Kinetic and thermodynamic determinants of trace metal partitioning at biointerphases: the role of intracellular speciation dynamics

A theory is elaborated for rationalizing the impacts of intracellular metal speciation dynamics on metal uptake in suspension of charged microorganisms beyond the classical thermodynamic representation.

Synthesis,64Cu-labeling and PET imaging of 1,4,7-triazacyclononane derived chelators with pendant azaheterocyclic arms

Efficient and stable⁶⁴Cu complexation by hexadentate TACN-derived chelators with pendant azaheterocyclic arms.

New microsecond intramolecular reactions of human telomeric DNA in solution

G-quadruplex equilibrium (type-1)-G-triplexes observed in the microsecond timescale in human telomeric DNA at pH 7.5, 0.15 M KCl. Other long-lived intermediates should not be ruled out. Similar behavior is observed in the presence of 0.15 M NaCl.

Precise determination of water exchanges on a mineral surface

Solvent exchanges on solid surfaces and dissolved ions are a fundamental property important for understanding chemical reactions, but the rates of fast exchanges are poorly constrained.

Intraparticulate speciation analysis of soft nanoparticulate metal complexes. The impact of electric condensation on the binding of Cd2+/Pb2+/Cu2+by humic acids

The drastic role of electrostatics in the binding of metal ions by soft charged nanoparticulate humic acid complexants is demonstrated.

Magnesium Transport and Magnesium Homeostasis

This review reviews the properties and regulation of the Salmonella enterica serovar Typhimurium and Escherichia coli transporters that mediate Mg2+ influx: CorA and the Mgt P-type ATPases. In addition, potential Mg2+ regulation of transcription and translation, largely via the PhoPQ two component system, is discussed. CorA proteins are a unique class of transporters and are widespread in the Bacteria and Archaea, with rather distant but functional homologs in eukaryotes. The Mgt transporters are highly homologous to other P-type ATPases but are more closely related to the eukaryotic H+ and Ca2+ ATPases than to most prokaryotic ATPases. Hundreds of homologs of CorA are currently known from genomic sequencing. In contrast, only when extracellular and possibly intracellular Mg2+ levels fall significantly is the expression of mgtA and mgtB induced. Topology studies using blaM and lacZ fusions initially indicated that the Salmonella serovar Typhimurium CorA contained three transmembrane (TM) segments; however, subsequent data obtained using a variety of approaches showed that the CorA superfamily of proteins have only two TMs at the extreme C terminus. PhoP-PhoQ is a two-component system consisting of PhoQ, the sensor/receptor histidine kinase, and PhoP, the response regulator/transcriptional activator. The expression of both mgtA and mgtCB in either E. coli or Salmonella serovar Typhimurium is markedly induced in a PhoPQ-dependent manner by low concentrations of Mg2+ in the medium. phoP and phoQ form an operon with two promoters in both E. coli and Salmonella serovar Typhimurium.

Synthesis and structures of cadmium carboxylate and thiocarboxylate compounds with a sulfur-rich coordination environment: carboxylate exchange kinetics involving tris(2-mercapto-1-t-butylimidazolyl)hydroborato cadmium complexes, [Tm(Bu(t))]Cd(O2CR)

A series of cadmium carboxylate compounds in a sulfur-rich environment provided by the tris(2-tert-butylmercaptoimidazolyl)hydroborato ligand, namely, [Tm(Bu(t))]CdO2CR, has been synthesized via the reactions of the cadmium methyl derivative [Tm(Bu(t))]CdMe with RCO2H. Such compounds mimic aspects of cadmium-substituted zinc enzymes and also the surface atoms of cadmium chalcogenide crystals, and have therefore been employed to model relevant ligand exchange processes. Significantly, both (1)H and (19)F NMR spectroscopy demonstrate that the exchange of carboxylate groups between [Tm(Bu(t))]Cd(κ(2)-O2CR) and the carboxylic acid RCO2H is facile on the NMR time scale, even at low temperature. Analysis of the rate of exchange as a function of concentration of RCO2H indicates that reaction occurs via an associative rather than dissociative pathway. In addition to carboxylate compounds, the thiocarboxylate derivative [Tm(Bu(t))]Cd[κ(1)-SC(O)Ph] has also been synthesized via the reaction of [Tm(Bu(t))]CdMe with thiobenzoic acid. The molecular structure of [Tm(Bu(t))]Cd[κ(1)-SC(O)Ph] has been determined by X-ray diffraction, and an interesting feature is that, in contrast to the carboxylate derivatives [Tm(Bu(t))]Cd(κ(2)-O2CR), the thiocarboxylate ligand binds in a κ(1) manner via only the sulfur atom.

