Vanadium(IV/V) speciation of pyridine-2,6-dicarboxylic acid and 4-hydroxy-pyridine-2,6-dicarboxylic acid complexes: potentiometry,EPR spectroscopy and comparison across oxidation states

Evaluation of stability of vanadium(IV) and (V) complexes under similar conditions is critical for the interpretation and assessment of bioactivity of various vanadium species. Detailed understanding of the chemical properties of these complexes is necessary to explain differences observed their activity in biological systems. These studies are carried out to link the chemistry of both vanadium(IV) and (V) complexes of two ligands, 2,6-pyridinedicarboxylic acid (dipicolinic acid, H(2)dipic) and 4-hydroxy-2,6-pyridinedicarboxylic acid (H(2)dipic-OH). Solution speciation of the two 2,6-pyridinedicarboxylic acids with vanadium(IV) and vanadium(V) ions was determined by pH-potentiometry at I=0.2 M (KCl) ionic strength and at T=298 K. The stability and the metal affinities of the ligands were compared. Vanadium(V) complexes were found to form only tridentate coordinated 1:1 complexes, while vanadium(IV) formed complexes with both 1:1 and 1:2 stoichiometries. The formation constant reflects hindered coordination of a second ligand molecule, presumably because of the relatively small size of the metal ion. The most probable binding mode of the complexes was further explored using ambient and low temperature EPR spectroscopy for vanadium(IV) and 51V NMR spectroscopy for vanadium(V) systems. Upon complex formation the pyridinol-OH in position 4 deprotonates with pK approximately 3.7-4.1, which is approximately 6 orders of magnitude lower than that of the free ligand. The deprotonation enhances the ligand metal ion affinity compared to the parent ligand dipicolinic acid. In the light of the speciation and stability data of the metal complexes, the efficiency of the two ligands in transporting the metal ion in the two different oxidation states are assessed and discussed.     

Nuclear magnetic resonance studies of the solution chemistry of metal complexes. 26. Mixed ligand complexes of cadmium, nitrilotriacetic acid, glutathione, and related ligands

The complexation of glutathione and related ligands by the nitrilotriacetic acid complex of Cd2+ (Cd(NTA)-) has been investigated by 1H NMR as a model for the coordination chemistry of Cd2+ and GSH in biological systems. Related ligands included glycine, glutamic acid, cysteine, N-acetylcysteine, penicillamine, N-acetylpenicillamine, mercaptosuccinic acid, and the S-methyl derivative of glutathione. The nature of the complexes formed was deduced from 1H NMR spectra of Cd(NTA)- and the ligands. Mixed ligand complexes (Cd(NTA)L) and single ligand complexes (CdLx) are formed with the thiol ligands, whereas only mixed ligand complexes form with glycine, glutamic acid and S-methylglutathione. Formation constants of the mixed and the single ligand complexes were determined from NMR data. The results indicate that formation constants for binding of a thiolate donor group by Cd2+, either as the free ion or in a coordinately unsaturated complex, are in the range 10(5)-10(6).     

Amide Coupling Reaction for the Synthesis of Bispyridine-based Ligands and Their Complexation to Platinum as Dinuclear Anticancer Agents

Amide coupling reactions can be used to synthesize bispyridine-based ligands for use as bridging linkers in multinuclear platinum anticancer drugs. Isonicotinic acid, or its derivatives, are coupled to variable length diaminoalkane chains under an inert atmosphere in anhydrous DMF or DMSO with the use of a weak base, triethylamine, and a coupling agent, 1-propylphosphonic anhydride. The products precipitate from solution upon formation or can be precipitated by the addition of water. If desired, the ligands can be further purified by recrystallization from hot water. Dinuclear platinum complex synthesis using the bispyridine ligands is done in hot water using transplatin. The most informative of the chemical characterization techniques to determine the structure and gross purity of both the bispyridine ligands and the final platinum complexes is ¹H NMR with particular analysis of the aromatic region of the spectra (7-9 ppm). The platinum complexes have potential application as anticancer agents and the synthesis method can be modified to produce trinuclear and other multinuclear complexes with different hydrogen bonding functionality in the bridging ligand.     

