Charged Metal Complexes Research Articles

Less efficient skin penetration of the metal allergen Pd2+ compared to Ni2+ and Co2+ from patch test preparations.

Contrary to Ni2+- and Co2+-induced allergic contact dermatitis (ACD), reactions against Pd2+ are rare. However, Pd2+ activates a larger T cell fraction in vitro, suggesting an inefficient skin penetration. This study compares Ni2+, Co2+ and Pd2+ skin penetration from commonly used diagnostic patch test preparations (PTPs) and aqueous metal salt solutions. Using Franz diffusion cell assays, we applied the metals in PTPs (5% NiSO4, 1% CoCl2, 2% PdCl2 and 3% Na2PdCl4) and in solution to pigskin for 48 h, thereby mirroring the time frame of a patch test. The different compartments were analysed individually by inductively coupled plasma mass spectrometry. Metal ions were mainly retained in the upper stratum corneum layers. After application of PTPs, concentrations in the viable skin were lower for Pd2+ (1 and 7 μM) compared to Ni2+ and Co2+ (54 and 17 μM). Ni2+ and Co2+ penetrated the skin more efficiently than Pd2+ and thus may sensitize and elicit ACD more easily. This was observed for ions applied in petrolatum and aqueous solutions. We hypothesize that the differently charged metal complexes are responsible for the varying skin penetration behaviours.

Single-ion magnetism in novel Btp-based cobalt complexes of different charge.

The magnetic behavior of single-ion metal complexes may be influenced by the nature and composition of the secondary coordination sphere that can be composed of solvent molecules and counterions bound through non-covalent interactions. However, achieving precise control over the outer-coordination sphere of these magnetic complexes to demonstrate its influence on their magnetic properties presents a challenge. A strategy for varying the number of counterions, while simultaneously preserving the arrangement of the ligand atoms around the metal center without altering its oxidation state, is to adjust the overall formal charge of the complex. This adjustment could lead to changes in the magnetic properties of single-ion metal complexes. In this study, we present two novel ligands featuring the coordinating unit Btp (2,6-bis(1,2,3-triazol-4-yl)pyridine). These ligands are equipped with functional groups that can potentially undergo deprotonation. By carefully selecting the solvents used during the crystallization process of the complexes, we can tune at will the charge of the complexes, thus modifying the composition of the CoII complexes' outer-coordination sphere. We show that, by modifying these conditions, we can tailor the secondary coordination sphere of both charged (mono- and dicationic) and neutral anisotropic CoII metal complexes to show field-induced single-ion magnetism, influencing in turn the size of the barrier to reversal of the magnetization and their slow relaxation process.

Radiometal Complexes as Pharmacokinetic Modifiers: A Potent 68Ga-Labeled Gastrin-Releasing Peptide Receptor Antagonist Based on the Macrocyclic Metal Chelator NODIA-Me.