Aqueous vanadium ion dynamics relevant to bioinorganic chemistry: A review

Aqueous solutions of the four highest vanadium oxidation states exhibit four diverse colors, which only hint at the diverse reactions that these ions can undergo. Cationic vanadium ions form complexes with ligands; anionic vanadium ions form complexes with ligands and self-react to form isopolyanions. All vanadium species undergo oxidation-reduction reactions. With a few exceptions, elucidation of the dynamics of these reactions awaited the development of fast reaction techniques before the kinetics of elementary ligation, condensation, reduction, and oxidation of the aqueous vanadium ions could be investigated. As the biological roles played by endogenous and therapeutic vanadium expand, it is appropriate to bring the results of the diverse kinetics studies under one umbrella. To achieve this goal this review presents a systematic examination of elementary aqueous vanadium ion dynamics.

Phosphonated chelates for nuclear imaging

A series of bis-, tris- and tetra-phosphonated pyridine ligands is presented. In view of their potential use as chelates for radiopharmaceutical applications, the physico-chemical properties of the ligands and of their Co(II), Ni(II), Cu(II), and Zn(II) complexes were studied by means of potentiometry and UV-Vis absorption spectroscopy. The pKa values of the ligands and of the complexes, as well as the stability constants for the formation of the complexes, are presented. The kinetic aspects of the formation of Cu(II) complexes and of their dissociation in acidic media were studied by means of stopped flow experiments, and the stability of the Cu(II) complex toward reduction to Cu(I) was investigated by cyclic voltammetry and by titration with different reducing agents. The different thermodynamic and kinetic aspects of the polyphosphonated ligands were compared with regard to the impact of the number of phosphonic acid functions. Considering the very promising properties for complexation, preliminary SPECT/CT imaging experiments were carried out on mice with (99m)Tc using the bis- and tetra-phosphonated ligands L(2) and L(1). Finally, a bifunctional version of chelate L(1), L*, was used to label MTn12, a rat monoclonal antibody with both specificity and relatively high affinity for murine tenascin-C. The labeling was monitored by MALDI/MS spectrometry and the affinity of the labeled antibody was checked by immunostaining experiments. After chelation with (99m)Tc, the (99m)Tc-L*-MTn12 antibody was injected into a transgenic mouse with breast cancer and the biodistribution of the labeled antibody was followed by SPECT/CT imaging.

Effects of magnesium deficiency--more than skin deep

Dead Sea and magnesium salt therapy are two of the oldest forms of treatment for skin disease and several other disorders, supported by a body of largely anecdotal evidence. In this paper we review possible pathways for penetration of magnesium ions through the epidermis to reach the circulation, in turn replenishing cellular magnesium levels. We also discuss mechanisms for intercellular movement of magnesium ions and possible mechanisms for the interaction between magnesium ions and inflammatory mediators. Upon addition of magnesium ions in vitro, the expression of inflammatory mediators such as tumour necrosis factor α (TNFα) and nuclear factor κβ (NFκβ) is down regulated. Dysregulation of these and other inflammatory mediators has been linked to several inflammatory disorders, including asthma, arthritis, atherosclerosis and neuroinflammation.

Preparation of N,N-dialkylcarbamato lanthanide complexes by extraction of lanthanide ions from aqueous solution into hydrocarbons

Lanthanides are easily extracted as N,N-dibutylcarbamato complexes from aqueous solutions of their chlorides into heptane solutions of dibutylamine saturated with CO2. The products are recovered in high yields and are soluble in hydrocarbons. The derivatives [Ln(O2CNBu2)3]n [Ln = Nd (1), Eu (2), Tb (3)], [NH2Bu2]2[Ln4(CO3)(O2CNBu2)12] [Ln = Tb (4), Sm (5), Eu (6)], and [Sm4(CO3)(O2CNBu2)10], 7, have thus been obtained. The crystal and molecular structure of 4 has been solved; the samarium and europium complexes 5 and 6 were found to be isostructural. Mass spectra of the complexes 1-3, 4, and 7 (in MeCN/toluene) reveal that equilibria are present in solution. Compound 2 has been reacted in toluene with NHBz2 in the presence of CO2 affording [NH2Bz2][Eu(O2CNBz2)4], 8, through a ligand exchange process. By thermal treatment, 8 afforded [Eu(O2CNBz2)3]n, 9. With a similar procedure [Sm(O2CNBz2)3]n, 10, was obtained from 5. According to the photoluminescence study carried out on solid samples of 2, 4, 5, 7, and 8, the metal centered f-f transitions represent the only effective way to induce lanthanide luminescence in these complexes.

