James P. Collman

A Cytochrome c Oxidase Model Catalyzes Oxygen to Water Reduction Under Rate-Limiting Electron Flux

We studied the selectivity of a functional model of cytochrome c oxidases active site that mimics the coordination environment and relative locations of Fea3, CuB, and Tyr244. To control electron flux, we covalently attached this model and analogs lacking copper and phenol onto self-assembled monolayer–coated gold electrodes. When the electron transfer rate was made rate limiting, both copper and phenol were required to enhance selective reduction of oxygen to water. This finding supports the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons, thus preventing the release of toxic partially reduced oxygen species.

Using a functional enzyme model to understand the chemistry behind hydrogen sulfide induced hibernation

The toxic gas H2S is produced by enzymes in the body. At moderate concentrations, H2S elicits physiological effects similar to hibernation. Herein, we describe experiments that imply that the phenomenon probably results from reversible inhibition of the enzyme cytochrome c oxidase (CcO), which reduces oxygen during respiration. A functional model of the oxygen-reducing site in CcO was used to explore the effects of H2S during respiration. Spectroscopic analyses showed that the model binds two molecules of H2S. The electro-catalytic reduction of oxygen is reversibly inhibited by H2S concentrations similar to those that induce hibernation. This phenomenon derives from a weak, reversible binding of H2S to the FeII porphyrin, which mimics heme a3 in CcOs active site. No inhibition of CcO is detected at lower H2S concentrations. Nevertheless, at lower concentrations, H2S could have other biological effects on CcO. For example, H2S rapidly reduces FeIII and CuII in both the oxidized form of this functional model and in CcO itself. H2S also reduces CcOs biological reductant, cytochrome c, which normally derives its reducing equivalents from food metabolism. Consequently, it is speculated that H2S might also serve as a source of electrons during periods of hibernation when food supplies are low.

Role of the metal-metal bond in transition-metal clusters. Phosphido-bridged diiron carbonyl complexes

The synthesis, characterization, and ligand substitution reactions of Fez(PPh2)z(C0)6 (1) and Na2Fez(PPhz)z(C0)6.5THF (4) are presented. Reaction of 4 with alkylating agents affords directly the acyl complexes NaFe,,(PPhz)z(CO)S[C(0)R].2THF, (6). A mechanism involving intramolecular, metal-promoted alkyl migration is proposed. Reaction of 1 with lithium reagents affords LiFez(PPh2)2(CO)5[C(0)R]-3THF.

Mössbauer spectroscopy of hemoglobin model compounds: Evidence for conformational excitation

Zero‐field Mossbauer spectra of a model compound for hemoglobin, capable of undergoing reversible oxygenation, were recorded at various temperatures. A pair of peaks with temperature‐dependent quadrupole splitting and linewidth were observed. The results have been interpreted in terms of a model which is consistent with previous x‐ray studies and which provides for relaxation effects as the molecule assumes two possible conformational states. In terms of the proposed model, the two conformational states have been characterized by their energy separation and their respective electric field gradient tensors. The relaxation rate at each temperature has also been determined.

Electrocatalytic four-electron reduction of dioxygen by iridium porphyrins adsorbed on graphite

The electrocatalytic reduction of dioxygen by macrocyclic transition-metal complexes adsorbed on electrodes has been studied extensively in conjunction with the search for an inexpensive cathode material for oxygen fuel cells. The authors and other laboratories have shown that dicobalt cofacial porphyrin dimers can catalyze dioxygen reduction to water without producing significant amounts of hydrogen peroxide. To our knowledge, however, no monomeric macrocyclic metal complex has been reported to catalyze the direct four-electron reduction of dioxygen in acidic solution. In a survey of electrocatalytic oxygen reduction by various metalloporphyrins adsorbed on activated carbon, iridium complexes were reported to be the most active catalysts. Since the reduction product and the reaction pathway had not been elucidated in that work, they were prompted to study the electrocatalytic activity of iridium porphyrins toward dioxygen reduction. Here they report the first observation of a direct four-electron reduction of dioxygen catalyzed by Ir(OEP)H adsorbed on graphite in acidic electrolyte solution.

Chemical vapor deposition of YBa2Cu3O7−x superconducting films

Superconducting thin films of YBa2Cu3O7−x have been produced by chemical vapor deposition. Volatile coordination compound precursors of the oxide components, β‐diketonates of Y, Ba, and Cu, are thermally decomposed on hot substrates to form crystalline films. The Ba compound was evaporated in the presence of externally added vapors of the β‐diketonate to obtain steady evaporative behavior. Superconducting films were obtained on SrTiO3 substrates at temperatures above 800 °C. The best films have onset temperatures of 90 K and loss of resistance as high as 68 K. Epitaxial growth was obtained.Superconducting thin films of YBa/sub 2/Cu/sub 3/O/sub 7/minus//ital x// have been producedby chemical vapor deposition. Volatile coordination compound precursors of theoxide components, ..beta..-diketonates of Y, Ba, and Cu, are thermally decomposed onhot substrates to form crystalline films. The Ba compound was evaporated in thepresence of externally added vapors of the ..beta..-diketonate to obtain steadyevaporative behavior. Superconducting films were obtained on SrTiO/sub 3/substrates at temperatures above 800 /degree/C. The best films have onsettemperatures of 90 K and loss of resistance as high as 68 K. Epitaxial growthwas obtained.

