Amphiphilic Iron Research Articles

Photoreduction of a Self-Assembled (Lipidporphyrinato)iron(III) Complex in Saline by LMCT Excitation: Co-Aggregated Hyaluronic Acid Allows an Irreversible Electron Transfer

Abstract An amphiphilic (tetraphenylporphyrinato)iron(III) derivative with four alkylphosphocholine groups and a proximal imidazole [(lipidporphyrinato)iron(III); 1a] was self-assembled in phosphate-buffered saline (pH 7.3, [NaCl] = 0.15 M) to form spherical micelles with a diameter of 10 nm. The obtained solution showed a distinct absorption band at 362 nm, which was assigned to the ligand-to-metal [Cl− to iron(III)] charge transfer (LMCT) transition. Light irradiation into this CT band under an Ar atmosphere did not induce any changes in the UV-vis absorption spectrum. On the other hand, the addition of glucose (150 mM) to the solution led to complete photoreduction of the central iron(III) ion, giving a five-N-coordinated high-spin iron(II) complex. It has also been found that a small excess amount of hyaluronic acid ([units] = 100 µM) showed the same effect. The photoreduction was only seen by LMCT irradiation in the presence of the saccharide. It probably occurred via intramolecular electron transfer from Cl− to iron(III), and the produced chlorine radical was scavenged by the saccharide, which prevented a back electron transfer reaction (the quantum yields; ca. 0.007). Interestingly, hyaluronic acid changed the morphology of the 1a assembly from the micelle to a thin fiber. This co-aggregated structure with hyaluronic acid would be responsible for the effective photoreduction of 1a. The viscosity of the fiber solution significantly decreased during the photoreduction, which suggests that radical trapping induces depolymerization of the hyaluronic acid. Laser flash photolysis experiments showed that the reduction and the imidazole association to the iron(II) center are completed within 50 ns after a laser pulse. The photoreduced (lipidporphyrinato)iron(II) fibers can reversibly bind and release O2 similar to the same fibers which were prepared by chemical reduction using ascorbic acid.

Spin crossover phenomenon of a semi-fluorinated iron (II) complex organized in a Langmuir–Blodgett film

A new amphiphilic iron (II) complex bearing semi-fluorinated chains has been organized in Langmuir and Langmuir–Blodgett (LB) films. This molecule forms a perfectly stable monolayer at the gas–water interface. Such a film can be transferred easily onto a solid substrate leading to well-defined multilayers. The spin crossover phenomenon occurring in this material has been studied by infrared spectroscopy and magnetization measurements. In the LB film architecture, the iron complex appears to be quenched in a high spin state. This quenching can be released after a thermal annealing and is therefore associated to the specific organization induced by the LB technique.

Spin Crossover of a Langmuir−Blodgett Film Based on an Amphiphilic Iron(II) Complex

An amphiphilic iron(II) complex [FeIIL2(NCS)2, where L is a substituted bipyridine] exhibiting a thermal spin-crossover phenomenon around room temperature is chemically unstable at the gas−water interface. However, it forms a perfectly stable Langmuir film when formamide−water mixtures are used as a subphase. After transfer onto a solid substrate, a well-defined Langmuir−Blodgett (LB) film was then obtained. Infrared spectroscopy at variable temperature showed that the spin conversion still occurs in this lamellar material. The specific organization imposed by the LB technique leads to a surroundings effect demonstrated, in particular, by thermal cycles. The understanding of this effect is crucial to control any thermal or light-induced spin conversion within an LB film.

Spin-crossover complex stabilized on a formamide/water subphase

Formamide/water mixtures were used as subphase for a Langmuir film of an amphiphilic iron(II) complex exhibiting a thermally-induced spin-crossover phenomenon around room temperature. The use of formamide enables the chemical stabilization of this complex (contrary to a pure water subphase) and leads to perfectly defined Langmuir–Blodgett (LB) films of this molecule. Variable temperature infrared spectrometry demonstrates that the low-spin to high-spin conversion occurs in this multilayer. Such a strategy based on a monolayer spread on organic solvent permits the formation of Langmuir and LB films from compounds chemically sensitive to water.

