We studied the selectivity of a functional model of cytochrome c oxidase's active site that mimics the coordination environment and relative locations of Fe(a3), Cu(B), and Tyr(244). 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.

The rate of electron transfer is measured to two ferrocene and one iron tetraphenylporphyrin redox species coupled through terminal acetylenes to azide-terminated thiol monolayers by the Cu(I)-catalyzed azide-alkyne cycloaddition (a Sharpless "click" reaction) to form the 1,2,3-triazole linkage. The high yield, chemoselectivity, convenience, and broad applicability of this triazole formation reaction make such a modular assembly strategy very attractive. Electron-transfer rate constants from greater than 60,000 to 1 s −1 are obtained by varying the length and conjugation of the electron-transfer bridge and by varying the surrounding diluent thiols in the monolayer. Triazole and the triazole carbonyl linkages provide similar electronic coupling for electron transfer as esters. The ability to vary the rate of electron transfer to many different redox species over many orders of magnitude by using modular coupling chemistry provides a convenient way to study and control the delivery of electrons to multielectron redox catalysts and similar interfacial systems that require controlled delivery of electrons.

The toxic gas H2S is produced by enzymes in the body. At

The electronic structure of the Fe–O2 center in oxy-hemoglobin and oxy-myoglobin is a long-standing issue in the field of bioinorganic chemistry. Spectroscopic studies have been complicated by the highly delocalized nature of the porphyrin and calculations require interpretation of multi-determinant wavefunctions for a highly covalent metal site. Here, iron L-edge X-ray absorption spectroscopy (XAS), interpreted using a valence bond configuration interaction (VBCI) multiplet model, is applied to directly probe the electronic structure of the iron in the biomimetic Fe–O2 heme complex [Fe(pfp)(1-MeIm)O2] (pfp = meso-tetra(α,α,α,α-o-pivalamidophenyl) porphyrin or TpivPP). This method allows separate estimates of σ-donor, π-donor, and π-acceptor interactions through ligand to metal charge transfer (LMCT) and metal to ligand charge transfer (MLCT) mixing pathways. The L-edge spectrum of [Fe(pfp)(1-MeIm)O2] is further compared to those of [FeII(pfp)(1-MeIm)2], [FeII(pfp)], and [FeIII(tpp)(ImH)2]Cl (tpp = meso-tetraphenylporphyrin) which have FeII S = 0, FeII S = 1 and FeIII S = 1/2 ground states, respectively. These serve as references for the three possible contributions to the ground state of oxy-pfp. The Fe–O2 pfp site is experimentally determined to have both significant σ-donation and a strong π-interaction of the O2 with the iron, with the latter having implications with respect to the spin polarization of the ground state.

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 N 2O 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 Fe B does not show NO reduction activity. These results are consistent with a so-called ''trans'' mechanism for the reduction of NO by bacterial NOR.functional model ͉ NO reduction ͉ N2O ͉ "trans" mechanism N itric oxide reductase (NOR) is a membrane-bound enzyme that catalyzes the 2e Ϫ reduction of nitric oxide (NO) to nitrous oxide (N 2 O), an obligatory step involved in the sequential reduction of nitrate to dinitrogen known as bacterial denitrification. The active site of NOR consists of a monohistidine ligated five-coordinate heme and a trisimidazole ligated nonheme Fe B . This structure strongly resembles the active site of oxygen reduction enzyme-cytochrome c oxidase (CcO), which possesses a heme-a 3 /Cu B center ( Fig. 1) (1, 2). Essentially, the distal metal Cu B in CcO is replaced by a nonheme Fe metal in NOR; NOR and CcO are thought to be distant relatives.The dinuclear iron active site in NOR was confirmed a decade ago by spectroscopic studies (3). Presumably, two NO molecules are turned over to give one molecule of N 2 O and one molecule of H 2 O at the diiron center with the consumption of two electrons and two protons. Although many enzyme studies of NOR have been focused on the intermediate trapping and elucidation of the reaction mechanism (4-15), the details of the catalytic cycle are still unresolved because of the lack of structural information and uncertainty regarding short-lived intermediates.In contrast to enzyme studies, synthetic biomimetic model complexes provide a straightforward and controlled method to understand how this chemical transformation proceeds at the enzyme active site. However, only a few synthetic models have been developed that mimic the active site of NOR; moreover, these compounds either lack a proximal imidazole ligand (16,17) or use pyridine as a replacement for the histidine ligands (18)(19)(20). No functional NOR models have been reported to date. Our CcO model complexes have proved to be functionally active for oxygen reduction reaction with minimal reactive oxygen species (ROS) formation (21-24). These appear to be promising NOR model candidates if the distal Cu metal is replaced by an iron because the resulting diiron compound has almost all of the key components in NOR: a heme Fe with a proximal imidazole ligand and a trisimidazole ligated nonheme Fe center.In this report, we disclose the first synthetic functional NOR model LFe II /Fe II (Fig. 2), which reacts with two equivalents of NO to give one equivalent of N 2 O and the bis-ferric product. We have shown that NO binds to both heme Fe and Fe B to form a possible bis-nitrosyl intermediate; subsequently, the two bound NO molec...