A functional nitric oxide reductase model

A functional heme/nonheme nitric oxide reductase (NOR) model is presented. The fully reduced diiron compound reacts with two equivalents of NO leading to the formation of one equivalent of N2O and the bis-ferric product. NO binds to both heme Fe and nonheme Fe complexes forming individual ferrous nitrosyl species. The mixed-valence species with an oxidized heme and a reduced nonheme FeB does not show NO reduction activity. These results are consistent with a so-called “trans” mechanism for the reduction of NO by bacterial NOR.

Synthetic models for the oxygen-binding hemoproteins

The oxygen binding hemoproteins hemoglobin (Hb), myoglobin (Mb), and cytochrome P-450 are important to the biological transport, storage, and metabolism of oxygen. Nevertheless the nature of the coordinate link between iron and dioxygen in these hemoproteins has not been defined at the atomic level. Furthermore the way in which the glogin proteinheme interaction directs reversible oxygen binding has been obscure. My students have addressed and partially clarified these issues by preparing crystalline iron(II) porphyrin-dioxygen complexes.1–5 These remarkable Mb models which reversibly bind oxygen in solution or in the solid state at ambient temperature have been characterized by Mossbauer1,5, ir spectra4 and X-ray crystallographic analysis.2

Mössbauer effect study of the magnetic properties of S=1 ferrous tetraphenylporphyrin

Mossbauer spectra of the polycrystalline form of the square planar compound α,β,γ,δ‐tetraphenylporphinato–iron (II) have been observed at a variety of temperatures. Analysis of the resulting spectra yield a positive electric field gradient interaction with quadrupole splitting ΔE=1.51 mm/s (4.2 K) and 1.52 mm/s (300 K). At low temperature, an external magnetic field induces a negligible internal field along the symmetry axis of the molecule. In the transverse direction, it gives rise to a positive internal field with little temperature dependence up to 30 K. The data can be interpreted in terms of a crystal field model involving a low‐lying 3A2g state which is split by spin–orbit coupling into a ground singlet and a doublet lying 80 cm−1 above it.

Dendritic Iron(II) Porphyrins as Models for Hemoglobin and Myoglobin: Specific Stabilization of O2 Complexes in Dendrimers with H-Bond-Donor Centers

Two types of dendritically functionalized iron(II) porphyrins were prepared (Scheme) and investigated in the presence of 1,2-dimethylimidazole (1,2-DiMeIm) as the axial ligand as model systems for T(tense)-state hemoglobin (Hb) and myoglobin (Mb). Equilibrium O2- and CO-binding studies were performed in toluene and aqueous phosphate buffer (pH 7). UV/VIS Titrations (Fig. 4) revealed that the two dendritic receptors 1⋅FeII-1,2-DiMeIm and 2⋅FeII-1,2-DiMeIm (Fig. 2) with secondary amide moieties in the dendritic branching undergo reversible complexation (Fig. 5) with O2 and CO in dry toluene. Whereas the CO affinity is similar to that measured for the natural receptors, the O2 affinity is greatly enhanced and exceeds that of T-state Hb by a factor of ca. 1500 (Table). The oxygenated complexes possess half-lives of several h (Fig. 6). This remarkable stability originates from both dendritic encapsulation of the iron(II) porphyrin and formation of a H-bond between bound O2 and a dendritic amide NH moiety (Fig. 11). Whereas reversible CO binding was also observed in aqueous solution (Fig. 10), the oxygenated iron(II) complexes are destabilized by the presence of H2O with respect to oxidative decay (Fig. 9), possibly as a result of the weakening of the O2⋅⋅⋅H−N H-bond by the competitive solvent. The comparison between the two dendrimers with amide branchings and ester derivative 3⋅FeII-1,2-DiMeIm (Fig. 2), which lacks H-bond donor centers in the periphery of the porphyrin, further supports the role of H-bonding in stabilizing the O2 complex against irreversible oxidation. All three derivatives bind CO reversibly and with similar affinity (Fig. 8) in dry toluene, but the oxygenated complex of 3⋅FeII-1,2-DiMeIm undergoes much more rapid oxidative decomposition (Fig. 7).