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inactivation of murine norovirus (MNV) and coliphage MS2 by ClO
2
were
lower than chlorine [139]. At 5°C, Ct values for 4-log reduction in MNV were
determined as 0.314 and 0.247 mg/l min for chlorine and ClO
2
, respectively.
Another example is the inactivation of
Cryptosporidium parvum
[140]. Ninety
percentage inactivation of
C. parvum
required 1.3 mg/l ClO
2
for 1 hour, but
80 mg/l of chlorine and monochloramine for 1 hour was needed to obtain same
percentage of inactivation [140]. ClO
2
was also shown to be equal or superior
to free available chlorine to inactivate bacterial threat agents [141].
An overview of the generation and the stability of ClO
2
in aqueous solution,
followed by its reactivity with amino acids, peptides, and proteins are discussed
below.
3.3.1 Generation of ClO
2
Reduction of the chlorate ion in acidic media with and without H
2
O
2
has been
used to produce excessive quantities of ClO
2
. generally, ClO
2
is generated by
the reaction of the chlorite ion with an acid and/or chlorine [132]. These
methods involve the use of concentrated acids and/or externally added oxi-
dants such as Cl
2
, OCl
−
, and H
2
O
2
. As an alternative, the electrochemical
generation by a one-electron transfer from
ClO
−
to ClO
2
requires a consider-
able input of electrical energy. Recently, a catalytic process using the water-
soluble manganese porphyrin, tetrakis-5,10,15,20-(
N
,
N
-dimethylimidazolium)
porphyrinatomanganese(III) ([Mn-(TDMImP)]), demonstrated the genera-
tion of ClO
2
efficiently from chlorite ion under mild ambient conditions [142].
These results may have implications in understanding the reactions of the
chlorite ion with heme proteins and metalloporphyrins. Water-soluble syn-
thetic iron porphyrin complexes have been shown to generate O
2
from the
chlorite ion [143, 144]. The heme-thiolate enzyme chloroperoxidase trans-
ferred
ClO
−
to a mixture of
ClO
−
, Cl
−
, and O
2
through an intermediate ClO
2
[145]. The chemistry of the O
2
-evolving enzyme chlorite dismutase (Cld) has
also been studied [142-144, 146-149]. For example, the decomposition of chlo-
rite ion by
Dechloromonas aromatica
chlorite dismutase (DA-Cld) initially
produce O
2
DA-Cld, which ultimately yields 1 mol of O
2
and 1 mol of Cl
−
for
each mole of chlorite consumption [148, 150]. A proposed mechanism is pre-
sented in Figure 3.14 [148]. Initially, a Cld-chlorite Michaelis complex formed,
which subsequently cleaved via heterolytic and hemolytic cleavages, respec-
tively. Heterolytic cleavage yielded compound I and hypochlorite as the
leaving group, whereas compound II and the hypochloryl radical were formed
in hemolytic cleavage. The peroxychlorite ion was produced before the forma-
tion of products of the reaction.
3.3.2 Decomposition of ClO
2
The decomposition rate of ClO
2
in neutral solution is slow. However, in alka-
line solution, the rate is enhanced with the formation of
ClO
−
and
ClO
−
(Eq.
3.16) [151]:
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