Impact of electrostatics on the chemodynamics of highly charged metal-polymer nanoparticle complexes

In this work, the impact of electrostatics on the stability constant, the rate of association/dissociation, and the lability of complexes formed between Cd(II), Pb(II), and carboxyl-modified polymer nanoparticles (also known as latex particles) of radius ∼ 50 nm is systematically investigated via electroanalytical measurements over a wide range of pHs and NaNO3 electrolyte concentrations. The corresponding interfacial structure and key electrostatic properties of the particles are independently derived from their electrokinetic response, successfully interpreted using soft particle electrohydrodynamic formalism, and complemented by Förster resonance energy transfer (FRET) analysis. The results underpin the presence of an ∼0.7-1 nm thick permeable and highly charged shell layer at the surface of the polymer nanoparticles. Their electrophoretic mobility further exhibits a minimum versus NaNO3 concentration due to strong polarization of the electric double layer. Integrating these structural and electrostatic particle features with recent theory on chemodynamics of particulate metal complexes yields a remarkable recovery of the measured increase in complex stability with increasing pH and/or decreasing solution salinity. In the case of the strongly binding Pb(II), the discrepancy at pH > 5.5 is unambiguously assigned to the formation of multidendate complexes with carboxylate groups located in the particle shell. With increasing pH and/or decreasing electrolyte concentration, the theory further predicts a kinetically controlled formation of metal complexes and a dramatic loss of their lability (especially for lead) on the time-scale of diffusion toward a macroscopic reactive electrode surface. These theoretical findings are again shown to be in agreement with experimental evidence.

Aqueous Mg2+ and Ca2+ ligand exchange mechanisms identified with 2DIR spectroscopy

Biological systems must discriminate between calcium and magnesium for these ions to perform their distinct biological functions, but the mechanism for distinguishing aqueous ions has yet to be determined. Ionic recognition depends upon the rate and mechanism by which ligands enter and leave the first solvation shell surrounding these cations. We present a time-resolved vibrational spectroscopy study of these ligand exchange dynamics in aqueous solution. The sensitivity of the CN-stretch frequency of NCS(-) to ion pair formation has been utilized to investigate the mechanism and dynamics of ligand exchange into and out of the first solvation shell of aqueous magnesium and calcium ions with multidimensional vibrational (2DIR) spectroscopy. We have determined that anion exchange follows a dissociative mechanism for Mg(2+) and an associative mechanism for Ca(2+).

Understanding the extraordinary ionic reactivity of aqueous nanoparticles

Nanoparticles (NPs) are generally believed to derive their high reactivity from the inherently large specific surface area. Here we show that this is just the trivial part of a more involved picture. Nanoparticles that carry electric charge are able to generate chemical reaction rates that are even substantially larger than those for similar molecular reactants. This is achieved by Boltzmann accumulation of ionic reactants and the Debye acceleration of their transport. The ensuing unique reactivity features are general for all types of nanoparticles but most prominent for soft ones that exploit the accelerating mechanisms on a 3D level. These features have great potential for exploitation in the catalysis of ionic reactions: the reactivity of sites can be enhanced by increasing the indifferent charge density in the NP body.

The aqueous Ca2+ system, in comparison with Zn2+, Fe3+, and Al3+: an ab initio molecular dynamics study

Herein, we report on the structure and dynamics of the aqueous Ca(2+) system studied by using ab initio molecular dynamics (AIMD) simulations. Our detailed study revealed the formation of well-formed hydration shells with characteristics that were significantly different to those of bulk water. To facilitate a robust comparison with state-of-the-art X-ray absorption fine structure (XAFS) data, we employ a 1st principles MD-XAFS procedure and directly compare simulated and experimental XAFS spectra. A comparison of the data for the aqueous Ca(2+) system with those of the recently reported Zn(2+), Fe(3+), and Al(3+) species showed that many of their structural characteristics correlated well with charge density on the cation. Some very important exceptions were found, which indicated a strong sensitivity of the solvent structure towards the cation's valence electronic structure. Average dipole moments for the 2nd shell of all cations were suppressed relative to bulk water.

