As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal–oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal–oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron–oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

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The heme-containkg monooxygenase, cytochrome P-450, has attracted considerable attention because of the central role this enzyme plays in oxidative mechanisms. Several forms of this protein found in the liver apparently protect the body from foreign chemical assult by oxidizing toxic compounds to more manageable, hydrophilic materials. Thus, for example, drugs that contain alkyl amines are oxidatively cleaved by cytochrome P-450 to give aldehydes. I t is now recognized that some of the metabolic transformations mediated bv cvtochrome P-450 nroduce toxic or carcinoeenic products. '['he epoxide prodived from 5,inyl chloride by liver cstochrome 1'-450 is known 11, Droduce tumors in that organ whereas benzpyrene, which is found in cigarette smoke, is oxidized in the lung to acarcinogenic diol-epoxide. Many of the oxidative transformations of organic compounds mediated by cytochrome P-450 are carefully designed metabolic events. A cascade of very specific forms of cytochrome P-450 found in the adrenal cortex, for example, is involved in manv of the imoortant stens of lioid metaho-~ ~ lism. After its hiosynthesis, cholesterol undergoes a series of oxidations in which cvtochrome P-430 is involved. The removal of the alkyl side chain from ring-D is known t o proceed bv two successive and stereoselective hvdroxvlations a t '2-20 and '2-22 followed by a glycol cleavage. ~h k eventual hydroxylation of C-11 in ring-C on the way to the corticosteroids is also a P-450 mediated process.

A series of cationic cobalt porphyrins was found to catalyze electrochemical water oxidation to O 2 efficiently at room temperature in neutral aqueous solution. 10,15,porphyrin, with a highly electron-deficient meso-dimethylimidazolium porphyrin, was the most effective catalyst. The O 2 formation rate was 170 nmol·cm −2 ·min −1 (k obs = 1.4 × 10 3 s −1 ) with a Faradaic efficiency near 90%. Mechanistic investigations indicate the generation of a Co IV -O porphyrin cation radical as the reactive oxidant, which has accumulated two oxidizing equivalents above the Co III resting state of the catalyst. The buffer base in solution was shown to play several critical roles during the catalysis by facilitating both redox-coupled proton transfer processes leading to the reactive oxidant and subsequent O-O bond formation. More basic buffer anions led to lower catalytic onset potentials, extending below 1 V. This homogeneous cobalt-porphyrin system was shown to be robust under active catalytic conditions, showing negligible decomposition over hours of operation. Added EDTA or ion exchange resin caused no catalyst poisoning, indicating that cobalt ions were not released from the porphyrin macrocycle during catalysis. Likewise, surface analysis by energy dispersive X-ray spectroscopy of the working electrodes showed no deposition of heterogeneous cobalt films. Taken together, the results indicate that Co-5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin is an efficient, homogeneous, single-site water oxidation catalyst.ater oxidation is a key step in photosynthesis that efficiently harvests and stores solar energy (1). The oxidation of H 2 O to O 2 is a four-electron, four-proton process in which O-O bond formation is the key chemical step. In photosystem II, these proton-coupled electron transfers (PCETs) occur via a tyrosine at the Mn 4 Ca oxygen-evolving complex (2). An important thermodynamic aspect of photosynthesis is the efficient conversion of photonic energy to electrical potential, thus providing 99% of the driving force required to convert CO 2 to carbohydrates (Eqs. 1 and 2):andThe development of synthetic catalysts that can mediate water oxidation under mild conditions with a minimal energy cost has become an appealing challenge for chemists (3-7). Among various approaches, homogeneous molecular catalysts have shown attractive features such as controllable redox properties, ease of investigating reaction mechanisms, and strategies for the characterization of reactive intermediates (8,9). Recently, such efforts have resulted in the development of a significant number of systems based on single-site and multinuclear transition metal complexes including Mn, Fe, Co, Cu, Ru, and Ir (9-15). Examples of cobalt-based molecular catalysts include a cobalt phthalocyanine (16), a cobalt "hangman" corrole (17), multipyridine cobalt complexes (18, 19), a dinuclear Co-peroxo species (20), and most recently, Co-porphyrins (21). Determining if these molecular complexes retain their homogenous nature during catalysis or ...

Biologically controlled mineralization features an orchestrated balance among various controlling factors such as spatial delineation, template promotion, crystal growth modification and cessation, and so on. Highly ordered calcium carbonate lamellae formed in the nacreous layers of mollusk (aragonite), the foliated calcitic layers of mollusk (calcite), or the semi-nacre of brachiopods (calcite) are excellent examples of the outcome of such synergistic control. Mimicking the concerted interplay of template promotion and growth inhibition as often utilized in biomineralization, we have synthesized macroscopic and continuous calcium carbonate thin films with thickness ranging from 0.4 to 0.6 μm. The thin films were prepared at air/subphase interfaces by promoting mineral deposition with amphiphilic porphyrin templates, coupled with growth inhibition by the use of poly(acrylic acid) as a soluble inhibitor. Films formed at 22 °C were found to have a biphasic structure containing both amorphous and crystalline calcium carbonate. The crystalline regions were identified to be calcite oriented with the (00.1) face parallel to the porphyrin monolayer at the air/subphase interface. Films obtained in the early stage of formation at lower temperature (4 °C) displayed characteristics of a single amorphous phase. These observations suggest that films formed through a multistage assembly process, during which an initial amorphous deposition was followed by a phase transformation into the ultimate crystalline phase and the orientation of the crystalline phase was controlled by the porphyrin template during the phase transformation. The results provide new insights into the template-inhibitor−biomineral interaction and a new mechanism for synthesizing ceramic thin film under mild conditions.