The gastrin-releasing peptide receptor (GRPr) is overexpressed in various cancer types including prostate and breast carcinomas, making it an attractive target for molecular imaging and therapy. In this work, we designed a novel GRPr antagonistic probe comprising metal chelator NODIA-Me. This 1,4,7-triazacyclononane-based chelator forms positively charged metal complexes due to its neutral methylimidazole arms. Because a positive charge at the N-terminus of GRPr conjugates is responsible for high receptor affinity as exemplified by the current gold standard DOTA-RM2, we investigated if a positively charged radiometal complex can be used as a pharmacokinetic modifier to also produce high-affinity GRPr conjugates. In this respect, the bioconjugate NODIA-Me-Ahx-JMV594 was prepared by a combination of solid-phase peptide synthesis and solution-based reactions in a 94% yield. Radiolabeling provided the 68Ga-labeled conjugate in radiochemical yields of >95% and radiochemical purities of >98% with mean molar activities of Am ∼17 MBq nmol-1. The competitive GRPr affinity of the metal-free and 69/71Ga-labeled conjugate was determined to be IC50 = 0.41 ± 0.06 and 1.45 ± 0.06 nM, respectively. The metal-free GRPr antagonist DOTA-RM2 and its corresponding 69/71Ga complex had IC50 values of 1.42 ± 0.07 and 0.98 ± 0.19 nM, respectively. Small-animal PET imaging of mice bearing GRPr(+) PC-3 tumors revealed high radioactivity accumulation in the tumors and in the pancreas as an organ with high levels of GRPr expression. These findings were corroborated by the corresponding ex vivo biodistribution data, in which the tumors and the pancreas exhibited the highest radioactivity accumulation. By coinjection of an excess of NODIA-Me-Ahx-JMV594, uptake in the tumors and GRPr(+) organs was significantly reduced, confirming specific receptor-mediated uptake. The estrogen receptor-positive tumor of a female breast cancer patient was clearly visualized by PET imaging using 68Ga-labeled NODIA-Me-Ahx-JMV594. To summarize, the positive charge at the N-terminus of the conjugate induced by the Ga(NODIA-Me) complex resulted in high GRPr affinity comparable to that of the potent antagonist DOTA-RM2. The conjugate NODIA-Me-Ahx-JMV594 is a promising probe for imaging of GRPr tumors that warrants further evaluation in larger patient cohorts as well as in combination with other radiometals.

Charge Inversion of Mono- and Dianions to Cations via Triply Charged Metal Complexes: Application to Lipid Mixtures.

Electrospray ionization (ESI) of mixtures can give rise to ions with different masses and charges with overlapping mass-to-charge (m/z) ratios. Such a scenario can be particularly problematic for the detection of low-abundance species in the presence of more highly abundant mixture components. For example, negative mode ESI of polar lipid extracts can result in highly abundant singly charged glyerophospholipids (GPLs), such as phosphatidylethanolamines (PE) and phosphatidylglycerols (PG), that can obscure much less abundant cardiolipins (CLs), which are complex phospholipids with masses roughly double those of GPLs that mostly form doubly charged anions. Despite their low relative abundance, CLs are lipidome components that perform vital biological functions. To facilitate the study of CLs in lipid mixtures without resorting to offline or online separations, we have developed a gas-phase approach employing ion/ion reactions to charge invert anionic lipid species using a trivalent metal-complex. Specifically, ytterbium(III) is shown to readily complex with three neutral ligands, N,N,N',N'-tetra-2-ethylhexyl diglycolamide (TEHDGA), to form [Yb(TEHDGA3)]3+ using ESI. Herein, we describe pilot studies to evaluate [Yb(TEHDGA)3]3+ as an ion/ion reagent to allow for chemical separation of doubly and singly charged anions, using lipid mixtures as examples, without neutralizing ions of either charge state.

Bio-removal of PtCl62− complex by Galdieria sulphuraria

Bio-removal of negative charged platinum complex is of great challenge owing to electrostatic repulsions between PtCl62− and general extracellular polymeric substance (EPS) of microorganism. Galdieria sulphuraria (GS) are thermophilic and acidophilic microalga with specific metabolism, which subsequently lead to their unique cellular compositions such as EPS and phycocyanin, possibly providing a strategy to deal with negative charged metal complex. Accordingly, G. sulphuraria are employed to remove negative charged PtCl62− complex with initial concentrations ranging from 0, 10, 20, 30, to 45 ppm. The growth rates of G. sulphuraria with microalgae named as GS-0, GS-10, GS-20, GS-30, and GS-45, respectively, and simultaneously bio-removal efficiencies of PtCl62− are investigated. G. sulphuraria are independent to PtCl62− within 0–30 ppm, while they are inhibited within 45 ppm of PtCl62−. The PtCl62− removal efficiencies of GS-10, GS-20, and GS-30 increase from 94.58%, 95.52%, to 95.92%, while decrease to 71.81% of GS-45. About 92.39%, 93.77%, 94.29%, and 75.21% of PtCl62− adsorbed are accumulated within GS-10, GS-20, GS-30, GS-45, with few in EPS. The PtCl62− complexes accumulated in EPS and algae cells are possibly decomposed to PtCl4 according to the increasing zeta potentials of EPS and algae cells. The results indicate that PtCl62− is efficiently removed by G. sulphuraria, achieving bio-removal of negative charged PtCl62− complex from wastewater.

