Page 196 - Chiral Separation Techniques

174 6 Enantiomer Separations using Designed Imprinted Chiral Phases

ethyl amide substituent has been replaced by an anilide group, an additional increase
in selectivity is seen. Combining the structural modifications in one molecule, p-
amino-phenylalanine-anilide (5), the highest separation factor was obtained. Similar
observations have been made in the imprinting of a number of different classes of
compounds and thermodynamic evidence for the existence of multiple additive inter-
actions in the sites have been provided [35]. In the search for optimal synthetic con-
ditions for MIPs, useful start-up information can be obtained from the vast literature
existing on solution studies of molecular interactions and molecular recognition [For
example see: 79-81].

Fig. 6-11. Stabilization of monomer template assemblies by thermodynamic considerations.

6.6.3 Thermodynamic Factors

An important part of the optimization process is the stabilization of the
monomer–template assemblies by thermodynamic considerations (Fig. 6-11). The
enthalpic and entropic contributions to the association will determine how the asso-
ciation will respond to changes in the polymerization temperature [18]. The change
in free volume of interaction will determine how the association will respond to
changes in polymerization pressure [82]. Finally, the solvent’s interaction with the
monomer–template assemblies relative to the free species indicates how well it will
stabilize the monomer–template assemblies in solution [16]. Here each system must
be optimized individually. Another option is simply to increase the concentration of
the monomer or the template. In the former case, a problem is that the crosslinking
as well as the potentially nonselective binding will increase simultaneously. In the