacetophenone to p-chlorophenylethanol. Hence optimization of culture conditions to get maximum enzyme expression and hence maximum conversion was thought off. The parameters considered for the study include effect of various Carbon sources, Nitrogen source, Metal ions, incubation Temperature and media pH on enzyme expression. The optimized culture a condition at which maximum bioconversion was achieved was maltose among various carbon sources. Tryptone was found to have maximum effect among the nitrogen sources. Media pH 7.6 and incubation temperature of 35 °C was found to be favourable for maximum enzyme activity. Among various divalent metal salts, addition of magnesium sulphate to the media significantly increased the enzyme activity.

Biocatalytic Ketone Reduction - A Study on Screening and Effect of Culture Conditions on the Reduction of Selected Ketones

Microbial conversions are gaining importance in the synthesis of important drug metabolites and their intermediates as they are good alternative to chemical synthesis since they are enantio-selective and regio-selective and even can be carried out at ambient temperature and atmospheric pressure. Till date, biocatalytic reduction of acetophenone and its derivatives has been widely reported. In the present study, we have made an attempt to carry out the microbial bioreduction of o-hydroxyacetophenone by screening some of the selected microorganisms which were obtained from culture collection centre as well as those which are isolated in our Microbiology lab. The selected microorganisms include Aspergillus ochraceous, Aspergillus flavus, Aspergillus tubingenesis, Aspergillus niger, Rhizopus stolanifer MTCC 162, Rhizopus stolanifer MTCC 2591 and Baker’s yeast.Among the seven microorganisms screened for the bioreduction of o-hydroxyacetophenone, Baker’s yeast and Aspergillus tubingenesis showed significant bioconversion where as Aspergillus ochraceous exhibited the least bioconversion.In our earlier study it was found that Aspergillus flavus has the required bioreductase enzyme, which showed the maximum conversion of p-chloroacetophenone to p-chlorophenylethanol. Hence optimization of culture conditions to get maximum enzyme expression and hence maximum conversion was thought off. The parameters considered for the study include effect of various Carbon sources, Nitrogen source, Metal ions, incubation Temperature and media pH on enzyme expression. The optimized culture a condition at which maximum bioconversion was achieved was maltose among various carbon sources. Tryptone was found to have maximum effect among the nitrogen sources. Media pH 7.6 and incubation temperature of 35 °C was found to be favourable for maximum enzyme activity. Among various divalent metal salts, addition of magnesium sulphate to the media significantly increased the enzyme activity.

Selective elution of nickel and cobalt from iminodiacetic acid cation exchange resin using ammoniacal solutions

Nickel and cobalt were eluted from iminodiacetic acid functionalized ion exchange resin by ammoniacal eluents. The highest recovery of nickel and cobalt was achieved using a mixture of aqueous ammonia, ammonium sulphate, and magnesium sulphate. The use of ammonium sulphate yielded a higher extent of nickel desorption than the use of ammonium carbonate. The presence of a divalent metal salt such as magnesium sulphate, calcium sulphate, or manganese sulphate was found to increase the extent of nickel desorption. Magnesium was found to be most suitable for this purpose. Nickel and cobalt on the resin are displaced by magnesium during this process. The selectivity of this elution process varies by metal. Aluminium and iron show little or no affinity for ammoniacal elution. Calcium and manganese desorb to a limited extent. The kinetics of nickel desorption in aqueous ammonia–ammonium sulphate–magnesium sulphate solution are slower than that of sulphuric acid elution. When resin loaded with magnesium is contacted with aqueous nickel sulphate, nickel sulphate will displace magnesium on the resin. This reaction occurs on a similar time scale as that of protonated resin.

Synthesis, characterization, and crystal structures of cobalt(II), copper(II), and zinc(II) complexes of a bidentate iminophenol

A bidentate iminophenol (HL = 2-((4-methoxyphenylimino)methyl)-4,6-di-tert-butylphenol derived from condensation of 4-methoxyaniline and 3,5-di-tert-butyl-2-hydroxybenzaldehyde) was mixed with divalent metal salts to form the corresponding mononuclear metal complexes [MII(L)2] (M = Co (1), Cu (2), and Zn (3)). The complexes are characterized by different spectroscopic and analytical tools. X-ray crystal structures of the complexes revealed homoleptic mononuclear complexes with MN2O2 coordination. The cobalt(II) (1) and zinc(II) (3) complexes display a pseudo-tetrahedral coordination geometry, whereas the copper(II) complex (2) exhibits a distorted square-planar coordination. The zinc(II) complex (3) emits at 460 nm with a twofold enhancement of emission with respect to the free iminophenol.

