In excess of phenylalanine
methyl ester the protected aspartame forms a poorly soluble carboxylate
anion which precipitates from the reaction mixture. This makes it
easy to remove by filtration.
The final
steps are:
More than 99.99%
of the aspartame produced by this enzyme-catalysed process is
the required sweet isomer. It is hard to imagine any method
other than enzyme
catalysis giving such a high selectivity.
