Nonheme and heme complexes

Iron centers that are more electron-deficient than iron(III) compounds are used for efficient and highly specific oxidation reactions in, for example, heme and nonheme enzymes [166-172]. Most iron(IV)-complexes found in biological reaction cycles possess terminal or bridging 0x0 groups as is known from a large number of structural and spectroscopic investigations. With the exception of iron(IV)-nitrido groups, nonoxo iron(IV) centers very rarely take part in such reactions. 

Stabilization of M(Por)HNO Complexes by Fe Proteins and Heme and Nonheme Platforms 122... 

High-valent iron(IV)-oxo complexes of heme and nonheme ligands in oxygenation reactions 07ACR522. 

The crystal stmcture of [(OEP)Fe]2(A<-ONNO) demonstrates one example of a iram-hyponitrite bridge bimetaUic complex proposed as an intermediate in the reduction of NO to N2O, except that our system is composed of two heme bimetaUic centers instead of the proposed heme and nonheme bimetallic centers. Regardless, we note here that the [(OEP)Fe]2(/<-ONNO) complex is, to date, the only reported crystal structure of a trares-hyponitrite bridge bimetaUic porphyrin complex. Key stmctural features of the crystal structure are worthy of note. 

The products of aU the nitrosylation reactions described and analyzed so far for both the heme- or nonheme complexes have been consistently described as diamagnetic low-spin (Fe NO ) species, independently of the high-spin or low-spin nature of the Fe(III) reactants. Strikingly, there is an evident contrast between the values of feoff that measure the lability of NO, between moderately labile heme and the undoubtedly inert nonheme nitrosylated... 

The chemical biology of nitric oxide (NO) and its derivatives rely heavily on the interaction of this diatom with heme and nonheme iron enzymes. Such interactions are important in the mammalian cardiovascular system as well as in the detoxification of NO in pathogenic microbes. As such, synthetic bioinorganic chemists have designed and constructed a variety of low molecular weight coordination complexes to understand the structural and reactive properties of FeNO systems as they relate to biological processes. Over the last several years, much synthetic work has focused on the construction of FeNO and FeNO complexes as representative models of NO reductase enzymes and as potential nitroxyl- or FINO-releasing molecules for new cardiovascular therapeutics. This review describes the synthesis, structure, spectroscopy, and reactivity of such FeNO systems published from 2011 to 2014. 

In equation 1.16, there are various ligands X /XH that can accept/donate the proton. These include the oxo group in the ferryl active sites in heme and nonheme oxygenase enzymes, the hydroxide at the active site in lipoxygenase, the aniline in equation (1.17), and an imidazolate, as in equation (1.18). As these examples illustrate, some metal-mediated HAT reactions involve a formal separation of the e and H that make up the transferred H . In the case of metal-imidazole complexes the proton and electron are 3 bonds or 4 A separated. Despite this separation, we refer to all of these reactions as HAT. Recent interest into the intimate details of HAT reactions has led to new thinking and new definitions. Still, we prefer the simplest definition of HAT, as reactions where... 

Metal complexes, especially involving transition metals, are known for their role as catalysts in a broad variety of chemical processes including isomerization, oxidization, hydrogenation, and polymerization. Such catalytic reactions play an important role not only in many industrial processes, such as petroleum and polymer industries, but also in many biological systems, e.g., a variety of selective oxidation catalysts with heme (1) and nonheme (2) iron centers. The transition metals in these systems usually constitute a fundamental part of the catalyst, due to their... 

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Rhodopseudomonas viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33,28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution288 319 323 (Fig. 23-31). In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first known structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

Cytochromes/( J from Latin frons for leaf) and were discovered almost half a century ago hy Hill and Scarisbrick and by Hill, respectively. In fact, the redox behavior of these chloroplast c)do-chromes led Robin Hill and Fay Bendall to formulate the so-called Z-scheme for oxygenic photosynthesis. In 1972, Nelson and Neumann" isolated a partially purified complex from a digitonin-fraction-ated PS-I particle obtained from lettuce chloroplasts. The complex was found to contain Cyt/, Cyt and non-heme iron, which led the authors to note its similarity to the Cyt-bci complex (i.e., complex III) of mitochondria. In 1975 Sugahara, Shaw and this author isolated a complex from spinach TSF-I particles and by investigation of its spectroscopic and EPR properties, showed that it also contained Cyt/, Cyt b and nonheme iron, consistent with its being a bjcomplex. The fraction also displayed a distinct EPR signal characteristic of a copper protein, apparently due to plastocyanin co-precipitated during fractionation. 

While it is impossible to review individual metal-oxo complexes here, one recent dramatic development, discovery of ferryl(IV) nonheme complexes,57 58 deserves to be mentioned. Iron(IV)-oxo intermediates were known in heme chemistry for quite some time, but ferryl(IV) in nonheme systems became readily available and well characterized only very recently, when the X-ray structure of the Ferv=0 complex with tetramethylcyclam was determined,139 and several similar complexes were identified spectroscopically and crystallographically (Figure 4.36).57,58... 

In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first knovm structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

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