Support materials for yeast immobilization affect the concentration of metal ions in the fermentation medium

Hydrogels are potential absorbers of ions present in the medium, but they can also release some cations (e.g. calcium in calcium alginates). Polyelectrolytes such as alginic acid and poly(acrylic acid) strongly bind polyvalent metal ions. In this paper, the impact of some materials used for the immobilization of yeast, in particular the concentration of certain ions, especially those that play a significant role in the process of fermentation such as calcium, magnesium, zinc and manganese, was assessed. The support for the immobilization of yeast (calcium alginate and polyacrylate) significantly affected the concentration of metal ions in a model solution. Calcium alginate decreased the concentration of Mg2+, Zn2+ and Mn2+ in the solution by about 10–25%, and greatly increased the Ca2+ concentration. Polyacrylate, while swelling, decreased the concentration of cations present by up to 50%. Based on the results in this work, no supplementation of wort with divalent metal salts is advised, since it may lead to a stronger cross-linking of polyelectrolytes used, and consequently result in a slower fermentation. Similarly, the relatively high concentration of calcium ions, caused by ion exchange, did not appear to affect the fermentation process. Copyright © 2013 The Institute of Brewing & Distilling

Parallel chain polyrotaxane, layer, and diamondoid divalent metal coordination polymers containing para aromatic dicarboxylate and bis(4-pyridylmethyl)piperazine ligands

Hydrothermal reaction of a closed-shell divalent metal salt, a para aromatic dicarboxylate precursor, and the long-spanning dipyridyl ligand bis(4-pyridylmethyl)piperazine (4-bpmp) afforded three new coordination polymers, which were structurally characterized by single-crystal X-ray diffraction. [Cd(tere)(4-bpmp)(H2O)]n (1, tere=terephthalate) exhibits a simple non-interpenetrated 66 diamondoid 3D network with pentagonal bipyramidally coordinated cadmium ions. [Zn2(tere)(4-bpmp)2Cl2]n (2), which possesses tetrahedrally coordinated zinc atoms, manifests an exceptionally rare 1D+1D→1D parallel interpenetrated threaded-loop polyrotaxane structure. Extension of the para aromatic dicarboxylate pendant arms by employing the 1,4-phenylenediacetate (1,4-phda) ligand generated [Zn(1,4-phda)(4-bpmp)(H2O)2]·2H2O}n (3), which shows a simple stacked (4,4) grid structure. Luminescent properties of these materials are also discussed.

Synthesis, characterization of metal-free, metallophthalocyanines and catalytic activity of cobalt phthalocyanine in cyclohexene oxidation

In this work, the new phthalonitrile derivative 3 bearing 4-pentylphenol 1, metal-free phthalocyanine 4, metallophthalocyanines 5 and 6 have been synthesized. Metal free phthalocyanine 4 and metallophthalocyanines 5 and 6 bearing four 4-(4-pentylphenoxy) groups on the periphery have prepared by cyclotetramerization of a new precursor, namely 4-(4-pentylphenoxy)phthalonitrile 3 in the presence of the corresponding divalent metal salts [Zn(II), Co(II)]. The new compounds have been characterized by IR, 1H NMR, 13C NMR and MS spectra data. The new cobalt phthalocyanine 6 has tested as a catalyst for the oxidation of cyclohexene with different oxidants, such as tert-butylhydroperoxide (TBHP), m-chloroperoxybenzoic acid (m-CPBA) and hydrogen peroxide (H2O2), in organic solvent. It is seen that complex 6 can selectively oxidize cyclohexene to give 2-cyclohexen-one as major product, 2-cyclohexen-ol and cyclohexene oxide as minor product. Finally, because of the minimal destruction of the catalyst, higher selectivity and conversion, TBHP has found to be best oxidant.

Synthesis, characterisation and electrochemical investigation of phthalocyanines with pendant 4‐{2‐[2‐(4‐tert‐butylphenoxy)ethoxy]ethoxy} substituents

In this study, a novel metal‐free phthalocyanine and three metallophthalocyanines carrying four 2‐[2‐(4‐tert‐butylphenoxy)ethoxy groups on the periphery were prepared by cyclotetramerisation of a dinitrile derivative in the presence of the corresponding divalent metal salts [zinc(ii), cobalt(ii), copper(ii)]. These new phthalocyanine compounds have been characterised by infrared, 1H and 13C nuclear magnetic resonance and electrospray mass spectroscopies and elemental analysis. The electrochemical properties of the metal‐free, zinc(ii) and cobalt(ii) phthalocyanines were investigated by cyclic voltammetry and differential pulse voltammetry methods. The cobalt complex showed a metal‐based reduction process, while the metal‐free and zinc(ii) phthalocyanines showed ligand‐based electron transfer processes. It has been found that the absorption spectra substantially depend on concentration. It has been shown that these changes are attributable to the association of the phthalocyanine molecule. The number of molecules in the associates and the equilibrium constants for this association are determined.

