Guy Serratrice

Calcein as a Fluorescent Probe for Ferric Iron APPLICATION TO IRON NUTRITION IN PLANT CELLS

The recent use of calcein (CA) as a fluorescent probe for cellular iron has been shown to reflect the nutritional status of iron in mammalian cells (Breuer, W., Epsztejn, S., and Cabantchik, Z. I. (1995) J. Biol. Chem. 270, 24209–24215). CA was claimed to be a chemosensor for iron(II), to measure the labile iron pool and the concentration of cellular free iron(II). We first study here the thermodynamic and kinetic properties of iron binding by CA. Chelation of a first iron(III) involves one aminodiacetic arm and a phenol. The overall stability constant log β111 of FeIIICAH is 33.9. The free metal ion concentration is pFeIII = 20.3. A (FeIII)2 CA complex can be formed. A reversible iron(III) exchange from FeIIICAH to citrate and nitrilotriacetic acid is evidenced when these ligands are present in large excess. The kinetics of iron(III) exchange by CA is compatible with metabolic studies. The low reduction potential of FeIIICAH shows that the ferric form is highly stabilized. CA fluorescence is quenched by 85% after FeIIIchelation but by only 20% using FeII. Real time iron nutrition by Arabidopsis thaliana cells has been measured by fluorimetry, and the iron buffer FeIIICAH + CA was used as source of iron. As a siderophore, FeIIICAH promotes cell growth and regreening of iron-deficient cells more rapidly than FeIIIEDTA. We conclude that CA is a good chemosensor for iron(III) in cells and biological fluids, but not for Fe(II). We discuss the interest of quantifying iron buffers in biochemical studies of iron, in vitro as well as in cells.

An ab initio LCAO-MO-SCF study of reaction paths for proton transfer in ammonium aqueous solution

The possible mechanisms for proton transfer in ammonium aqueous solutions are discussed through ab initio LCAO-MO-SCF calculations for the following hydrogen-bonded complexes : [NH4 + … NH3] ; [NH4 + … OH2] ; [NH4 + … OH2 … OH2] ; [NH4 + … OH2 … NH3] and [H2O … NH4 + … OH2 … OH2]. The energy curve along the reaction coordinate is drawn for the first three systems. A double well potential curve is obtained for the two symmetrical systems with a very low barrier to proton transfer : 2·9 kcal/mole for the system [NH4 + … NH3] and 4·3 kcal/mole for the system [NH4 + … H2O … NH3]. For both systems the exchange mechanism involves three successive steps : association, transfer and dissociation. Solvation may affect the energetics of the first and third steps. For the unsymmetrical system NH4 + + H2O, the energy would increase continuously during the steps of proton transfer and dissociation. Hence the process of proton transfer between an ammonium ion and a water molecule may take place in solution only if assis...

Synthesis and NMR study of two lipophilic iron(III) sequestering agents based on 8-hydroxyquinoline; H-bonding and conformational changes

Abstract The synthesis of two tripodal iron chelating agents based on 8-hydroxyquinoline is described. The ligands consist of tris(2-aminoethylamine) (spacer) linked in 2- or 7- position to three 8-hydroxyquinoline units (allowing the complexation of iron). NMR study of these ligands in DMSO-d6 solutions evidence intramolecular H-bond networks inducing conformational changes in relation to the protonation state of the tertiary amine.

Fluorocarbons as oxygen carriers. I. An NMR study of oxygen solutions in hexafluorobenzene

Relaxation times of 13C and 19F (or 1H) nuclei of hexafluorobenzene (or benzene) solutions of oxygen at 25° C are measured at several frequencies using variable concentrations of oxygen (up to 0.09 M under 6 atm). The influence of paramagnetic oxygen on neighboring solvent nuclei can be explained by dipolar electronic spin–nuclear spin interactions modulated by translation diffusion in the case of C6H6 solutions only. The results for C6F6 solutions suggest a discontinuous diffusion through the C6F6 liquid ’’lattice’’ with a short residence time (≲10−12 sec) in privileged positions of the lattice.

Host–guest interaction between cyclen based macrotricyclic ligands and phosphate anions. A potentiometric investigation

The host-guest interaction between orthophosphate, pyrophosphate and triphosphate anions and three cyclen based macrotricyclic ligands possessing ortho- (TOC), meta- (TMC) and para-xylenyl (TPC) linkers was investigated by potentiometric measurements. The ternary species present in solution and their stability constants have been determined. The different behaviours are explained in terms of hydrogen bond formation and coulombic attraction between the organic host and the inorganic guest. The selectivity, illustrated with species distribution diagrams, is discussed. The results unambiguously showed the importance of the distance between the two cyclen cores and emphasized the increasing of the triphosphate species selectivity together with the cavity size of the ligand. A comparison of the present results with those obtained with their mono-bridged homologues is also discussed.

