HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 304

are in strong alkaline solutions. All of the species, except MnO 2 , absorb strongly
in the visible region. There are distinct absorption maxima for Mn(VII) and
Mn(VI) ions with high molar extinction coefficients at 522 and 426 nm, respec-
tively. The spectral differences were used to study the photochemistry of
aqueous Mn(VII) in acidic and basic solutions [177]. The final product was
determined to be MnO − under neutral conditions. In the case of basic solution,
the product was identified to be MnO 2− . The suggested mechanism involved a
formation of the Mn(V)-peroxo complex as an intermediate in the light-
induced decomposition of MnO − .
The spectra of manganese porphyrin complexes, MnTM-2PyP (TM-
2PyP = chlorotetra(N-Me-2-pyridyl)porphyrin), are shown in Figure 6.14.
Mn(V) had a strong band at 434 nm. The oxoMn(IV) porphyrin complex had
a lower absorbance with a maximum at 426 nm. The oxo Mn(III) porphyrin
complex had a characteristic band at 450 nm.
The potential diagrams for Mn are shown in Figure 6.15. A discussion on
the reduction potentials of different species and their impact on the oxidation/
reduction (redox) reactions have been thoroughly reviewed [174]. The redox
potential values can be used to determine the stability and redox reactions of
the different oxidation states of Mn. Of the several species, permanganate is
the most important compound. The MnO − ion is thermodynamically unstable
in aqueous solution and its potential is greater than the evolution of oxygen
over the entire pH range. In acidic solutions, it is reduced to the Mn 2+ ion.
MnO 2 is the reduced species of the MnO − ion in alkaline solutions.
Figure 6.14. uV−vis spectra MnTM-2-PyP porphyrins. All three are 5 µm in a 50 mM
pH 7.4 phosphate buffer. OxoMn(V)TM-2-PyP (λ max = 434 nm) was prepared by oxida-
tion of Mn(III)TM-2-PyP (λ max = 454 nm) by 1 equiv of oxone. In situ reduction of
Mn(V) by NaNO 2 (10 equiv) produced oxoMn(IV)TM-2-PyP (λ max = 426 nm) (adapted
from Jin and Groves [195] with permission of the American Chemical Society).