Mono-vitamin B6 complexes of palladium(II) and their interactions with nucleosides

The interactions of potassium tetrachloropalladate(II) with the B6 vitamins pyridoxal, pyridoxine, and pyridoxamine in 1:1 molar ratio have been studied. From DMF solutions, the ionic trichloro (pyridoxal or pyridoxine) palladates(II) were isolated. Pyridoxamine, on the other hand, in aqueous solutions gave the dimeric complex bis [mu-chloro-pyridoxaminato-palladium(II)]. In the first two complexes, the ligands coordinated to palladium through their pyridine nitrogen while, in the last one, pyridoxamine acted as a chelating ligand through its phenolic oxygen and aminomethyl nitrogen. All three complexes reacted with nucleosides, yielding the complexes [Pd(PL)(Nucl)Cl2], [Pd(PN)(Nucl)Cl2], and [Pd(PM-H+)(Nucl)Cl], respectively. Those complexes with one ionizable N(1)H imino proton underwent deprotonation, and the new mixed ligand complexes [Pd(PL)(Nucl-H+)Cl], [Pd(PN)(Nucl-H+)], and [Pd(PM-H+)(Nucl-H+)] were formed. In all mixed ligand complexes, the B6 vitamins maintained their coordination modes. The nucleosides, on the other hand, exhibited their usual coordination sites, i.e., in the nondeprotonated complexes, purine nucleosides coordinated only through their N7 atom. In the deprotonated complexes, they acted as bidentate ligands and coordinated through their N7 and O6 atoms. All complexes were characterized with elemental analyses, conductivity measurements, and various spectroscopic techniques.     

Multiple forms of the cobalt(II)-carnosine complex.

Cobalt(II) ion and L-carnosine produce two different complexes when mixed in aqueous solution at pH 7.2. One complex has coordination of N-3 of the imidazole ring to the cobalt(II) and is produced when the concentration of peptide exceeds that of cobalt(II). The second complex has chelation of three nitrogen atoms of a single carnosine. This second complex produces a reversible oxygen carrier by making stable mixed chelates with additional carnosine, histidine or cysteine. These results indicate that cobalt complexes with mixed ligands should be of more importance $\text{in}\text{vivo}$ than those with carnosine as the only ligand. They provide an explanation for the high activity and substrate specificity of carnosinase in kidney.     

Increasing the luminescence of lanthanide complexes.

This review compares the chemical and physical properties of lanthanide ion complexes and of other narrow-emitting species that can be used as labels for cytometry. A series of luminescent lanthanide ion macrocyclic complexes, Quantum Dyes, which do not release or exchange their central lanthanide ion, do accept energy transfer from ligands, and are capable of covalent binding to macromolecules, including proteins and nucleic acids, is described and their properties are discussed.Two methods are described for increasing the luminescence intensity of lanthanide ion complexes, which intrinsically is not as high as that of standard fluorophores or quantum dots. One method consists of adding a complex of a second lanthanide ion in a micellar solution (columinescence); the other method produces dry preparations by evaporation of a homogeneous solution containing an added complex of a second lanthanide ion or an excess of an unbound antenna ligand. Both methods involve the Resonance Energy Transfer Enhanced Luminescence, RETEL, effect as the mechanism for the luminescence enhancement.     

Metal Complexation in Xylem Fluid : II. THEORETICAL EQUILIBRIUM MODEL AND COMPUTATIONAL COMPUTER PROGRAM

Theoretical considerations of metal complex formation in aqueous solutions were used to develop a computer program (CHELATE) to calculate all equilibrium species (free metal ions, metal complexes, etc.) in any user-defined system, such as xylem fluid. Mass-balance equations were established to describe each free metal ion and each free ligand concentration as a function of solution pH, total metal or total ligand, hydrogen-association constants, and the stability constants of known metal complexes. A default data base can be altered by the user to define any desired system covered by the stored equilibrium data. The program can currently handle nine metal ions, 35 ligands, and 500 complex species. The validity of the program was confirmed by using experimental test systems in which free-metal ion activity measurements were made with ion-selective electrodes.     