Single step hydrothermal based synthesis of M(II)Sb 2 O 6 (M = Cd and Zn) type antimonates and their photocatalytic properties

Experiments involving single step hydrothermal reactions of the divalent metal (Zn2 + , Cd2 + , Pb2 + , Cu2 + , Ni2 + and Mn2 + ) salts with ilmenite NaSbO3 yielded pure divalent antimonates in the case of CdSb2O6 crystallizing in the PbSb2O6 type structure and ZnSb2O6 crystallizing in the trirutile structure type. In the case of Pb2 + , Cu2 + , Ni2 + and Mn2 + divalent cations, phase pure product could not be obtained. The obtained powders were characterized by powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and UV-visible diffuse reflectance spectroscopy. The oxide powders obtained possessed lower crystallite size as compared to their solid-state synthesized counterparts. This was evident from the broadening of the powder X-ray diffraction peaks. The antimonates were photocatalytically active for the decomposition of methylene blue (MB) dye under UV light irradiation.

Employing Dianionic Macrocyclic Tetracarbenes To Synthesize Neutral Divalent Metal Complexes

Macrocyclic tetraimidazolium diborate ligand precursors with two different ring sizes have been synthesized by ring-forming reactions between diimidazoles and haloboranes. Deprotonation of the macrocyclic tetraimidazoliums with n-butyllithium followed by the addition of divalent metal salts of palladium or nickel leads to macrocyclic tetracarbene complexes with an 18-atom macrocycle, but not the 16-atom variant. These neutral palladium and nickel complexes are the first examples of macrocyclic tetracarbene diborate complexes, and unlike their cationic counterparts, they are highly soluble in nonpolar solvents. All macrocyclic tetraimidazoliums and their corresponding metal complexes were characterized by single-crystal X-ray diffraction and spectroscopic techniques.

Thermal solid-state polymerization of a divalent metal salt of an unsaturated carboxylic acid and the effects of additives

Thermal solid-state polymerization of zinc diacrylate (ZDA) alone and with additives, such as diphenyl disulfide (DPDS) and thiophenol (TP) derivatives and dicumyl peroxide (DCP), was investigated. The solid-state polymerization of ZDA was confirmed to be non-topochemical, and the main polymer structure was found to be polyacrylate with acrylate or polyacrylate connected by Zn2+ although a part of the monomer structure was eliminated during the polymerization. The polymerization rates were estimated by conversion from ZDA to its polymer, which was determined gravimetrically. The qualitative order of the polymerization rate was ZDA<ZDA-DPDS, ZDA-TP << ZDA-DPDS-DCP, ZDA-TP-DCP<ZDA-DCP, in which each additive was mixed with a 1/60 molar ratio of ZDA. This trend could be explained by the initiation and chain-transfer abilities for radical polymerization of the additives. Mixing these additives in a small amount to the solid monomers is a facile and efficient method to accelerate the solid-state polymerization. Zinc diacrylate (ZDA) gave the polymer as shown in the scheme by heating in the solid state. When additives like diphenyl disulfide (DPDS), thiophenol (TP) and dicumyl peroxide (DCP) were mixed in 1/60 molar ratio to ZDA, qualitative order of the polymerization rate was obtained to be ZDA < ZDA-DPDS(or TP) << ZDA-DPDS(or TP)-DCP < ZDA-DCP. Namely, mixing these additives to ZDA accelerated the solid-state polymerization.

A Recyclable Trinuclear Bifunctional Catalyst Derived from a Tetraoxo Bis‐Zn(salphen) Metalloligand

A tetraoxo bis-Zn(salphen) supramolecular host can bind various divalent metal salts, thereby providing access to trinuclear bifunctional systems that incorporate both Lewis acid sites and dynamically bound nucleophilic anions. The formation of these trinuclear species was investigated and their stability features were also determined. The application of these trinuclear complexes as bifunctional catalysts was evaluated in the formation of cyclic organic carbonates from epoxides and CO(2). The catalytic data, in combination with control experiments, clearly demonstrate that these trinuclear compounds show much higher recycling potential compared to various control compounds and they can be used in up to five cycles without an observable loss in activity. Furthermore, this new recyclable catalytic system does not require any additives and can be applied under solvent-free conditions.

