Hydroxamic acids interactions with metals in aqueous and micellar media: a mechanistic study of complexation reactions and metallacrown formation

This thesis is focused on the different aspects of hydroxamic acids coordination, in particular on their ability to give rise to metal complexes of different stoichiometry and geometry. An in depth analysis of the binding mechanisms is carried out with the help of complementary approaches, ranging from classical spectrophotometry to a more peculiar kinetic analysis by fast techniques. Hydroxamic acids have a high binding affinity to a range of transition metal ions, particularly Fe(III), and are ligands ubiquitous in coordination chemistry and chemical biology. They have been widely employed as chelating agents for a long time and studies of their metal complexes still attract great attention. Since the possible biological activity of hydroxamic acids must be correlated with the formation of their metal complexes, interest in these systems has increased extensively over the past decade. Furthermore, hydroxamic acids can be used in extractive metallurgy as flotation agents, because of their ability to form very stable chelates with various metal ions. Hence, studies on hydroxamic acids metal complexes formation and dissociation reactions are object of vast interest, as they conduct to the comprehension of the involved microscopic mechanisms and to the evaluation of their physical and chemical requirements. In this context this thesis focuses on the equilibria and mechanisms involved in this class of complexation reactions mainly in aqueous solution, but also in micellar media. Reactions of some hydroxamic acids with three relevant and common cations in biological, industrial and environmental fields are investigated. In particular, examples of three different modes of coordination of hydroxamic acids with metal ions are reported and analyzed: from mononuclear complexes (Ni(II)) to supramolecular compounds (Cu(II)), passing through a dinuclear complex (Fe(III)). The hydroxamic acids taken into account are salicylhydroxamic acid (SHA), benzohydroxamic acid (BHA) and (S)-α-alaninehydroxamic acid (Alaha). This study is mainly based on a combination of kinetic and thermodynamic methods in order to analyze the investigated systems in different perspectives. In this thesis a new method is developed that enables the evaluation of the separate contributions to the equilibrium and rate parameters in the aqueous and micellar phase (Nickel-SHA system). Furthermore, the formation of a dinuclear complex (M2L) in aqueous solution, formed by the reaction of Fe(III) with SHA, is demonstrated for the first time. Such a complex provides the rationale for the building of supramolecular structures as metallacrowns, whose self-assembly mechanism is also worked out in this thesis work, together with metallacrowns host-guest solution equilibria (Copper-Alaha and Copper-Lanthanum-Alaha systems). Special attention is also paid to the complex speciation of the Fe(III) ion in aqueous solution. In particular, the non negligible contribution of the Fe(III) trimer is proved and its structure is clarified. This thesis, together with putting into light the richness of hydroxamic acids coordination mechanisms, reveals the importance of a complementary approach, based on both kinetics and thermodynamics, for the treatment of complex systems involved in multiple equilibria

Caratterizzazione cinetica, termodinamica e spettroscopica di un complesso dinucleare del ferro(III) con l'acido salicilidrossammico.

Alquanto recentemente è stata sintetizzata e caratterizzata una interessante classe di composti nota con il nome di metallacrown. Questi composti inorganici imitano i più noti crownethers nell’architettura chimica e, come i crownethers, sono in grado di alloggiare uno ione metallico nella cavità centrale. Il motivo di base della struttura dei metallacrown è costituito dalla sequenza M-N-O-M che, ripetuta quattro volte, costituisce l’anello che delimita la cavità della molecola. L’acido salicilidrossammico (SHA) è particolarmente adatto a fornire l’unità base ed infatti esistono molti metallacrown che contengono l’SHA come parte dell’anello macrociclico. Bisogna però notare che, per formare il motivo di base, una singola molecola di SHA deve essere in grado di legarsi a due ioni metallici. Ciò comporta, inoltre, la doppia deprotonazione del gruppo idrossammato dell’SHA. Un complesso binucleare di questo tipo dovrebbe dunque agire da precursore della formazione dei metallacrown. Fino ad oggi non si conoscono complessi binucleari dell’acido salicilidrossammico stabili in soluzione acquosa, così come non è mai stata osservata la doppia deprotonazione del gruppo idrossammato, per cui la formazione di complessi binucleari dell’SHA è stata solo ipotizzata, ma non dimostrata. Questa tesi rientra nel quadro di un progetto teso ad individuare il meccanismo di formazione dei metallacrown ed il suo scopo consiste nella ricerca di un complesso precursore con le caratteristiche sopra delineate, nonché nella caratterizzazione del suo comportamento in soluzione