The relationship between metal-metal distance of two metal ions chelated complex and RNA cleavage activity.

We prepared a series of ligands possessing two binding sites for metal coordination: in each ligand molecule, two binding sites with the same functionality (2,2'-dipicolylamino group) were placed at the of various methyl arenes. Thus, the distances between the metal binding sites were different from ligand to ligand. We examined the rate of the hydrolysis of RNA dimer catalyzed by La3+ ion binuclear complexes of the ligands. The catalytic activity of the binuclear complexes increased as the distance between the metal binding sites was decreased.     

Chelating and bridging bis(diphenylphosphino)aniline complexes of copper(I)

The ligand bis(diphenylphosphino)aniline (dppan) has been shown to be a versatile ligand sporting different coordination modes and geometries as dictated by copper(I) and the counter ion. The molecular structures of its Cu(I) complexes were characterized by X-ray crystallography. The ligand was found in a chelating mode and monomeric complexes were formed when the ligand to copper ratio was 2:1 and the anion was non-coordinating. However, with thiocyanate as the counter anion, the ligand was found to adopt two different modes, with one ligand chelating and the other acting as a monodentate ligand. With CuX (X = Cl, Br), dppan formed a tetrameric complex when the ligand and metal were reacted in the ratio of 1:1. But reactions containing ligand and metal in the ratios of 1:2 or 2:1, resulted in the formation of a mixture of species in solution. Crystallization however, led to the isolation of the tetrameric complex. Variable temperature ³¹P{¹H} NMR spectra of the isolated tetramers did not show the presence of chelated structures in solution. Tetra-alkylammonium salts were added to solutions of various complexes of dppan and studied by ³¹P{¹H} NMR to probe the effect of anions on the stability of complexes in solution. The Cu-dppan complexes were robust and did not interconvert with other structures in solution unlike the bis(diphenylphosphino)isopropylamine complexes.     

A rapid non-equilibrium critical precipitation assay to assess aluminium-ligand interactions

Abstract

In many natural situations (e.g. environmental and biological) aqueous metal-ligand interactions occur in complex, dynamic solutions and do not adhere to true equilibrium. Nonetheless, equilibrium-based assays in simple solutions are generally used to model metal-ligand interactions of natural systems. Moreover, these are time consuming and not easily applied or understood by many applied scientists. Here, a ‘critical precipitation assay’ was used to investigate the interaction of common ligands with aluminium at pH 7.0, under non-equilibrium conditions. Results obtained were correlated with literature-derived stability constants for the aluminium-ligand interactions, while high-resolution ¹H nuclear magnetic resonance spectroscopy (¹H NMR) was used to confirm the nature of observed interactions. Weak interaction with aluminium was confirmed for traditional weak ligands (e.g. bicarbonate) as these were unable to compete with the hydroxide ion for aluminium at pH 7.0. Two types of interaction were seen for the ‘stronger’ ligands that could compete with hydroxy-polymerisation. Firstly, distinct aluminium:ligand stoichiometric ratios were observed for ligands such as ethylenediaminetetra-acetic acid (1:1) or 1,3,5-trideoxy-1,3,5-tris( dimethylamino)-cis-inositol (1:2). Secondly, most ligands, including citrate and maltol, did not prevent hydroxy-polymerisation but did maintain more aluminium ‘in solution’ (approximately 2.5:1 aluminium:ligand) than permitted by acceptable aluminium:ligand stoichiometric ratios, suggesting the formation of dynamic metastable hydroxy-bridged aluminium-ligand complexes. ¹H NMR with aluminium and maltol or citrate, supported this idea as complex spectral patterns were observed prior to precipitation. Aluminium maintained in solution at pH 7.0 correlated, with literature-derived stability constants suggesting that non-equilibrium aluminium-ligand interactions approximate to equilibrium and that this assay could be used as a quick screening method for investigation of aluminium-ligand interactions.     