Origin of a High Overpotential of Co Electrodeposition in a Room-Temperature Ionic Liquid.

Metal electrodeposition in room-temperature ionic liquids (RTILs) often shows high overpotentials. Although this is often explained by the formation of a negatively charged metal complex due to the coordination of RTIL anions and the hindrance of its close approach onto the negatively charged electrode, we propose an alternative model based upon surface-enhanced infrared absorption spectroscopy measurements under Co electrodeposition. We found that the anionic first layer exists on the negatively charged electrode, and its replacement with a cationic one and Co electrodeposition both begin at an identical onset potential. The correlation between the interfacial structure and the electrodeposition reaction that can be modified by additives indicated that the high overpotential can be mainly attributed to the restructuring of the characteristic interfacial multilayer structure stabilized by its charge order, which is required for the reorganization of solvent ions after the reduction of Co2+.

Positively and Negatively Charged Porphyrin Metal Complexes That Form Ion-Pairing Assemblies

An ordered arrangement of π-electronic species is crucial for the fabrication of functional organic materials such as organic electronic devices including field-effect transistors, light-emitting diodes, and photovoltaic cells. Synergetic uses of electrostatic interactions and other noncovalent interactions, including π–π stacking, is very important for the alignment of π-electronic charged species (cations and anions) and the formation of dimension-controlled assemblies including fiber and sheet solid materials, supramolecular gels, and liquid crystals.[1,2] An advantage of using electrostatic interactions is the formation of various ion-pairing materials by combining constituent π-electronic ions: for example, ten kinds of cations and ten kinds of anions are mixed to ideally provide one hundred kinds of ion pairs. Furthermore, assembling modes can be modulated by constituent ions as well as by the environment, thus exhibiting particular properties according to the arrangement of charged building units even in an ion pair. In this study, porphyrin–AuIII complexes were found to act as π-electronic cations which can combine with various counteranions, including π-electronic anions such as negatively charged porphyrin metal complexes. In the solid-state and soft materials, the formation of assemblies with contributions of charge-by-charge and charge-segregated assemblies, whose ionic components were highly organized by π–π stacking and electrostatic interactions, depended on the geometries and electronic states of constituent ionic species.[3,4]

Electrokinetic approach to assess the behaviour of a contaminated marine sediment

The present study focuses on the laboratory-scale application of an electrokinetic technique for the remediation of a contaminated marine sediment dredged from a harbour. The challenge of the study was the treatment of the silty sediment containing heavy metals and organic matter, as methods to remove heavy metals or organic matter separately do not usually yield satisfactory results for the mixture of both. The sediment was characterised in terms of elemental and mineralogical composition, heavy metals, total organic carbon, particle size distribution and zeta-potential. Five configurations of remediation tests have been explored. Four of these tests were employing electric field, two of which were carried out with carbon steel electrodes (CSE) and the other two with activated Ti electrodes (ATE). One test was conducted without electrical current, and it was used as a benchmark. In some tests, acetic acid was added to the catholyte to enhance heavy metal mobility. The pH, conductivity and composition of catholyte and anolyte were characterized as function of test duration. Electrophoretic measurements showed that the zeta-potential of raw, untreated sediment samples was negative across all the range of pH. In the tests with CSE, electroosmotic flow (EOF) towards the cathode was observed when acetic acid was added. In other tests, i.e. CSE without acetic acid and ATE with acetic acid, the flow initially directed towards the cathode but then reversed. In the case of ATE without acetic acid, the EOF aimed only at the anode. Without current, no heavy metals were removed. In the tests without acetic acid, the metals concentrated in the anolyte, most probably due to electroosmotic flow or migration of negatively charged aqueous metal complexes. Arsenic and a small amount of Cu and Zn were found in the catholyte. When acetic acid was added to the catholyte, and with ATE electrodes, all the metals were found in the catholyte, whilst Ni was not detected in the anolyte. The formation of an acidic front through the sediment and the control of the EOF appear to be the key factors for the successful remediation via electrokinetic technique. It has been demonstrated that electroremediation can be efficient for removal of all studied metals as all of them could be mobilised using Ti electrodes. The emission of metals such as Ni and Cr from dissolution of carbon steel anode must be taken into account in electrochemical remediation tests.

