Oxene structures

Ferrvl rebound mechanisms suggests that the formation of the ferryl-oxene structure is similar to those for Compound I (Por-FeIV-0) in the peroxidase reaction (Fig. 3.8). This mechanism (Groves and McClusky, 1976 Groves and Subramanian,... 

As mentioned in the introduction, Mb also forms an Fe =0 heme on reaction with peroxides (7). Of particular interest is the mechanism of proton delivery to the oxene ligand on reduction because the distal heme pocket of Mb lacks proton donors (Figure 2b) and is isolated from the bulk solvent (43), unlike CCP where the hydrated substrate channel connects the distal heme cavity to the solvent (44). Thus, it is expected that solvent-assisted PT to the heme of Mb at physiological pH is severely restricted. A spectacular example of solvent-assisted PT to a buried redox center has been highlighted in the X-ray structure of the bacterial reaction centers from Ehodobacter sphaeroides (45). A narrow hydrated channel extends from the cytoplasmic side of the reaction center to quinone Qb, which is buried -23 A in the L-subunit. 

Parallel chemical oxidation of deuterium labeled biphenylacetylene with metachloro-perbenzoic acid yielded the same product as was obtained in the enzymatic reaction. These observations exclude the possible oxidation of the acetylenic terminal C—H bond, because such a reaction would not be compatible with complete conservation of the deuterium atom. Thus, the formation of a ketene derivative was postulated (Scheme 5). However, the nature of the intermediate from which the ketene derivative is formed remains to be elucidated. An oxirene structure was suggested in parallel to alkene epoxidation however, oxirenes are very unstable species due to electronic and steric factors It is currently believed that a complex between the acetylenic n electrons and cytochrome P450 iron-oxene moiety is formed, leading to the formation of the ketene. ... 

These derivatives (Type B) are at least formally prepared by the insertion of a fragment into a M-N bond to yield a new M-X-N unit. Such species have been suggested as possible intermediates in the insertion of an oxygen atom into a G-H bond by cytochrome P-450. Several of these derivatives have formally had a carbene fragment inserted into a M-N bond. Such derivatives include a Ni derivative and a cobalt(III) species that has undergone two such insertion reactions. Other species represent the formal reaction of a vinylidene with iron(III) (two different crystalline forms). This iron derivative has an intermediate-spin state. Other complexes result from the insertion of a nitrene or an oxene This last derivative can also be considered to be a porphyrin N-oxide derivative and the structure of a free base species of a porphyrin N-oxide has also been reported". Appropriate stereochemical parameters for the members of this class are found in Table IX. 

The details of the structures formed depend on multiple factors, including temperature, salt concentration, and molecular geometry. Large, electrostatically repulsive head groups tend to cause formation of micelles, with a hydrophobic interior. For example, in a spherical micelle, the nonpolar components are locally aggregated and the polar components become the interface with the bulk water phase, like a herd of musk oxen assembling in a circle with their horns facing... 

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