Coordination polymers of tetrazole-yl acylamide with octahedrally coordinated divalent transition metals: the effects of metal centers and side-groups on the structural topologies and symmetries

Hydrothermal reactions of divalent transition metal salts with two different-sized tetrazole-yl acylamide ligands, N-(1H-tetrazol-5-yl)acetamide (H-NTAA) and N-(1H-tetrazol-5-yl)propionamide (H-NTPA), led to six crystalline coordination polymers, named Cd(NTAA)2 (1), Zn(NTAA)2·∼H2O (2), Cu(NTAA)2 (3), Mn(NTAA)2 (4), Cd(NTPA)2 (5), Mn(NTPA)2 (6), respectively. The X-ray diffraction analysis revealed that compound 2 is a 1D zigzag chain, compounds 3 and 4 exhibit an interesting 2D 44-sql network, while compounds 1, 5 and 6 possess a 3D non-interpenetrated diamondoid framework. Wherein, compounds 1, 4, 5 and 6 crystallized in an acentric space group with significant second harmonic generation (SHG) efficiencies. The effect of metal ions and that of small aliphatic acyl side-groups in semirigid tetrazole-yl acylamide ligands on the structural topologies and symmetries of generated coordination polymers are discussed.

Mechanics and molecular filtration performance of graphyne nanoweb membranes for selective water purification

Two-dimensional carbon materials such as the 2D nanoweb-like graphyne membrane are promising as molecular sieves for energy and environmental applications. Based on the application of water purification - the removal of contaminants from wastewater and seawater - here we use molecular dynamics (MD) simulations to investigate the interplay between mechanical forces, filtration mechanisms, and overall performance for graphyne membranes with different pore sizes. We carry out biaxial tensile tests and verify the superior mechanical robustness and tolerance of graphyne membranes against possible deformations from the membrane installation process. A possible ultimate stress in excess of 15 GPa and an ultimate strain of 1.2-2.7% are determined. We also demonstrate their excellent filtration performance with barrier-free water permeation and perfect rejection of the representative contaminants considered here, including divalent heavy metal salts (copper sulfate), hydrophobic organic chemicals (benzene and carbon tetrachloride), and inorganic monovalent salts (sodium chloride). We find that graphtriyne, with an effective pore diameter of 3.8 Å, exhibits an optimal purification performance, because the contaminant rejection rate is more sensitive to pore size than water permeability. In addition, we find that the hydrophobic graphyne membranes exhibit higher rejection rates for hydrophilic contaminants compared to the hydrophobic ones. This size exclusion effect is a result of the larger hydrated radii of hydrophilic species due to stronger interactions between them and water molecules. Finally, we find that the maximum deformation of graphtriyne at the ultimate strain before material failure has only a minor impact on its filtration performance. One of the advantages of using graphyne for water purification is that no chemical functionalization or defects need to be introduced, which maintains the structural integrity of the membrane, and possibly, the long-term device performance.

Adsorption of the Cationic Surfactant Benzyldimethylhexadecylammonium Chloride at the Silica–Water Interface and Metal Salt Effects on the Adsorption Kinetics

The adsorption of the cationic surfactant benzyldimethylhexadecylammonium (BDMHA(+)) chloride has been studied at the hydrophilic silica-water interface by Raman spectroscopy in total internal reflection geometry (TIR Raman). This Raman spectroscopic technique takes advantage of an evanescent electric field that is generated at the silica-water interface in TIR mode with specific probing depth. The present study demonstrates the capabilities of the TIR Raman sampling configuration to provide structural information and simultaneously serve as an experimental platform for studying thermodynamic and kinetic properties of BDMHA(+)Cl(-) at the silica-water interface at neutral pH and compare its adsorption behavior with the modified adsorption properties in the presence of four different concentrations of a divalent metal salt. Spectral analysis of the Raman scattering intensities as a function of time and concentration provided the input data for evaluating adsorption properties of the surfactant in the absence and presence of the metal salt additive. Addition of the magnesium metal salt lowered the cmc, altered the surface excess of the surfactant, and increased the Langmuir adsorption constants, as well as the magnitude of the free energy of adsorption, and adsorption kinetics, proportional to the concentrations of the metal salt. Adsorption isotherms based on a modified Langmuir adsorption model were established for five systems: the pure surfactant in aqueous solution, and the surfactant in the presence of 5, 10, 50, and 100 mM of magnesium chloride. The metal salt did not enhance surfactant adsorption at very low surfactant concentrations below 5 μM, where adsorption occurs by electrostatic attraction; the divalent metal salt, however, favorably influenced the adsorption behavior in the aggregate formation region by reducing the electrostatic repulsion between the polar surfactant head groups, and enhancing the hydrophobic effect between the hydrophobic surfactant alkyl chains and the polar water molecules.

