Hydrogen atom transfer alkyl hydroperoxides

As for the peroxidases, Compound I and water are formed in the first step from one equivalent of hydrogen peroxide and the resting state of the catalase. The back-reaction, however, does not proceed via Compound II but rather via a two-electron-two-proton transfer cascade, in which both hydrogen atoms of a second molecule of hydrogen peroxide are transferred to the ferryl subunit of the porphyrin cofactor. Due to the similarity of catalases and peroxidases, it is not too surprising that this reaction is also catalyzed by most peroxidases. Alternatively, catalases and some peroxidases react with alkyl hydroperoxides via the respective alkanol to an aldehyde or ketone (Scheme 2.17). A requirement for this reaction is an easily accessible active site for the hydroperoxide, so that only those peroxidases with open access such as CPO or CiP are able to promote this reaction. 

Termination of hydrocarbon radicals is not the only recombination reaction. Other possibilities (depending on the structure of the radicals) are (i) the termination by disproportionation, where hydrogen is eliminated, which yields an olefin (R-CHj-CHj R-CH=CH2-I-H ), (ii) termination by transfer (e.g. in reaction with antioxidants), (ill) termination by recombination with a hydroxyl radical, which leads to an alcohol (R-CHj-CHj -I- HO R-CH -GHj-OH) and (iv) further oxidation of the terminal hydroperoxide (R-CHj-CHj -I- Oj -> R-CHj-CHj-O-O -> R-CHj-CHj-O-OH). In the terminal hydroperoxide, the link between oxygen atoms is then cleaved, which gives an alkanal (R-CHj-CHj-O-OH R-CH2-CH=0- -H20). Alternatively, the link between carbon atoms can be cleaved, which results in a shorter alkyl radical (R-CHj ). Some of these reactions are discussed in relation to the secondary reactions of hydroperoxides. 

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