Structures and Dissociation Products of Ce/Peptide Complexes: Competition between Coordination and Charge Delocalization.

Structures of [Ce(GGG)]3+ and [Ce(GGG ? H)]2+ have been investigated by DFT calculations. The two lowest-energy structures of the triply charged metal complex have the peptide in either the iminol or conventional zwitterionic form, and these ions have almost identical energies. In the doubly charged complex, the iminol and charge-solvated structures are the best structures on the potential energy surface, but the latter is favored. In both iminol structures, the metal ion coordinates to the iminol oxygen rather than to the nitrogen, unlike in previously reported iminol-containing complexes. Triply charged [Ce(peptide)]3+ complexes are fragile and not easily isolated in a mass spectrometer, whereas the doubly charged [Ce(peptide ? H)]2+ complexes are more robust. Here, we studied the fragmentations of 37 [Ce(peptide ? H)]2+ and 30 [Ce(peptide)(peptide ? H)]2+ complexes and the results are systematically summarized. Losses of CO and/or H2O are the most commonly observed fragmentation channels for [Ce(peptide ? H)]2+ complexes and these dissociation pathways are modeled by DFT calculations. For [Ce(peptide)(peptide ? H)]2+ complexes the neutral peptide plays the role of a solvent molecule but, unlike in the dissociations of [Ce(CH3CN)(peptide ? H)]2+ complexes, the loss of the solvent molecule is not observed. Instead, fragmentation occurs by cleavage of the second amide bond of the solvating peptide molecule.

Charged Porphyrin Metal Complexes As Building Units of Ion-Pairing Assemblies and Materials

π-Electronic ions with appropriate geometries and peripheral substituents provide assemblies through interactions between building subunits, resulting in fascinating electronic properties. Structures and properties of the assemblies can be controlled by the combined negatively and positively charged species in the assemblies. Thus far, diverse ion-responsive π-electronic systems and π-electronic ions have afforded dimension-controlled ion-pairing assemblies as crystals, supramolecular gels, and thermotropic liquid crystals (LCs).[1,2] Highly ordered arrangement of charged species has been found to be a key factor to exhibit the enhanced performance as fascinating electronic materials. In fact, ion pairs of porphyrin-AuIII complexes as π-electronic cations, prepared with the combination of various anions including a π-electronic anion, formed dimension-controlled assemblies as thermotropic liquid crystals, whose ionic components were highly organized by electrostatic and π–π stacking interactions.[3–5] References A recent review: Haketa, Y.; Maeda, H.Bull. Chem. Soc. Jpn. 2018, 91,420.Recent examples: a) Maeda, H.;Takeda, Y.; Haketa, Y.; Morimoto, Y.; Yasuda, N. Chem. Eur. J. 2018, 24, 8910; b) Kaname, S.; Haketa, Y.; Yasuda, N.; Maeda, H. Org. Lett. 2018, 20, 3268; c) Maeda, H.; Okubo, T.; Haketa, Y.; Yasuda, N. Chem. Eur. J. 2018, 24, 16176; d) Yamakado, R.; Ishibashi, H.; Motoyoshi, Y.; Yasuda, N.; Maeda, H. Chem. Commun. 2019, 55, in press.Haketa, H.; Bando, Y.; Sasano, Y.; Tanaka, H.; Yasuda, N.; Hisaki,I.; Maeda, H.to be submitted.Assemblies of anionic porphyrins: Sasano, Y.; Yasuda, N.; Maeda, H. to be submitted.As related species, chiral metal complexes of dipyrrins, with half structures of porphyrin, have exhibited long lifetime photo-excited triplet states: Hirata, G.; Kobayashi, Y.; Yasuda, N.; Maeda, H. to be submitted.

