A193Ir Mössbauer study of Pt−Ir catalysts prepared by impregnantion of silica with H2MCl6

The state of iridium on Pt?Ir catalysts prepared by impregnation of amorphous silica with H₂IrCl₆ and H₂PtCl₆ was studied by¹⁹³Ir Mössbauer spectroscopy after different steps of preparation. The Ir is adsorbed in its trivalent state, presumably as [IrCl₆]^3?. Calcination in air at 450°C converts this to IrO₂. The metallic clusters formed by subsequent reduction in H₂ at 200°C show a strong tendency towards segregation of Ir and Pt and re-oxidize partially when exposed to air at ambient temperature. In both respects the behaviour is similar to that of samples prepared by co-exchange from [Ir(NH₃)₅Cl]Cl₂ and Pt(NH₃)₄Cl₂. H₂O.     

Modulation of fragmental charge transfer via hydrogen bonds. Direct measurement of electronic contributions

Hydrogen bonds play an important role in an overwhelming variety of fields from biology to surface and supramolecular chemistry. The term "hydrogen bond" refers to a wide range of interactions with various covalent and polar contributions. In particular, hydrogen bonds have an important role in the folding and packing of peptides and nucleic acids. Recent studies also point to the importance of hydrogen bonding in the context of second-shell interactions, in metal binding and selectivity in metalloproteins, and in controlling the dynamics of membrane proteins. In this study, we demonstrate and quantify the modulation of fragmental charge transfer from hydrogen-bonded ligands to a metal center, by employing our recently introduced molecular potentiometer. The molecular details that affect this type of fragmental charge transfer are presented and a path for transferring chemical information is demonstrated. We found that H-bond interactions in the extended positions of axial ligands provide an effective means of modulating the amount of fragmental charge transfer to a metal center, thereby dramatically influencing the electronic properties of the ligand, the binding affinity, and the binding of additional ligands. The magnitude of fragmental charge-transfer modulation induced by a single ligand-solvent H-bond interaction is comparable to those induced by covalent substitution, although H-bond enthalpy is only on the order of several kilojoules per mole. Importantly, we find a significant change in the ligand electronic properties, even for weak C-H...O=C H-bond formation, where the bond enthalpy is substantially lower than for conventional H-bond interactions. The excess fragmental charge transferred to the metal center, deduced from the spectroscopic measurements, correlates well with the computationally determined values. Our findings underscore the importance of second-shell interactions in the active sites of enzymes, beyond the structural and electrostatic importance that is widely recognized today.     

Mössbauer analysis of the state of iridium and platinum in supported catalysts

The state of iridium and platinum in supported noble metal catalysts was investigated by Mössbauer spectroscopy. With iridium, conventional absorption experiments with the 73 KeV γ-rays of¹⁹³Ir were preformed; platinum was studied in source experiments using the 77 KeV resonance in¹⁹⁷Au. First results for pure Pt catalysts on silica supports and for a used automotive catalytic converter are presented. In Pt-rich bimetallic Pt?Ir catalysts on amorphous silica, both Pt and Ir were studied after different treatments.     

Protein-A-mediated Targeting of Bacteriochlorophyll-IgG to Staphylococcus aureus: A Model for Enhanced Site-Specific Photocytotoxicity

Abstract— A model for studying the efficiency of photodynamic action with a photosensitizer placed exclusively on the bacterial cell wall has been used. Bacteriochlorophyllide molecules, conjugated to rabbit immunoglobulin G (IgG), were synthesized. The conjugated pigment bacteriochlo-rophyll (Bchl)-IgG bound with high specificity to protein-A residues naturally exposed on the cell wall of the bacterium Staphylococcus aureus Cowan I. In bacterial suspensions the phototoxicity of the targeted conjugates (0.5-2.5 pigment per IgG molecule) was dose dependent (LD₅₀= 1.7 μ M ) in the presence of light (Λ > 550 nm) and inhibited by native IgG but not by ovalbumin, suggesting selective interaction with protein-A on the bacterial cell wall. No dark toxicity was noticed even with the highest conjugate concentration tested. In contrast, the photocytotoxicity of bacteriochlorophyll-serine (Bchl-Ser, LD₅₀= 0.07 μ M ) used as a nontargeted control was not inhibited by IgG. In spite of its lower apparent potency, Bchl-IgG was found to be 30 times more efficacious than Bchl-Ser: At LD₅₀, only 66000 Bchl-IgG molecules were bound per bacterium compared to 1900 000 molecules of Bchl-Ser. The higher efficacy of Bchl-IgG is explained by its exclusive position on the bacterial cell wall. Consequently, photogeneration of oxidative species is confined to the cell wall and its vicinity, a seemingly highly susceptible domain for photodynamic action. In considering the design of cell-specific sensitizers for bacterial and cancer therapies, it would be beneficial to identify the more discretely sensitive subcellular domains as targets.     

