Industrial Biotransformations

where r p = selectivity to component p (–); n s0 = amount of substrate s at the start of the
reaction (mol); n s = amount of substrate s at the end of the reaction (mol); n p0 = amount
of product p at the start of the reaction (mol); n p = amount of product p at the end of the
reaction (mol); m s = stoichiometric factor for substrate s (–); and m p = stoichiometric factor
for product p (–).
The selectivity describes the extent to which the product molecules are synthesized in
relation to the substrate molecules converted. The selectivity must be as close to unity as
possible, in order to avoid wastage of the starting material. If the unreacted substrate can
be recovered during down stream processing and recycled into the reactor, the selectivity
becomes a very crucial factor in the economic operation of the plant.
If only a very short reaction course is considered, the selectivity assumes a differential
form. This is interesting for gaining information on the synthesis of by-products at every
step of the conversion. This information is important to estimate whether a premature
halting of the reaction would be more useful in improving the overall yield of the process.
The combination of conversion, yield and selectivity leads to the equation:
g ˆ r X (4)
5.1.1.4 Enantiomeric Excess
The enantiomeric excess (ee) is the difference between the numbers of both enantiomers
per sum of the enantiomers:
ee R ˆ n R n S
n R ‡n S
5.1 Definitions
where ee R = enantiomeric excess of the (R)-enantiomer (–); n R = amount of the (R)-enantiomer
(mol); and n S = amount of the (S)-enantiomer (mol).
The enantiomeric excess describes the enantiomeric purity of an optically active molecule.
Minor differences in the spatial sequence of the binding partners of one central
atom (particularly in the active site region) can lead to dramatic differences in chemical
behavior, biological pathways and (substrate) recognition. It is absolutely essential to
choose biotransformation processes that lead to products with high enantiomeric excess.
This is because the pharmacological activities of the enantiomers might not be identical.
Each enantiomer could even have a completely different pharmacological activity profile,
including serious side effects in certain cases. The enantiomers often have different organoleptic
properties, such as taste, flavor and odor. Many pharmaceuticals and agrochemicals,
which were previously sold as racemates, are now sold as single enantiomer products
(“racemic switches”). The US FDA (Food and Drug Administration) makes it mandatory
for drug companies to carry out clinical trials of the individual enantiomers before
selection of the correct enantiomer as the active pharmaceutical ingredient [1]. The use
of enantiomerically pure drugs instead of racemates avoids the intake of inactive (or even
toxic) compounds. Similarly, this applies to the use of enantiomerically pure agrochemicals.
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