As expected biodegradation was the main transformation route of MTP with
metoprolol acid (MTPA) being the major identified TP. However, photodegradation
also contributed to the degradation of MTP even though in a minor extent than
biodegradation [88]. For this specific route of degradation, O-desmethyl-MTP and
α-hydroxy-MTP were identified in addition to MTPA. The bioreduction of CBZ into
CBZ alcohol was the unique biodegradation route identified for CBZ, and biodegradation was the major transformation route of CBZ, while phototransformation also
occurred in light conditions but similarly to MTP in a much lesser extent than
biodegradation. Reductive dechlorination, hydroxylation, and cleavage of the ether
bond were the major transformation routes observed under photolysis [87]. Results
for MTP biodegradation kinetic experiments in soil slurry experiment system are
shown in Fig. 4a, b.
Experiments in the light degraded MTP quicker than their dark equivalents due to
MTP photolysis. No stereoselectivity in MTP degradation was observed in any
abiotic experiments. Stereoselective degradation of MTP leading to a (S)-enrichment
was exclusively observed under biotic conditions, confirming the specificity of ER
variations to biodegradation processes. Enantiomeric fractionation (ER t /ER 0 ) was
plotted against the MTP residual fraction (C t /C 0 ) according to the Rayleigh equation
for all biotic experiments. The linear fit to the Rayleigh approximation was obtained
under both dark and light conditions (R
2
> 0.99 and > 0.98, respectively). Both
enantiomers followed the same first-order kinetic and probably obey the same
degradation mechanisms. In these conditions, the ER-conversion relationship does
not depend on reaction conditions but mainly depends on reaction time. The
enantiomeric enrichment factor was calculated to be 22% and 56% in light and
dark conditions, respectively.
In anoxic soil slurry experiments, the transformation of CBZ into CBZ-OH was
nearly quantitative, and biodegradation gave a first-order kinetic fit (see Fig. 5a, b).
When the enantiomeric fractionation was plotted against the CBZ residual fraction,
the linear fit to the Rayleigh equation was obtained with r squared above 0.98 as
quality control parameter. The enantiomeric enrichment factor was given by the
slope of linear regression line and was found to be 33% in dark conditions and 26%
in light conditions. This lower value in light condition is due to the slight contribution of phototransformation to the whole degradation of CBZ.
The environmental significance of these results is that on the basis of the
knowledge of the enantiomeric enrichment factors (ε R ), which are specific to one
biotransformation reaction, and on the basis of the experimental measurement of ER
of MTP and CBZ by chiral LC-MS, it is possible to derive the percentage of the
dissipation of selected PhACs in soil which can be directly attributed to biodegradation processes without establishing a mass balance during the course of the
degradation. This approach avoids to carry out analysis in soil leachates and in
plant or to evaluate the formation of non-extractable residues to evaluate the
biodegradation rate of a chiral pharmaceutical.
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M. Brienza et al.
metoprolol acid (MTPA) being the major identified TP. However, photodegradation
also contributed to the degradation of MTP even though in a minor extent than
biodegradation [88]. For this specific route of degradation, O-desmethyl-MTP and
α-hydroxy-MTP were identified in addition to MTPA. The bioreduction of CBZ into
CBZ alcohol was the unique biodegradation route identified for CBZ, and biodegradation was the major transformation route of CBZ, while phototransformation also
occurred in light conditions but similarly to MTP in a much lesser extent than
biodegradation. Reductive dechlorination, hydroxylation, and cleavage of the ether
bond were the major transformation routes observed under photolysis [87]. Results
for MTP biodegradation kinetic experiments in soil slurry experiment system are
shown in Fig. 4a, b.
Experiments in the light degraded MTP quicker than their dark equivalents due to
MTP photolysis. No stereoselectivity in MTP degradation was observed in any
abiotic experiments. Stereoselective degradation of MTP leading to a (S)-enrichment
was exclusively observed under biotic conditions, confirming the specificity of ER
variations to biodegradation processes. Enantiomeric fractionation (ER t /ER 0 ) was
plotted against the MTP residual fraction (C t /C 0 ) according to the Rayleigh equation
for all biotic experiments. The linear fit to the Rayleigh approximation was obtained
under both dark and light conditions (R
2
> 0.99 and > 0.98, respectively). Both
enantiomers followed the same first-order kinetic and probably obey the same
degradation mechanisms. In these conditions, the ER-conversion relationship does
not depend on reaction conditions but mainly depends on reaction time. The
enantiomeric enrichment factor was calculated to be 22% and 56% in light and
dark conditions, respectively.
In anoxic soil slurry experiments, the transformation of CBZ into CBZ-OH was
nearly quantitative, and biodegradation gave a first-order kinetic fit (see Fig. 5a, b).
When the enantiomeric fractionation was plotted against the CBZ residual fraction,
the linear fit to the Rayleigh equation was obtained with r squared above 0.98 as
quality control parameter. The enantiomeric enrichment factor was given by the
slope of linear regression line and was found to be 33% in dark conditions and 26%
in light conditions. This lower value in light condition is due to the slight contribution of phototransformation to the whole degradation of CBZ.
The environmental significance of these results is that on the basis of the
knowledge of the enantiomeric enrichment factors (ε R ), which are specific to one
biotransformation reaction, and on the basis of the experimental measurement of ER
of MTP and CBZ by chiral LC-MS, it is possible to derive the percentage of the
dissipation of selected PhACs in soil which can be directly attributed to biodegradation processes without establishing a mass balance during the course of the
degradation. This approach avoids to carry out analysis in soil leachates and in
plant or to evaluate the formation of non-extractable residues to evaluate the
biodegradation rate of a chiral pharmaceutical.
166
M. Brienza et al.