3884 J. Agric. Food Chem., Vol. 47, No. 9, 1999
Ludv ´ı k et al.
Sch em e 3
anaerobic, reductive conditions are preferred for the
complete cleavage of metamitron.
Preliminary experiments indicated that structurally
related metribuzin, more frequently used in the U.S.,
shows a pattern of reduction (Ludv ´ı k et al., 1998b) and
hydrolysis similar to that of metamitron. More detailed
investigation of its hydrolysis is in progress.
ACKNOWLEDGMENT
We thank to D. B. Knaebel (Clarkson University) for
discussion of microbial processes.
LITERATURE CITED
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(
Scheme 3). We can offer no evidence if in this hydrolytic
process the 2,3 or 4,5 bond is cleaved first. This excludes
the possibility to offer an explanation why the hydrolysis
of the diprotonated species is slower than that of the
monoprotonated form.
Ring opening has been observed also for 1,2,4-tri-
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CONCLUSIONS
As metamitron (1) is resistant to an acid-catalyzed
hydrolysis, but 1,6-dihydrometamitron (2) is relatively
easily hydrolyzed, it is proposed that in nature the
hydrolysis of the pesticide is preceded by a reduction of
the 1,6-azomethine bond. This conclusion is supported
by the fact that the reduction of the 1,6-azomethine bond
in metamitron (1) occurs at rather positive potentials
comparable to those at which the reduction of nitro
compounds occurs. A potential range between -0.2 and
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0.8 V (depending on pH) means that reduction occurs
relatively easily. It seems plausible that reductases
which act on organic nitro compounds and contribute
to the conversion of organic compounds in the ecosystem
could reduce the 1,6-CdN bond in metamitron as well.
Effective microbial metabolism of nitroaromatics (e.g.,
trinitrotoluene) and nitroheterocycles (hexahydro-1,3,5-
trinitro-1,3,5-triazine) occurs reductively (Boopathy et
al., 1993; McCormick et al., 1981; Harvey et al., 1990;
Preuss et al., 1993). Therefore, it is expected that similar
nonspecific (Preuss et al., 1993) or specific reductive
microbial pathways (Preuss et al., 1993) could promote
the effective hydrolysis of the metamitron molecule.
The only suggestion of accumulation of this chemical
in the environment is that the chemical is found in some
surface waters. However, this is normally associated
with immediate runoff events following spraying, not
due to lack of metabolism in surface soils (Lundberg et
al., 1995). Since this chemical is not known to ac-
cumulate in agricultural soils, it is expected that mi-
croaerophilic or localized anaerobic microenvironments
in the soil foster the microbial degradation of this
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6
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It is generally known that hydrolyses which in vitro
occur only under extreme conditions can be facilitated
by hydrolases under physiological conditions. It is thus
possible that the hydrolysis of the 1,6-dihydrometamitron,
which occurs in vitro at pH <3, takes place enzymati-
cally in soil under natural conditions. Thus, in nature,
8
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