Focus on speciation assessment in marine radiochemistry with Xray Absorption Spectroscopy

International audienceThe solubility, migration behavior and bioavailability of radionuclides in the marine environment strongly depend on their speciation. This focus article reviews the state-of-the-art and recent advances in the determination of radionuclide speciation in seawater, which is still challenging because of the very high ionic strength of the medium associated with ultra-trace concentrations of these elements in the oceans. In particular, we have highlighted the contribution and usefulness of synchrotron-based techniques such as X-ray Absorption Spectroscopy. Within this scope, we overview some major radionuclides in seawater, their natural or anthropogenic origin, their reactivity and natural concentrations. We outline the theoretical speciation models currently used, based on thermodynamic stability constants, and compare them to published experimental data recently obtained from spectroscopic investigation of radionuclides in natural seawater samples. Finally, we discuss some leading perspectives on radionuclide speciation using X-ray Absorption Spectroscopy in environmental samples at concentrations that must deal with spectroscopy detection limits

Perylene bisimide metal complexes as new MWCNTs dispersants: Role of the metal ion in stability and temperature sensing

We report on new dispersants based on metal complexes of an extended polycylic aromatic perylene bisimides (PeryC_M), suitable for the exfoliation of MWCNTs. The formal 1:1 metal complexes provided well-disentangled and undamaged MWCNTs and with a content at least 35 wt.% higher than that given by the bare ligand. The ionic charges at the periphery of PeryC_M endowed the π − conjugated perylene core with electrostatic repulsions, thus enhancing MWCNTs dispersibility. MWCNT-based mixtures displayed electrical resistance depending on the MWCNTs content and a typical semiconducting (activated) electrical transport with decreased resistance when heated within the physiological temperature range, i.e. from 20 to 40 °C. While perypheral charges are meaningful for MWCNTs deboundling and stabilization, they do not play any significant role in determining resistance sensitivity to temperature variations within the physiological regime. A negative temperature coefficient of about 10−2 K−1 and comparable to that of common thermistors was found for the investigated sensors

Comparative analysis of the perception of nuclear risk among two populations (expert/non-expert) in France

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Uranium speciation in a chalky soil

International audienceIn this article, the speciation and behavior of anthropogenic metallic uranium deposited on natural soil is approached by combining EXAFS (Extended X-ray Absorption Fine Structure), and TRLFS (Time-Resolved Laser-induced Fluorescence Spectroscopy). First, uranium (uranyl) speciation was determined along the vertical profile of the soil and bedrock by linear combination fitting of the EXAFS spectra. It shows that uranium migration is strongly limited by the sorption reaction onto soi

Interaction of hydroxamic acids with metals: from simple complexes to metallacrowns

The interaction of hydroxamic acids with metal ions, giving rise to bound forms of increased complexity, is described through three examples, making use of the kinetic approach. In the first example the results of an investigation on the binding of Ni(II) to salicylhydroxamic acid (SHA) and phenylbenzohydroxamic acid (PBHA) in water and in sodium dodecylsulphate (SDS) solution are presented. Besides observing the so-called “micellar catalytic effect” a change of mechanism has been detected on changing the medium from water to micelle. Actually, while in water the principal path involves reaction (1) Ni2+ + L- ⇌ NiL+ (1) in SDS the main path is represented by reaction (2) NiOH+ + HL ⇌ NiL+ + H2O (2) where HL denotes the unprotonated hydroxamic acid. The analysis of the results shows that the acid strength of the Ni(H2O)6 2+ ion increases by more than two orders of magnitude on going from water to SDS. The second example is concerned with a stopped-flow investigation of SHA and benzohydroxamic acid (BHA) with Fe(III). It is shown for the first time that SHA is able to bind two Fe(III) ions. The important feature of this complex is the binding of the second Fe(III) ion at the O,N site. This binding implies total deprotonation of SHA, a process not observable with the free acid. Moreover and more important, this complex presents the O-Fe-N-O sequence which characterizes the metallacrown structure. The complex does constitute the fundamental repeating unit of metallacrowns. In the third example are described the main reactive steps that lead to metallacrown formation. The kinetics of the Cu(II)-α-alanine hydroxamic acid metallacrown (12MC4) formation have been investigated by mixing Cu(II) and ligand (L) in the 5:4 ratio. Different stoichiometric ratios do not lead to formation of any complex structure. The kinetic behaviour shows that mononuclear ML an dinuclear M2L2 structures are rapidly formed and two dimers, reacting together, form a tetrameric structure which, upon adding further Cu(II) ion, originates the cyclic species 12MC4. In the presence of a slight excess of L and lanthanum, 12MC4 is converted to 15MC5. The binding of La(III) to the cavity of the metallacrown has been investigated as well