Chemodynamics of soft nanoparticulate complexes: Cu(II) and Ni(II) complexes with fulvic acids and aquatic humic acids

The dynamics of metal complexation by small humic substances (fulvic acid and aquatic humic acid, collectively denoted as “fulvic-like substance”, FS) are explored within the framework of concepts recently developed for soft nanoparticulate complexants. From a comprehensive collection of published equilibrium and dissociation rate constants for CuFS and NiFS complexes, the association rate constant, ka, is determined as a function of the degree of complexing site occupation, θ. From this large data set, it is shown for the first time that ka is independent of θ. This result has important consequences for finding the nature of the rate limiting step in the association process. The influence of electric effects on the rate of the association process is described, namely (i) the accelerating effect of the negatively charged electrostatic field of FS on the diffusion of metal ions toward it, and (ii) the extent to which metal ions electrostatically accumulate in the counterionic atmosphere of FS. These processes are discussed qualitatively in relation to the derived values of ka. For slowly dehydrating metal ions such as Ni(H2O)6 2+ (dehydration rate constant, kw), ka is expected to derive straight from kw. In contrast, for rapidly dehydrating metal ions such as Cu(H2O)6 2+, transport limitations and electric effects involved in the formation of the precursor outer-sphere associate appear to be important overall rate-limiting factors. This is of great significance for understanding the chemodynamics of humic complexes in the sense that inner-sphere complex formation would not always be the (sole) rate limiting step.

Electric relaxation processes in chemodynamics of aqueous metal complexes: from simple ligands to soft nanoparticulate complexants

The chemodynamics of metal complexes with nanoparticulate complexants can differ significantly from that for simple ligands. The spatial confinement of charged sites and binding sites to the nanoparticulate body impacts on the time scales of various steps in the overall complex formation process. The greater the charge carried by the nanoparticle, the longer it takes to set up the counterion distribution equilibrium with the medium. A z+ metal ion (z > 1) in a 1:1 background electrolyte will accumulate in the counterionic atmosphere around negatively charged simple ions, as well as within/around the body of a soft nanoparticle with negative structural charge. The rate of accumulation is often governed by diffusion and proceeds until Boltzmann partition equilibrium between the charged entity and the ions in the medium is attained. The electrostatic accumulation proceeds simultaneously with outer-sphere and inner-sphere complex formation. The rate of the eventual inner-sphere complex formation is generally controlled by the rate constant of dehydration of the metal ion, k(w). For common transition metal ions with moderate to fast dehydration rates, e.g., Cu(2+), Pb(2+), and Cd(2+), it is shown that the ionic equilibration with the medium may be the slower step and thus rate-limiting in their overall complexation with nanoparticles.

Characterization of DNA polymerase β from Danio rerio by overexpression in E. coli using the in vivo/in vitro compatible pIVEX plasmid

BACKGROUND

Eukaryotic DNA polymerase β (pol β), the polymerase thought to be responsible for DNA repair synthesis, has been extensively characterized in rats and humans. However, pol β has not been purified or enzymatically characterized from the model fish species Danio rerio (zebrafish). We used the in vitro/in vivo dual expression system plasmid, pIVEX, to express Danio rerio pol β (Danio pol β) for biochemical characterization.

RESULTS

Danio pol β encoded by the in vitro/in vivo-compatible pIVEX plasmid was expressed in E. coli BL21(DE3), BL21(DE3)pLysS, and KRX, and in vitro as a C-terminal His-tagged protein. Danio pol β expressed in vitro was subject to proteolysis; therefore, bacterial overexpression was used to produce the protein for kinetic analyses. KRX cells were preferred because of their reduced propensity for leaky expression of pol β. The cDNA of Danio rerio pol β encodes a protein of 337 amino acids, which is 2-3 amino acids longer than other pol β proteins, and contains a P63D amino acid substitution, unlike mammalian pol βs. This substitution lies in a hairpin sequence within an 8-kDa domain, likely to be important in DNA binding. We performed extensive biochemical characterization of Danio pol β in comparison with rat pol β, which revealed its sensitivity to metal ion activators (Mn2+ and Mg2+), its optimum salt concentration (10 mM KCl and 50 mM NaCl), alkaline pH optimum (pH 9.0), and low temperature optimum (30°C). Substituting Mn2+ for Mg2+ resulted in 8.6-fold higher catalytic efficiency (kcat/Km).

CONCLUSIONS

Our characterization of pol β from a model fish organism contributes to the study of the function and evolution of DNA polymerases, which are emerging as important cellular targets for chemical intervention in the development of anticancer agents.