Gas-Phase Ion/Ion Reactions Involving Tris-Phenanthroline Alkaline Earth Metal Complexes as Charge Inversion Reagents for the Identification of Fatty Acids.

Fatty acids (FA) play vital biological roles as energy sources, signaling molecules and key building blocks of complex lipids in cell membranes. Modifications to FA structure and composition are associated with the onset and progression of a number of chronic diseases. Consequently, the sensitive detection and unambiguous structure elucidation of FA is integral to the advancement of biomedical sciences. Recent advances in FA analysis have taken advantage of wet chemical derivatization to enhance detection and drive unique fragmentation in tandem mass spectrometry protocols. Here, we significantly further this approach through demonstrating gas-phase charge inversion of singly deprotonated FA ions, [M - H]-, using doubly charged tris-phenanthroline alkaline earth metal complexes, [Cat(Phen)3]2+ (Cat = Mg2+, Ca2+, Sr2+, or Ba2+). Metal cationized FA, [M - H + Cat]+ are obtained after the gas-phase ion/ion reaction. Low-energy collision-induced dissociation (CID) of the [M - H + Cat]+ cations facilitates double bond localization for a variety of monounsaturated and polyunsaturated FAs. Ultimately, detailed characterization presented unambiguous distinction among FA double bond positional isomers, such as n-3 and n-6 isomers. The method was successfully used to identify the FA profile of corn oil, including double bond localization for unsaturated FAs present.

The transformation from 2°-amine to 3°-amine of cyclam ring alters the fragmentation patterns of 1-tosylcytosine-cyclam conjugates.

The novel N-1-sulfonylcytosine-cyclam conjugates 1 and 2 conjugates are ionized by electrospray ionization mass spectrometry (ESI MS) in positive and negative modes (ES+ and ES- ) as singly protonated/deprotonated species or as singly or doubly charged metal complexes. Their structure and fragmentation behavior is examined by collision induced experiments. It was observed that the structure of the conjugate dictated the mode of the ionization: 1 was analyzed in ES- mode while 2 in positive mode. Complexation with metal ions did not have the influence on the ionization mode. Zn2+ and Cu2+ complexes with ligand 1 followed the similar fragmentation pattern in negative ionization mode. The transformation from 2°-amine in 1 to 3°-amine of cyclam ring in 2 leads to the different fragmentation patterns due to the modification of the protonation priority which changed the fragmentation channels within the conjugate itself. Cu2+ ions formed complexes practically immediately, and the priority had the cyclam portion of the ligand 2. The structure of the formed Zn2+ complexes with ligand 2 depended on the number of 3° amines within the cyclam portion of the conjugate and the ratio of the metal:ligand used. The cleavage of the cyclam ring of metal complexes is driven by the formation of the fragment that suited the coordinating demand of the metal ions and the collision energy applied. Finally, it was shown that the structure of the cyclam conjugate dictates the fragmentation reactions and not the metal ions.

Behavior of Ionic Liquids Around Charged Metal Complexes: Investigation of Homogeneous Electron Transfer Reactions Between Metal Complexes in Ionic Liquids

The second-order electron transfer reaction between the photo-excited triplet state of [Zn(TPP)]* (TPP = 5,10,15,20-tetraphenylporphyrin) and [Co(sep)]3+ (sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) was investigated in three ionic liquids (ILs, 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with R = butyl, pentyl, and hexyl) and in acetonitrile. Results of electrochemical and kinetic measurements indicated that ILs dissociate in the vicinity of charged metal complexes and at electrodes, although the dissociated anionic and cationic components of the ILs seem to exist as pairs around the metal complexes. Second-order rate constants for the electron transfer reaction are 1.88 × 109, 3.65 × 107, 2.63 × 107, and 2.01 × 107 kg·mol−1·s−1 in acetonitrile and in the butyl, pentyl and hexyl ILs, respectively, at 298 K, after correction of the contribution of diffusion. The average slope of the plot of the logarithmic second-order rate constants observed in acetonitrile and ILs against the logarithmic viscosity of each solvent was − 0.84. However, the slope of the same plot was much steeper (− 4.1) when data for only the three ILs were used. Detailed analyses of the experimental results on the basis of the Latner–Levin cross relation and the Marcus theory lead to the conclusion that the solvent properties such as the dielectric constant and refractive index around the polarized/charged transition states are different from those for the bulk ILs: observed self-exchange rate constants did not exhibit the Pekar factor dependence when dielectric constants and refractive indices for bulk ILs are used.