Amphiphilic peroxynitrite decomposition catalysts in liposomal assemblies

Background: Peroxynitrite (ONOO −), a toxic biological oxidant, has been implicated in many pathophysiological conditions. The water-soluble porphyrins 5,10,15,20-tetrakis( N-methyl-4′-pyridyl)porphinato iron(III) (FeTMPyP) and manganese(III) (MnTMPyP) have recently emerged as potential drugs for ONOO − detoxification, and FeTMPyP has demonstrated activity in models of ONOO − related disease states. We set out to develop amphiphilic analogs of FeTMPyP and MnTMPyP suitable for liposomal delivery in sterically stabilized liposomes (SLs). Results: Three amphiphilic iron porphyrins (termed 1a-c) and three manganese porphyrins (termed 2a-c) bound to liposomes and catalyzed the decomposition of ONOO −. The polyethylene-glycol-linked metalloporphyrins 1b and 2b proved the most effective of these catalysts, rapidly decomposing ONOO − with second-order rate constants (k cat) of 2.9 × 10 5 M −1s −1 and 5.0 × 10 5 M −1 s −1, respectively, in dimyristoylphosphatidylcholine liposomes. Catalysts 1b and 2b also bound to SLs, and these metal loporphyrin-SL constructs efficiently catalyzed ONOO − decomposition (k cat ≈ 2 × 10 5 M −1 s −1). The analogous metalloporphyrins 1a and 2a, which are not separated from the vesicle membrane surface by polyethylene glycol linkers, were significantly less effective (k cat ≈ 3.5 × 10 4 M −1 s −1). Conclusions: For these amphiphilic analogs of FeTMPyP and MnTMPyP, the polarity of the environment of the metalloporphyrin headgroup is intimately related to the efficiency of the catalyst; a polar aqueous environment is essential for effective catalysis of ONOO − decomposition. Thus, catalysts 1b and 2b react rapidly with ONOO − and are potential therapeutic agents that, unlike their water-soluble TMPyP analogs, could be administered as liposomal formulations in SLs. These SL-bound amphiphilic metalloporphyrins may prove to be highly effective in the exploration and treatment of ONOO − related disease states.

Investigations of iron coordination and redox reactions in seawater using 59Fe radiometry and ion-pair solvent extraction of amphiphilic iron complexes

Iron coordination and redox reactions in synthetic and coastal seawater were investigated at nanomolar concentrations using 59Fe radiometry and ion-pair solvent extraction of iron chelated by sulfoxine (8-hydroxyquinoline-5-sulfonate) and BPDS (bathophenanthroline disulfonate). Using sulfoxine, we determined the rate at which the monomeric Fe(III) hydroxide species present in seawater of pH 8 are complexed by the microbial siderophore deferriferrioxamine B and the synthetic chelator EDTA (ethylenediaminetetraacetic acid). Forward rate constants of 2 × 10 6M −1s −1 and 20 M −1s −1, respectively, were obtained. The kinetics of these reactions have not been measured previously at pH values near that of seawater. Conditional equilibrium constants measured for the Fe(III)-EDTA system are consistent with published stability constants for EDTA complexes and for Fe(III) hydrolytic equilibria minus the neutral Fe(OH) 3 o species, suggesting it is not quantitatively significant near pH 8. Commercial humic acid was found to have sufficient affinity for iron to compete with Fe(III) hydrolysis in seawater, and limited evidence was obtained for an interaction with dissolved organic matter in coastal seawater. In our investigations of redox reactions using BPDS to trap Fe(II) produced in the medium, we observed enhanced photoreduction of Fe(III) by humic acid as well as reduction induced by solutes released from phytoplankton in seawater of pH 8. Although the method is sensitive enough to work at near-oceanic levels of iron, the difficulty in distinguishing Fe(II) generated by Fe(III)-BPDS interactions from Fe(II) produced by other means limits its utility. This analytical ambiguity may be generalizable to other methods which measure ferrous iron in seawater using Fe(II)-specific ligands.

Interaction between cofacially oriented metalloporphyrins in aqueous media. Unusual acidolytic properties of an amphiphilic iron porphyrin μ-oxo dimer carrying poly(oxyethylene) side chains

The acid titration of the amphiphilic iron porphyrin μ-oxo dimer carrying poly(oxyethylene) side chains demonstrates the pronounced dependence of the acidolytic property of the internal μ-oxo bridge on the length of the water-soluble polyether side chains.