Thermodynamics of Calcium binding to the Calmodulin N-terminal domain to evaluate site-specific affinity constants and cooperativity

International audienceCalmodulin (CaM) is an essential Ca(II)-dependent regulator of cell physiology. To understand its interaction with Ca(II) at a molecular level, it is essential to examine Ca(II) binding at each site of the protein, even if it is challenging to estimate the site-specific binding properties of the interdependent CaM-binding sites. In this study, we evaluated the site-specific Ca(II)-binding affinity of sites I and II of the N-terminal domain by combining site-directed mutagenesis and spectrofluorimetry. The mutations had very low impact on the protein structure and stability. We used these binding constants to evaluate the inter-site cooperativity energy and compared it with its lower limit value usually reported in the literature. We found that site I affinity for Ca(II) was 1.5 times that of site II and that cooperativity induced an approximately tenfold higher affinity for the second Ca(II)-binding event, as compared to the first one. We further showed that insertion of a tryptophan at position 7 of site II binding loop significantly increased site II affinity for Ca(II) and the intra-domain cooperativity. ΔH and ΔS parameters were studied by isothermal titration calorimetry for Ca(II) binding to site I, site II and to the entire N-terminal domain. They showed that calcium binding is mainly entropy driven for the first and second binding events. These findings provide molecular information on the structure-affinity relationship of the individual sites of the CaM N-terminal domain and new perspectives for the optimization of metal ion binding by mutating the EF-hand loops sequences

Nuclear risk, from Radioisotope (bio)chemistry to Public Perception

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Inter-Site Cooperativity of Calmodulin N-Terminal Domain and Phosphorylation Synergistically Improve the Affinity and Selectivity for Uranyl

International audienceUranyl–protein interactions participate in uranyl trafficking or toxicity to cells. In addition to their qualitative identification, thermodynamic data are needed to predict predominant mechanisms that they mediate in vivo. We previously showed that uranyl can substitute calcium at the canonical EF-hand binding motif of calmodulin (CaM) site I. Here, we investigate thermodynamic properties of uranyl interaction with site II and with the whole CaM N-terminal domain by spectrofluorimetry and ITC. Site II has an affinity for uranyl about 10 times lower than site I. Uranyl binding at site I is exothermic with a large enthalpic contribution, while for site II, the enthalpic contribution to the Gibbs free energy of binding is about 10 times lower than the entropic term. For the N–terminal domain, macroscopic binding constants for uranyl are two to three orders of magnitude higher than for calcium. A positive cooperative process driven by entropy increases the second uranyl-binding event as compared with the first one, with ΔΔG = −2.0 ± 0.4 kJ mol−1, vs. ΔΔG = −6.1 ± 0.1 kJ mol−1 for calcium. Site I phosphorylation largely increases both site I and site II affinity for uranyl and uranyl-binding cooperativity. Combining site I phosphorylation and site II Thr7Trp mutation leads to picomolar dissociation constants Kd1 = 1.7 ± 0.3 pM and Kd2 = 196 ± 21 pM at pH 7. A structural model obtained by MD simulations suggests a structural role of site I phosphorylation in the affinity modulation