Investigation of Non-covalent Interactions of Metal Complexes with DNA in Cell-free Systems.

Non-covalent interactions of metallo compounds with DNA range from the simple, unspecific electrostatic binding of a positively charged metal complex to the sequence-selective recognition of DNA binding sites due to shape, size, symmetry and hydrogen bonding complementarity of a rationally designed system. Metal complexes that recognize and target specific DNA sequences or particular structures are of considerable interest as therapeutics, diagnostics or structural probes. To gain molecular level insight into DNA-metal complex interactions, binding studies are carried out in cell-free systems using isolated DNA or short oligonucleotides. For this, a powerful toolbox of complementary spectroscopic and biophysical techniques is available. This review focuses on the most frequently applied spectroscopic methods; UV/Vis, CD, LD, fluorescence emission and NMR spectroscopy and is aimed at giving the reader an overview of the qualitative and/or quantitative information that can be obtained. After a short introduction into DNA structures and non-covalent metal complex-DNA interactions, each spectroscopic method will be discussed. In the last section a few selected studies will be described as illustrative examples for the potential of the various spectroscopic methods.

Uncertainty Principle in the Elucidation of the Extraction Mechanism of Ions from Aqueous towards Ionic Liquid Phases. PtCl62− and [C1C8IM][NTf2] as a Textbook Case

AbstractThe extraction mechanism of Pt(IV) from aqueous HCl solutions towards the ionic liquid 1‐octyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide ([C1C8IM][NTf2]) is deciphered. To that end, aqueous speciation of Pt(IV) as a function of HCl was studied by 195Pt NMR. Influence of the concentration of HCl (ranging from 1 to 12 M) and of the initial concentration of Pt(IV) (ranging from 10−5 to 10−2 M) in the aqueous phase on the distribution coefficients of Pt(IV) was systematically investigated. The decrease in D with the concentrations of HCl and Pt(IV) observed is quantitatively very well described with a model using three chemical equilibria, namely the solubility of the ionic liquid in water, the extraction of PtCl62− through an ion‐pair formation and that of Cl− towards the ionic liquid phase. D values were then predicted over a wider range of concentrations for HCl and PtCl62−. Alternatively, the extraction of PtCl62− was described using an anion exchange hypothesis and it is demonstrated that these two apparently different mechanisms are actually equivalent within hydrophobic ionic liquids. Because the two versions of the extraction model presented here are not specific to PtCl62− or the ionic liquid, they can be generalized to the extraction of any charged (metal) complex and any hydrophobic ionic liquid, allowing us to outline a general uncertainty principle in the determination of extraction mechanisms in systems involving ionic liquids

Labilities of aqueous nanoparticulate metal complexes in environmental speciation analysis