The microenvironment effect on the generation of reactive oxygen species by Pd-bacteriopheophorbide

Generation of reactive oxygen species (ROS) is the hallmark of important biological processes and photodynamic therapy (PDT), where ROS production results from in situ illumination of certain dyes. Here we test the hypothesis that the yield, fate, and efficacy of the species evolved highly depend on the dye's environment. We show that Pd-bacteriopheophorbide (Pd-Bpheid), a useful reagent for vascular targeted PDT (VTP) of solid tumors, which has recently entered into phase II clinical trials under the code name WST09 (trade name TOOKAD), forms appreciable amounts of hydroxyl radicals, superoxide radicals, and probably hydrogen peroxide in aqueous medium but not in organic solvents where singlet oxygen almost exclusively forms. Evidence is provided by pico- and nanosecond time-resolved spectroscopies, ESR spectroscopy with spin-traps, time-resolved singlet oxygen phosphorescence, and chemical product analysis. The quantum yield for singlet oxygen formation falls from approximately 1 in organic solvents to approximately 0.5 in membrane-like systems (micelles or liposomes), where superoxide and hydroxyl radicals form at a minimal quantum yield of 0.1%. Analysis of photochemical products suggests that the formation of oxygen radicals involves both electron and proton transfer from (3)Pd-Bpheid at the membrane/water interface to a colliding oxygen molecule, consequently forming superoxide, then hydrogen peroxide, and finally hydroxyl radicals, with no need for metal catalysis. The ability of bacteriochlorophyll (Bchl) derivatives to form such radicals upon excitation at the near infrared (NIR) domain opens new avenues in PDT and research of redox regulation in animals and plants.     

WST11, a novel water-soluble bacteriochlorophyll derivative; cellular uptake, pharmacokinetics, biodistribution and vascular-targeted photodynamic activity using melanoma tumors as a model

WST11 is a novel negatively charged water-soluble palladium-bacteriochlorophyll derivative that was developed for vascular-targeted photodynamic therapy (VTP) in our laboratory. The in vitro results suggest that WST11 cellular uptake, clearance and phototoxicity are mediated by serum albumin trafficking. In vivo, WST11 was found to clear rapidly from the circulation (t1/2=1.65 min) after intravenous bolus injection in the mouse, whereas a longer clearance time (t1/2=7.5 min) was noted in rats after 20 min of infusion. The biodistribution of WST11 in mouse tissues indicates hepatic clearance (t1/2=20 min), with minor (kidney, lung and spleen) or no intermediary accumulation in other tissues. As soon as 1 h after injection, WST11 had nearly cleared from the body of the mouse, except for a temporal accumulation in the lungs from which it cleared within 40 min. On the basis of these results, we set the VTP protocol for a short illumination period (5 min), delivered immediately after WST11 injection. On subjecting M2R melanoma xenografts to WST11-VTP, we achieved 100% tumor flattening at all doses and a 70% cure with 9 mg/kg and a light exposure dose of 100 mW/cm2. These results provide direct evidence that WST11 is an effective agent for VTP and provide guidelines for further development of new candidates.     

Zinc-bacteriochlorophyllide dimers in de novo designed four-helix bundle proteins. A model system for natural light energy harvesting and dissipation

Photosynthetic organisms utilize interacting pairs of chlorophylls and bacteriochlorophylls as excitation energy donors and acceptors in light harvesting complexes, as photosensitizers of charge separation in reaction centers, and maybe as photoprotective quenching centers that dissipate excess excitation energy under high light intensities. To better understand how the pigment's local environment and spatial organization within the protein tune its ground- and excited-state properties to perform different functions, we prepared and characterized the simplest possible system of interacting bacteriochlorophylls within a protein scaffold. Using HP7, a high-affinity heme-binding protein of the HP class of de novo designed four-helix bundles, we incorporated 13(2)-OH-zinc-bacteriochlorophyllide-a (ZnBChlide), a water-soluble bacteriochlorophyll derivative, into specific binding sites within the four-helix bundle protein core. We capitalized on the rich and informative optical spectrum of ZnBChlide to rigorously characterize its complexes with HP7 and two variants, in which a single heme-binding site is eliminated by replacing histidine residues at positions 7 or 42 by phenylalanine. Surprisingly, we found the ZnBChlide binding capacity of HP7 and its variants to be higher than for heme: up to three ZnBChlide pigments bind per HP7, or two per each single histidine variant. The formation of dimers within HP7 results in dramatic quenching of ZnBChlide fluorescence, reducing its quantum yield by about 80%, and the singlet excited-state lifetime by 2 orders of magnitudes compared to the monomer. Thus, HP7 and its variants are the first examples of a simple protein environment that can isolate a self-quenching pair of photosynthetic pigments in pure form. Unlike its complicated natural analogues, this system can be constructed from the ground up, starting with the simplest functional element, increasing the complexity as needed.