Dynamics and heterogeneity of Pb(II) binding by SiO2 nanoparticles in an aqueous dispersion

Pb(II) binding by SiO(2) nanoparticles in an aqueous dispersion was investigated under conditions where the concentrations of Pb(2+) ions and nanoparticles are of similar magnitude. Conditional stability constants (log K) obtained at different values of pH and ionic strength varied from 4.4 at pH 5.5 and I = 0.1 M to 6.4 at pH 6.5 and I = 0.0015 M. In the range of metal to nanoparticle ratios from 1.6 to 0.3, log K strongly increases, which is shown to be due to heterogeneity in Pb(II) binding. For an ionic strength of 0.1 M the Pb(2+)/SiO(2) nanoparticle system is labile, whereas for lower ionic strengths there is loss of lability with increasing pH and decreasing ionic strength. Theoretical calculations on the basis of Eigen-type complex formation kinetics seem to support the loss of lability. This is related to the nanoparticulate nature of the system, where complexation rate constants become increasingly diffusion controlled. The ion binding heterogeneity and chemodynamics of oxidic nanoparticles clearly need further detailed research.

Chemodynamics of soft nanoparticulate metal complexes in aqueous media: basic theory for spherical particles with homogeneous spatial distributions of sites and charges

A theoretical discussion is presented to describe the formation and dissociation rate constants for metal ion binding by soft nanoparticulate complexants. The well-known framework of the Eigen mechanism for metal ion complexation by simple ligands in aqueous systems is the starting point. Expressions are derived for the rate constants for the intraparticulate individual outer-sphere and inner-sphere association and dissociation steps for the limiting cases of low and high charge densities. The charge density, binding site density, and size of the nanoparticle play crucial roles. The effects of the electrostatic potential and particle radius on the overall complexation reaction are compared with those for simple ligands. The limitations of the proposed approach for nanoparticulate ligands are discussed, and key issues for future developments are identified.

The dynamics of Ca2+ ions within the solvation shell of calbindin D9k

The encounter of a Ca²⁺ ion with a protein and its subsequent binding to specific binding sites is an intricate process that cannot be fully elucidated from experimental observations. We have applied Molecular Dynamics to study this process with atomistic details, using Calbindin D9k (CaB) as a model protein. The simulations show that in most of the time the Ca²⁺ ion spends within the Debye radius of CaB, it is being detained at the 1st and 2nd solvation shells. While being detained near the protein, the diffusion coefficient of the ion is significantly reduced. However, due to the relatively long period of detainment, the ion can scan an appreciable surface of the protein. The enhanced propagation of the ion on the surface has a functional role: significantly increasing the ability of the ion to scan the protein's surface before being dispersed to the bulk. The contribution of this mechanism to Ca²⁺ binding becomes significant at low ion concentrations, where the intervals between successive encounters with the protein are getting longer. The efficiency of the surface diffusion is affected by the distribution of charges on the protein's surface. Comparison of the Ca²⁺ binding dynamics in CaB and its E60D mutant reveals that in the wild type (WT) protein the carboxylate of E60 function as a preferred landing-site for the Ca²⁺ arriving from the bulk, followed by delivering it to the final binding site. Replacement of the glutamate by aspartate significantly reduced the ability to transfer Ca²⁺ ions from D60 to the final binding site, explaining the observed decrement in the affinity of the mutated protein to Ca²⁺.

Quantitative Determination of Chromium Valence in Environmental Samples Using Xafs Spectroscopy

Synchrotron x-ray absorption fine structure (XAFS) spectroscopy provides quantitative information on the speciation of Cr in complex oxide samples, including the Cr(VI):Cr(III) ratio, without any sample pretreatment. We discuss several XAFS methods that can be used for this purpose, including: (1) measurement of the height of the pre-edge peak of Cr K-edge spectra which is caused by ls→3d electronic transitions occurring predominately in the Cr(VI)O4 tetrahedron; (2) fitting observed XANES spectra using combinations of experimental spectra from Cr(III)- and Cr(VI)-containing model compounds; (3) fitting observed EXAFS spectra using combinations of experimental spectra from Cr(III)- and Cr(VI)-containing model compounds; and (4) ab-initio modeling of observed Cr K-EXAFS spectra using theoretical phase-shift and amplitude parameters for Cr(III)O6 and Cr(VI)O4 models. The first method is shown to be the simplest and most accurate. The samples examined using these methods include highly pure model compounds, Cr3+2O3 and Na2Cr6+O4·4H42O, as well as more complex soil samples taken from Cr-contaminated industrial sites and Cr waste disposal pits. We present XAFS evidence for the reduction of Cr(VI) to Cr(III) after reaction with the Fe(Il)-bearing oxide magnetite, Fe3O4, and show that Cr associated with magnetite in soils is reduced, whereas that associated with non-Fe(II)-bearing silicate minerals is not. Also presented are ab-initio multiplescattering calculations of XANES spectra of an Cr(VI)O4 tetrahedron, which help explain the XANES features of this Cr species in solids and aqueous solutions.