Synthesis and electrochemical investigation of phthalocyanines with dendritic bulky ethereal substituents

Metal-free and metallophthalocyanines carrying four dendritic 3,4-bis(benzyloxy)benzyloxy groups on the periphery were prepared by cyclotetramerization of a new precursor, namely 4-[3,4-bis(benzyloxy)benzyloxy]phthalonitrile (2) in the presence of the corresponding divalent metal salts (Zn(II), Co(II)). The novel phthalonitrile derivative 2 was synthesized by the reaction of 4-nitrophthalonitrile and 3,4-dibenzyloxybenzyl alcohol (1), which was prepared by the catalytic reduction of 3,4-dibenzyloxybenzaldehyde with NaBH4 in ethanol. The new compounds were characterized by elemental analysis, together with FT-IR, 1H NMR and UV–Vis spectra. The electrochemical and in situ spectroelectrochemical measurements showed that while the cobalt phthalocyanine complex (5) gives both metal-based and ring-based redox processes, the metal-free and zinc phthalocyanine complexes (3 and 4) give only ring-based electron transfer processes. An in situ electrocolorimetric method was applied to investigate the colour of the electrogenerated anionic and cationic forms of the novel phthalocyanine complexes carrying bulky ethereal aryloxy substituents.

Novel water‐soluble metallophthalocyanines supported on cotton fabric

In this study, two novel phthalonitrile derivatives were firstly prepared starting from 4‐nitrophthalonitrile: substitution of the nitro group with 3‐mercaptopropionic acid and 4‐mercaptobenzoic acid gave two precursors, namely, 4‐(carboxyethylsulphanyl) phthalonitrile and 4‐(carboxyphenylsulphanyl)phthalonitrile. Zinc(ii), copper(ii) and cobalt(ii) metallophthalocyanines bearing four carboxylic acid groups on the periphery were prepared by cyclotetramerisation of the new precursors in the presence of the corresponding divalent metal salts. The new compounds were characterised by elemental analyses, infrared, ultraviolet–visible, mass and proton nuclear magnetic resonance techniques. All these newly synthesised water‐soluble metallophthalocyanine derivatives were supported on cationic cotton fabrics in the presence of ammonia solution and sodium hydroxide and the dyeing effectiveness, levelness and fastness of these novel dyed fabrics were investigated. These novel dyed fabrics are considered to be useful in the textile industry and also in optical and catalytic applications, especially for air‐purifying materials in air conditioners.

A New Strategy To Stabilize Oxytocin in Aqueous Solutions: II. Suppression of Cysteine-Mediated Intermolecular Reactions by a Combination of Divalent Metal Ions and Citrate

A series of studies have been conducted to develop a heat-stable liquid oxytocin formulation. Oxytocin degradation products have been identified including citrate adducts formed in a formulation with citrate buffer. In a more recent study we have found that divalent metal salts in combination with citrate buffer strongly stabilize oxytocin in aqueous solutions (Avanti, C.; et al. AAPS J.2011, 13, 284-290). The aim of the present investigation was to identify various degradation products of oxytocin in citrate-buffered solution after thermal stress at a temperature of 70 °C for 5 days and the changes in degradation pattern in the presence of divalent metal ions. Degradation products of oxytocin in the citrate buffer formulation with and without divalent metal ions were analyzed using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). In the presence of divalent metal ions, almost all degradation products, in particular citrate adduct, tri- and tetrasulfides, and dimers, were greatly reduced in intensity. No significant difference in the stabilizing effect was found among the divalent metal ions Ca(2+), Mg(2+), and Zn(2+). The suppressed degradation products all involve the cysteine residues. We therefore postulate that cysteine-mediated intermolecular reactions are suppressed by complex formation of the divalent metal ion and citrate with oxytocin, thereby inhibiting the formation of citrate adducts and reactions of the cysteine thiol group in oxytocin.