Analytical determination of apparent stability constants using a copper ion selective electrode

Copper(II) complexes of di-, tri- and tetra peptides with previously published protonation constants were re-investigated using pH and copper ion selective electrode (ISE) potentiometry in conjunction with a modified version of HYPERQUAD computer program. The purpose was to demonstrate the suitability of the ISE approach for the determination of apparent stability constants for copper(II) complexes with ligands for which proton stability constants were not available. The interactions of Cu²⁺ with oligopeptides were also analysed using surface enhanced laser desorption/ionisation time-of-flight mass spectrometry (SELDI-ToF-MS). The results provide an insight into the metal complex species formed, their apparent stabilities under selected conditions and the effect of the relative positions of certain amino acids within the peptide sequence.     

Characterization of the monovalent cation activator binding site of S-adenosylmethionine synthetase by 205Tl NMR of enzyme-bound Tl+

The structure of the binding site for the monovalent cation activator of S-adenosylmethionine (AdoMet) synthetase from Escherichia coli has been characterized by 205Tl NMR of enzyme-bound Tl+. The chemical shift of the enzyme-Tl+ complex is 176 ppm downfield from aquo Tl+, a shift which is typical only of Tl+ complexes with solely oxygen ligands. The 205Tl resonance shifts upfield to 85 ppm in the enzyme-Mg(II)-Tl+ complex, to 38 ppm in the enzyme-Tl+-AdoMet complex and to 34 ppm in the enzyme-Tl+-AdoMet-Mg(II) complex. The 205Tl chemical shift of enzyme-bound Tl+ was not altered by binding of either methionine, or the Mg(II)-ATP analog Mg(II)-adenyl-5'-yl imidodiphosphate, or Mg(II)-pyrophosphate to the enzyme-Tl+-Mg(II) complex. The NMR data suggest that the substrates or products of the enzyme do not coordinate to the monovalent cation activator and imply that monovalent cation activation results from alterations in protein conformation.     

Crystal structures, antioxidation and DNA binding properties of Eu(III) complexes with Schiff-base ligands derived from 8-hydroxyquinoline-2-carboxyaldehyde and three aroylhydrazines

Eu(III) and every newly synthesized ligand can form a binuclear Eu(III) complex with a 1:1 metal to ligand stoichiometry and nine-coordinate at Eu(III) center. Every ligand acts as a dibasic tetradentate ligand, binding to Eu(III) through the phenolate oxygen atom, nitrogen atom of quinolinato unit, the CN group (methylene) and ⁻O-CN- group (enolized and deprotonated from OC-NH- group) of the aroylhydrazine side chain. One DMF (N,N-dimethylformamide) molecule is binding orthogonally to the ligand-plane from one side to the metal ion, while another DMF and a nitrate anion (bidentate) are binding from the other. Dimerization of the monomeric unit occurs through the phenolate oxygen atoms leading to a central planar four-membered (EuO)₂ ring. On the other hand, all the ligands and Eu(III) complexes may be used as potential anticancer drugs, binding to Calf thymus DNA through intercalations at the order of magnitude 10⁵-10⁷ M⁻¹. All the ligands and Eu(III) complexes are strong scavengers of hydroxyl radicals and superoxide radicals, but Eu(III) complex containing active phenolic hydroxyl group shows stronger scavenging effects for hydroxyl radicals than others, and Eu(III) complex containing N-heteroaromatic substituent shows stronger scavenging effects for superoxide radicals than others.     

π-Bonded alkene and arene complexes of silver(I): an electrospray mass spectrometric study

Electrospray mass spectra have been observed for a number of alkene and arene complexes of Ag(I) formed by the interaction of AgNO₃ and the organometallic ligand in water/methanol solution. The ES mass spectra show that almost all the alkene and arene ligands in stoichiometric excess form labile 1:2 cationic complexes with Ag(I) which are easily decomposed by collisional activation to the 1:1 species. However, with a deficiency of organic ligand polymeric species are observed. The cation [Ag(cod)₂]⁺ (cod=1,5-cyclooctadiene) was reacted with a variety of other potential ligands, such as PPh₃, AsPh₃, PhSCH₂SPh etc. In most cases, mixed complexes [Ag(cod)(ligand)]⁺ were observed, and excess ligand usually produced [Ag(ligand)₂]⁺.     

