Chemistry Reference
In-Depth Information
6.2.4 Conclusions
The high-valent manganese species undergo disproportionation and compro-
portionation reactions. The nature of transient species in the reaction of high-
valent manganese species can be understood by their absorption spectra. A
relatively simple and rapid pulse radiolysis technique may be applied to iden-
tify the intermediate Mn species. Moreover, this technique is useful in cases
where the concentrations of species are low (micromolar) and the conven-
tional techniques are not suitable. The permanganate ion has been extensively
used in the oxidation of organic compounds in aqueous and organic phases,
and current focus is on heterogeneous and solvent-free conditions (i.e., solid
support). The reaction mechanisms under these conditions need to be explored
further, which will provide the importance of high-valent Mn species in indus-
trial and biochemical reactions.
The oxidation of organic substrates by Mn(VII) under alkaline conditions
occurred through Mn(VI) and Mn(V) as intermediates, and Mn(IV) and
Mn(II) were not involved in the oxidation mechanism. However, MnO
2
was
involved as the autocatalytic reaction pathway in the oxidation by permanga-
nate under acidic and neutral conditions. The oxidation of α-amino acids by
Mn(VII) was autocatalyzed by MnO
2
colloidal particles. When reactions were
conducted in phosphate-buffered solutions, the concentration of phosphate
ions influenced the rates of oxidation of α-amino acids by the permanganate
ion. Studies on the effects of MnO
2
colloidal particles and phosphate ions may
also be extended for other amino acids to understand how side chains and
aromaticity control the rates of oxidation of amino acids by Mn(VII). More-
over, studies on high-valent manganese compounds may also provide informa-
tion for the role of high-valent iron species in oxidation reactions of various
enzymes, which is discussed in the next section. High-valent Mn species and
amino acids are involved in the water oxidation of PSII. The water oxidation
mechanism needs detailed understanding and requires further experimental
and theoretical elucidation.
6.3
IRON
The activation of O
2
and H
2
O
2
by iron complexes in industrial, environmental,
and biological redox processes has been of great interest [256-258]. Several
reviews on this subject has been reported [258-272]. In the biological environ-
ment, Fe(IV)- and Fe(V)-oxo species have been suggested as the active oxi-
dants of numerous heme and nonheme enzymes [260, 263, 273]. A general
scheme showing the formation of oxo species of Fe(IV) and Fe(V) is shown
in Figure 6.23 [260]. Iron(IV)-oxo complexes have the capability to hydroxyl-
ate C-H bonds of substrates efficiently. The heme-based enzyme P450 is
involved in key biochemical reactions in the body such as the biosynthesis of
hormones and drug metabolism as well as the detoxification of the liver. The
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