Highly stable acyclic bifunctional chelator for 64Cu PET imaging

Ligand L1, based on a pyridine scaffold, functionalized by two bis(methane phosphonate)aminomethyl groups, was shown to display a very high affinity towards Cu(II) (log K CuL=22.7) and selectivity over Ni(II), Co(II), Zn(II) and Ga(III) (Δ log K ML>4) as shown by the values of the stability constants obtained from potentiometric measurements. Insights into the coordination mode of the ligand around Cu(II) cation were obtained by UV-Vis absorption and EPR spectroscopies as well as density functional theory (DFT) calculations (B3LYP model) performed in aqueous solution. The results point to a pentacoordination pattern of the metal ion in the fully deprotonated [CuL1]6− species. Considering the beneficial thermodynamic parameters of this ligand, kinetic experiments were run to follow the formation of the copper(II) complexes, indicating a very rapid formation of the complex, appropriat e for 64Cu complexation. As L1 represents a particularly interesting target within the frame of 64Cu PET imaging, a synthetic protocol was developed to introduce a labeling function on the pyridyl moiety of L1, thereby affording L2, a potential bifunctional chelator (BFC) for PET imaging.

Catalytic activity of thiourea and its selected derivatives on electroreduction of In(III) in chlorates(VII)

It was found that thiourea, N-methylthiourea, N,N′-dimethylthiourea and N-allylthiourea accelerate the electroreduction process of In(III) ions in chlorates(VII). These substances are adsorbed on mercury from chlorates(VII). The relative surface excesses of thiourea and its derivatives increase with the increase of their concentrations and electrode charge. After adding thiourea, N-methylthiourea, N,N′-dimethylthiourea and N-allylthiourea to the solution an acceleration of the electroreduction process of In(III) ions occurs. This process depends on two factors: the adsorption of an accelerating substance on mercury and on the formation of complexes between a depolarizer and an accelerating substance on the electrode surface. The equilibrium of this complexing reaction determines the magnitude of the catalytic effect.

Protonation effects on dynamic flux properties of aqueous metal complexes

The degree of (de)protonation of aqueous metal species has significant consequences for the kinetics of complex formation/dissociation. All protonated forms of both the ligand and the hydrated central metal ion contribute to the rate of complex formation to an extent weighted by the pertaining outer-sphere stabilities. Likewise, the lifetime of the uncomplexed metal is determined by all the various protonated ligand species. Therefore, the interfacial reaction layer thickness, μ, and the ensuing kinetic flux, Jkin, are more involved than in the conventional case. All inner-sphere complexes contribute to the overall rate of dissociation, as weighted by their respective rate constants for dissociation, kd. The presence of inner-sphere deprotonated H2O, or of outer-sphere protonated ligand, generally has a great impact on kd of the inner-sphere complex. Consequently, the overall flux can be dominated by a species that is a minor component of the bulk speciation. The concepts are shown to provide a good description of experimental stripping chronopotentiometric data for several protonated metal–ligand systems.

Metal and substrate binding to an Fe(II) dioxygenase resolved by UV spectroscopy with global regression analysis

The addition of divalent metal ions or substrate taurine to TauD, an alpha-ketoglutarate-dependent dioxygenase, alters its UV absorption, as clearly observed by monitoring the protein's difference spectra. Binding of metal ions leads to a decrease in absorption at approximately 297 nm and modulation of other features. A separate signature with enhanced absorption at approximately 295 nm is identified for binding of taurine. These narrow ( approximately 700 cm(-1)) and intense ( approximately 0.5mM(-1) cm(-1)) spectral changes are attributed to ligand-induced protein conformational changes affecting the environment of aromatic residues. The changes in the UV difference spectra were exploited to assess directly the thermodynamics and kinetics of ligand interactions in wild-type TauD and selected variants. This approach holds promise as a new tool to probe ligand-induced conformational changes in a wide range of other proteins. Experimental and quantification approaches for a reliable analysis of protein absorption below 320 nm are presented.