Mn2+, Co2+, Cu2+ and Zn2+ complexes with two macrocyclic ligands bearing L-lactate-like functions: potentiometric studies and evaluation of superoxide-scavenging properties of the Mn2+ complex

Some aerobic organisms devoid of SOD use Mn2+ chelates to scavenge the O2- radical. Since the Mn2+-bis(lactato)diaquo complex is known as having a high SOD-like activity, we prepared manganese(II) complexes with triazamacrocyclic ligands bearing L-lactate-like functions in order to obtain model compounds able to disproportionate the superoxide radical. Thus, two macrocyclic ligands, N,N',N"-tris[2(S)-hydroxybutyric acid]-1,4,7-triazacyclononane, L1, and N,N',N"-tris[2(S)-hydroxybutyric acid]-1,5,9-triazacyclododecane, L2, were prepared and their capacity to retain the Mn2+ ion in aqueous solution was determined from potentiometric experiments. The chelating properties in aqueous solution of each ligand towards Co2+, Cu2+ and Zn2+ ions were also determined. L1 forms complexes with Mn2+, Co2+, Cu2+ and Zn2+ ions with stability constants of 8.33(5), 15.78(5), 17.65(3) and 14.32(1), respectively. L2 forms complexes with Cu2+ and Zn2+ ions with stability constants of 10.67(1) and 6.98(3), respectively. But the constants related to the Mn2+ and Co2+ complexes were too low to be determined by the method used. The stability constants values calculated for L2 complexes are significantly lower than those for the corresponding complexes of L1. Additional spectroscopic measurements were carried out on the Mn2+-L1 system. The electronic spectrum of this system showed a pH-dependence that may be consistent with the formation of hydroxo-species as the ESR spectra recorded at 120 K did not show oxidation of the Mn2+ ion in the pH range studied. The superoxide-scavenging activity of the manganese(II)-L1 complex was investigated using the cytochrome c assay. The Mn2+-L1 system showed an IC50 value of 1.7 microM which indicates that it appears as a potent SOD mimic.     

The observation of chaperone-ligand noncovalent complexes with electrospray ionization mass spectrometry.

Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) was applied for the study of noncovalent chaperone SecB-ligand complexes produced in solution and examined in the gas phase with the aid of electrospray ionization (ESI). Since chaperone proteins are believed to recognize and bind only with ligands with nonnative tertiary structure, this work required careful unfolding of the ligand and subsequent reaction with the intact chaperone (the noncovalent tetrameric protein, SecB). A high denaturant concentration was employed to produce nonnative structures of the OppA, and microdialysis of the resulting solutions containing the chaperone-ligand complexes was carried out to rapidly remove the denaturant prior to analysis. Multistage mass spectrometry was essential to the successful study of these complexes since the initial mass spectra indicated extensive adduction that precluded mass measurements, even after microdialysis. However, low energy collisional activation of the ions in the FTICR trap proved useful for adduct removal, and careful control of excitation level preserved the intact complexes of interest, revealing a 1:1 SecB:OppA stoichiometry. To our knowledge, these results present the first direct observation of chaperone-ligand noncovalent complexes and the highest molecular weight heterogeneous noncovalent complex observed to date by mass spectrometry. Furthermore, these results highlight the capabilities of FTICR for the study of such complex systems, and the development of a greater understanding of chaperone interactions in protein export.     

Solid-state and proton NMR characterization of an iron(II) complex of a tridentate, facially coordinating N,N,O donor ligand

Despite the many enzymes that use 2-His-1-carboxylate facial triads to bind iron(II), there are few crystallographically characterized synthetic iron(II) complexes of tridentate ligands that bind through two imidazoles and one carboxylate. We report ¹H NMR characterization of the equilibrium between one such ligand and aqueous Fe²⁺. The formation of 1:1 and 2:1 complexes is evident, but the 1:1 complex is never the exclusive compound in solution. This behavior has not been reported previously for N,N,O ligand-iron(II) complexes. The 2:1 ligand/iron complex crystallizes from solution, and it has been completely characterized including an X-ray crystal structure.     

