Enantioselective metabolism has been deeply investigated in pharmacokinetics and
pharmacodynamics because biological activity and toxicity are very often largely
dependent on chirality. Less is known about environmental enantioselective transformations because chirality has not been included in any regulatory risk assessment
procedure yet. As enantioselectivity is mainly specific to biodegradation processes,
PhAC chirality could be used to track the pollutant sources in treated and nontreated
wastewater [81] as well as to better understand their environmental fate. Indeed,
enantioselectivity reflects biological processes, since nonbiological enantioselective
catalysis is rare in nature. Abiotic enantioselective processes such as sorption on
environmental surfaces have been found in some cases [82] but usually of minor
significance with respect to biodegradation-related enantioselectivity. The major
advantage of the enantiomeric fractionation approach has been the relative simple
analysis of ER using chiral chromatography coupled to conventional mass spectrometry (LC-MS) with appropriate LODs of ng L
À1 in waters or of ng g
À1 in solid
matrices for environmental studies. The concept of applying the Rayleigh equation,
which was developed for isotopic fractionation, to enantioselective processes was
suggested by Jammer et al. [83, 84] for in vitro enzymatic transformations under
laboratory controlled conditions. In this case, the isotope ratio was replaced by the
enantiomeric enrichment (ERt), expressed as a ratio between two enantiomers. A
linear relationship between the evolution of the log of enantiomeric ratio and the log
of the evolution of a chiral compound concentration can be still obtained, and the
slope of the straight line gives the enantiomeric enrichment factor ε ER (see Fig. 1b).
This factor is usually larger than the isotopic enrichment factor. Actually, it is
expressed in percent unit and not in per mill unit and can be used as a characteristic
tool for an enzymatic reaction. This extension of the Raleigh model to
enantioselective processes is only valid providing that degradation kinetics of each
enantiomer fit to a first-order kinetic model. Enzymatic reactions are frequently
described by MichaelisÀMenten kinetics (Eq. 1), which are nonlinear. However,
the equation gives linear dependence when the concentration of the substrate C is
much lower than the Michaelis–Menten constant, K M :
dC
dt
¼
ÀkC
K M þ C
½
ð1Þ
This approximation is usually correct in environmental studies because contaminants are found in very low concentrations. However, the first-order kinetic model
can be disturbed by sorption processes because sorption processes are nonlinear
processes [85]. Indeed, when PhAC concentrations decrease, their bioavailability
often decreases leading to slower declines. This can be a source of uncertainty in the
determination of the biodegradation extent by using the enantiomeric fractionation
making it less accurate than the CSIA approach. Till now, the parallel process of
comparing the enrichment of one enantiomer relative to the other has been applied in
enantiomeric analysis to prove the existence of biodegradation and to try to quantify
the extent of this process exclusively in activated sludge treatments both at the
lab-scale [86] and in biological wastewater treatment plants [87, 88]. The contribution of this work was to expand the current knowledge of enantioselective processes
162
M. Brienza et al.
pharmacodynamics because biological activity and toxicity are very often largely
dependent on chirality. Less is known about environmental enantioselective transformations because chirality has not been included in any regulatory risk assessment
procedure yet. As enantioselectivity is mainly specific to biodegradation processes,
PhAC chirality could be used to track the pollutant sources in treated and nontreated
wastewater [81] as well as to better understand their environmental fate. Indeed,
enantioselectivity reflects biological processes, since nonbiological enantioselective
catalysis is rare in nature. Abiotic enantioselective processes such as sorption on
environmental surfaces have been found in some cases [82] but usually of minor
significance with respect to biodegradation-related enantioselectivity. The major
advantage of the enantiomeric fractionation approach has been the relative simple
analysis of ER using chiral chromatography coupled to conventional mass spectrometry (LC-MS) with appropriate LODs of ng L
À1 in waters or of ng g
À1 in solid
matrices for environmental studies. The concept of applying the Rayleigh equation,
which was developed for isotopic fractionation, to enantioselective processes was
suggested by Jammer et al. [83, 84] for in vitro enzymatic transformations under
laboratory controlled conditions. In this case, the isotope ratio was replaced by the
enantiomeric enrichment (ERt), expressed as a ratio between two enantiomers. A
linear relationship between the evolution of the log of enantiomeric ratio and the log
of the evolution of a chiral compound concentration can be still obtained, and the
slope of the straight line gives the enantiomeric enrichment factor ε ER (see Fig. 1b).
This factor is usually larger than the isotopic enrichment factor. Actually, it is
expressed in percent unit and not in per mill unit and can be used as a characteristic
tool for an enzymatic reaction. This extension of the Raleigh model to
enantioselective processes is only valid providing that degradation kinetics of each
enantiomer fit to a first-order kinetic model. Enzymatic reactions are frequently
described by MichaelisÀMenten kinetics (Eq. 1), which are nonlinear. However,
the equation gives linear dependence when the concentration of the substrate C is
much lower than the Michaelis–Menten constant, K M :
dC
dt
¼
ÀkC
K M þ C
½
ð1Þ
This approximation is usually correct in environmental studies because contaminants are found in very low concentrations. However, the first-order kinetic model
can be disturbed by sorption processes because sorption processes are nonlinear
processes [85]. Indeed, when PhAC concentrations decrease, their bioavailability
often decreases leading to slower declines. This can be a source of uncertainty in the
determination of the biodegradation extent by using the enantiomeric fractionation
making it less accurate than the CSIA approach. Till now, the parallel process of
comparing the enrichment of one enantiomer relative to the other has been applied in
enantiomeric analysis to prove the existence of biodegradation and to try to quantify
the extent of this process exclusively in activated sludge treatments both at the
lab-scale [86] and in biological wastewater treatment plants [87, 88]. The contribution of this work was to expand the current knowledge of enantioselective processes
162
M. Brienza et al.