Environmental context Sorbing nanoparticles can have a significant effect on the speciation of small ions and molecules in the environment. The reactivity of nanoparticulate-bound species can differ significantly from that of their molecular or colloidal counterparts. We present a conceptual framework that describes the chemodynamics and lability of nanoparticulate metal complexes over a wide range of experimental timescales and environmental conditions. Abstract An inherent property of a dispersion of charged nanoparticles is that their charges and reactive sites are spatially confined to the particle body which is at a different potential from that in the bulk medium. This feature has important consequences for the reactivity of nanoparticulate complexants: the diffusive rate of reactant supply is lower as compared to molecular complexants, whereas the local concentration of reactant ions may be enhanced if the particle’s electric field has the opposite charge sign. These effects are most dramatic for soft nanoparticles for which the electrostatic accumulation mechanisms operate on a 3-D level. We show how the interplay of these effects governs the reactivity of charged nanoparticulate metal complexes (M-NPs) at the surface of an analytical speciation sensor. A theoretical framework is presented that describes the lability of M-NP species over a range of effective timescales for different electrochemical and other dynamic speciation analysis techniques. The concepts are illustrated by electrochemical stripping data on metal complexes with natural soft nanoparticles of humic acid.

A Novel Mechanism for the Extraction of Metals from Water to Ionic Liquids

We present a novel mechanism for the extraction of metals from aqueous phases to room-temperature ionic liquids (ILs) by use of a high-temperature salt as an extraction agent. The mechanism capitalizes on the fact that charged metal complexes are soluble in ILs; this allows for extraction of charged complexes rather than the neutral species, which are formed by conventional approaches. The use of a well-chosen extraction agent also suppresses the competing ion-exchange mechanism, thus preventing degradation of the ionic liquid. The approach permits the use of excess extractant to drive the recovery of metals in high yield. This work presents both a thermodynamic framework for understanding the approach and experimental verification of the process in a range of different ILs. The method has great potential value in the recovery of metals, water purification and nuclear materials processing.

Analysis of the interaction of the molybdenum hydroxylase PaoABC from Escherichia coli with positively and negatively charged metal complexes

An unusual behavior of the periplasmic aldehyde oxidoreductase (PaoABC) from Escherichia coli has been observed from electrochemical investigations of the enzyme catalyzed oxidation of aromatic aldehydes with different mediators under different conditions of ionic strength. The enzyme has similarity to other molybdoenzymes of the xanthine oxidase family, but the catalytic behavior turned out to be very different. Under steady state conditions the turnover of PaoABC is maximal at pH4 for the negatively charged ferricyanide and at pH9 for a positively charged osmium complex. Stopped-flow kinetic measurements of the catalytic half reaction showed that oxidation of benzaldehyde proceeds also above pH7. Thus, benzaldehyde oxidation can proceed under acidic and basic conditions using this enzyme, a property which has not been described before for molybdenum hydroxylases. It is also suggested that the electron transfer with artificial electron acceptors and PaoABC can proceed at different protein sites and depends on the nature of the electron acceptor in addition to the ionic strength.

The parent tetrathiafulvalene–terpyridine dyad: synthesis and metal binding properties

The still unknown parent tetrathiafulvalene–terpyridine- ligand 5 (namely TTF–terpy), was synthesized through a straightforward strategy. The dyad exhibits an intramolecular charge transfer (ICT) which was evidenced by UV–vis electronic absorption. Complexation of various transition metal cations by this redox-active ligand was studied by UV–vis absorption spectroscopy as well as by cyclic voltammetry titration studies, supporting the dual functional character of this system. In addition, these results demonstrate that ligand 5 is a suitable building block for the preparation of electroactive neutral as well as charged metal complexes.

Infrared Multiple-Photon Dissociation Spectroscopy of Tripositive Ions: Lanthanum–Tryptophan Complexes

Collision-induced charge disproportionation limits the stability of triply charged metal ion complexes and has thus far prevented successful acquisition of their gas-phase IR spectra. This has curtailed our understanding of the structures of triply charged metal complexes in the gas phase and in biological environments. Herein we report the first gas-phase IR spectra of triply charged La(III) complexes with a derivative of tryptophan (N-acetyl tryptophan methyl ester), and an unusual dissociation product, a lanthanum amidate. These spectra are compared with those predicted using density functional theory. The best structures are those of the lowest energies that differ by details in the π-interaction between La(3+) and the indole rings. Other binding sites on the tryptophan derivative are the carbonyl oxygens. In the lanthanum amidate, La(3+) replaces an H(+) in the amide bond of the tryptophan derivative.