Fluorocarbons as oxygen carriers. II. An NMR study of partially or totally fluorinated alkanes and alkenes

Highly fluorinated compounds of the general type RFRH or RFRH′RF, with RF: n−CnF2n+1 n = 6, 7, or 8; RH: C2H5, CHCH2, n-C8H17; RH′: CHCH, CH2CH2, are studied either as pure degassed liquids or as solvents of oxygen, using 13C relaxation times T1 measurements in each case. Comparison of the relaxation data for the degassed liquids with those relative to the analogous n-alkanes provides evidence for slower internal segmental motions in the perfluoroalkyl chains. This rate decrease is shown to arise mainly from purely inertial effects and not from increased rotational potential barriers, thus suggesting similar flexibilities of both hydrocarbon and perfluorocarbon chains. Solubilities of oxygen (in mole fractions) are higher in fluoroalkanes than in previously studied hexafluorobenzene (J-J. Delpuech, M. A. Hamza, G. Serratrice, and M. J. Stebe, J. Chem. Phys. 70, 2680 (1979)). Relaxation data are expressed by the variation rates qx of relaxation rates T1−1 per mole fraction of dissolved oxygen. Values of qx. roughly decrease with the total length of the aliphatic chains, and from the ends of the center of each chain, except for C6F13CHCHC6F13. These results are not consistent with specific attractive oxygen-fluorine forces, the major factor for solubility being the liquid structure of the solvent, mainly determined by the shape of molecules, according to Chandlers viewpoint.

Synthesis and iron(III) complexing ability of CacCAM, a new analog of enterobactin possessing a free carboxylic anchor arm. Comparative studies with TRENCAM

A new tricatecholate, CacCAM, has been synthesized by attachment of three catecholamide subunits to a CO2H-functionalized triamine backbone. The solution coordination chemistry of the ligand and its iron(III) complex have been investigated by potentiometric, spectroscopic and kinetic methods. The results are compared to those obtained with TRENCAM, which possesses the same catecholamide subunits, but a TREN [tris(2-aminoethyl)-amine] backbone. The stability constant (log β110=42.9, pFe=27.5) is close to that of TRENCAM (log β110=43.6, pFe=27.8), showing that a change of backbone does not significantly alter the iron chelating ability of the ligand. Kinetics of iron exchange between Fe(III)–EDTA and CacCAM or TRENCAM involves similar rate constants (16±1 and 14±2 M−1 s−1, respectively). EPR data show that the iron(III) does not have the same distorted rhombic geometry in the two complexes. Fe(III)–CacCAM has been tested as a source of iron in nutritional experiments with Arabidopsis thaliana plant cells: its efficiency is good, comparable to that of Fe(III)–EDTA, the most widely used iron complex for plant nutrition.

Iron-citrate complexes and free radicals generation: is citric acid an innocent additive in foods and drinks?

The generation of free radicals (Fenton chemistry) from various iron citrate complexes has been studied. Spin trapping methods have been used. The results can question concerning the innocence of added citric acid in foods and cold drinks. We concluded that in absence of pathological situation citric acid is probably not dangerous but it may become dangerous in situation of oxidative stress and/or iron overload.

Design of iron chelators: syntheses and iron (III) complexing abilities of tripodal tris-bidentate ligands.

The interest in synthetic siderophore mimics includes therapeutic applications (iron chelation therapy), the design of more effective agents to deliver Fe to plants and the development of new chemical tools in order to study iron metabolism and iron assimilation processes in living systems. The design of ligands needs a rational approach for the understanding of the metal ion complexing abilities. The octahedral arrangement of donor atoms is the most favourable geometry, allowing the maximum possible distance between their formal or partial negative charges. Hexadentate chelators, usually of the tris-bidentate type, can accommodate the metal coordination sphere and are well-suited to obtain high pFe values. The first part of this review is dedicated to selected synthetic routes, taking into account (i) the nature of the chelating subunits, connecting groups and spacers, (ii) the water-solubility and hydrophilic/lipophilic balance, (iii) the chirality and (iv) the possibility of grafting probes or vectors. In the second part, we discuss the role of the molecular design on complexing abilities (thermodynamics and kinetics). The bidentate 8-hydroxyquinoline moiety offers an alternative to the usual coordinating hydroxamic acids, catechols and/or α-hydroxycarboxylic acids groups encountered in natural siderophores. The promizing results obtained with the tris-hydroxyquinoline-based ligand O-TRENSOX are summarized. O-TRENSOX exhibits a high and selective affinity for Fe(III) complexation. Its efficiency in delivering Fe to plants, iron mobilization, cell protection, and antiproliferative effects has been evidenced. Other chelators derived from O-TRENSOX (mixed catechol/8-hydroxyquinoline ligands, lipophilic ligands) are also described. Some results question the relevance of partition coefficients to foresee the activity of iron chelators. The development of probes (fluorescent, radioactive, spin labelled) based on the O-TRENSOX backbone is in progress in order to get insights in the complicated iron metabolism processes.

New Polypodal Polycarboxylic Ligands − Complexation of Rare-Earth Ions in Aqueous Solution

The complexation ability of new polypodal carboxylic ligands prepared from 2,2-bis(hydroxymethyl)-1-alkanols was investigated in aqueous solution with Y3+ and various lanthanide ions (La3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+). The values of the stability constants for ML complexes are of the order of 104 for the hexadentate trioxypropionates, 107 for the hexadentate trioxyacetates and 1014 for the nonadentate triaminohexaacetates. In these systems, the nitrogens of the amino groups seem to be better chelating atoms than the oxygens of the ether groups. The hydration numbers of dysprosium(III) complexes have been determined from the DyIIIinduced 17O NMR water shifts.