Inhibition of yeast growth by molybdenum-hydroxylamido complexes correlates with their presence in media at differing pH values

The effects of Mo-hydroxylamido complexes on cell growth were determined in Saccharomyces cerevisiae to investigate the biological effects of four different Mo complexes as a function of pH. Studies with yeast, an eukaryotic cell, are particularly suited to examine growth at different pH values because this organism grows well from pH 3 to 6.5. Studies can therefore be performed both in the presence of intact complexes and when the complexes have hydrolyzed to ligand and free metal ion. One of the complexes we examined was structurally characterized by X-ray crystallography. Yeast growth was inhibited in media solutions containing added Mo-dialkylhydroxylamido complexes at pH 3-7. When combining the yeast growth studies with a systematic study of the Mo-hydroxylamido complexes' stability as a function of pH and an examination of their speciation in yeast media, the effects of intact complexes can be distinguished from that of ligand and metal. This is possible because different effects are observed with complex present than when ligand or metal alone is present. At pH 3, the growth inhibition is attributed to the forms of molybdate ion that exist in solution because most of the complexes have hydrolyzed to oxomolybdate and ligand. The monoalkylhydroxylamine ligand inhibited yeast growth at pH 5, 6 and 7, while the dialkylhydroxylamine ligands had little effect on yeast growth. Growth inhibition of the Mo-dialkylhydroxylamido complexes is observed when a complex exists in the media. A complex that is inert to ligand exchange is not effective even at pH 3 where other Mo-hydroxylamido complexes show growth inhibition as molybdate. These results show that the formation of some Mo complexes can protect yeast from the growth inhibition observed when either the ligand or Mo salt alone are present.     

EPR and O2.- scavenger activity: Cu(II)-peptide complexes as superoxide dismutase models.

Several copper(II) complexes with aminoacids and peptides are known to show superoxide dismutase (SOD)-like activity. EPR spectroscopy has proved to be a useful tool for studying the complex equilibria of the copper(II) ion and various ligands of biological importance in solution. In the present work, a variety of copper(II) complexes with di-, tri- and tetra-peptides containing only glycine residues (GG, GGG and GGGG) and others containing a histidyl residue in different positions (HGG, GHG, GGH and GGHG) have been investigated. EPR parameters obtained by extensive use of computer simulation of spectra lead to reliable spin Hamiltonian EPR parameters at both room temperature and in frozen solution. The molecular orbital coefficients computed from the anisotropic EPR data and the d-d electronic energies are used to characterize different arrangements of the complexes. Estimation of the scavenger activity of the complexes due to the particular environment created by the ligands around copper is discussed in the frame of the structure-activity relationship.     

X-Ray crystallographic, NMR and antimicrobial activity studies of magnesium complexes of fluoroquinolones - racemic ofloxacin and its S-form, levofloxacin

The magnesium complexes of racemic ofloxacin (oflo) and its pure S-form levofloxacin (S-oflo) have been studied by X-ray crystallography and NMR spectroscopy. Two compounds, [Mg(R-oflo)(S-oflo)(H(2)O)(2)].2H(2)O (1) and [Mg(S-oflo)(2)(H(2)O)(2)].2H(2)O (2), respectively, have been prepared by hydrothermal reactions and their crystal structures have been determined. In both structures the anionic fluoroquinolone ligands are coordinated through the keto and carboxylate oxygens forming 1:2 Mg:oflo complexes. The two structures are practically identical except for the orientation of one of the oxazine methyl groups at the chiral center of 2 which was found in equatorial position, the other oxazine methyl groups in 1 and 2 being axial. This difference affects the stacking pattern of quinolone molecules in the cell. (1)H NMR chemical shift data and Mn(II) paramagnetic line broadening measurements on the free ofloxacin suggest that the coordination of the ligands in solution involves the keto and carboxylate oxygens. However, it is not possible to decide whether the complexes in aqueous solution have 1:1 or 1:2 stoichiometry. The methylated piperazine nitrogen does not interact with the metal ion. Magnesium-quinolone interaction is discussed in relation to the biological activity of quinolones. The antimicrobial activity of the complexes against various microorganisms was tested and it was established that their activity is similar to that of free quinolone drugs.