Page 183 - Chiral Separation Techniques

6.3 Structure–Binding Relationships 161

6.3.2 Low Selectivity

Numerous examples of MICSPs that are capable of resolving more than the race-
mate corresponding to the template have been reported [17, 32]. In these cases some
structural variations are tolerated without seriously compromising the efficiency of
the separation. For instance, a polymer imprinted with L-phenylalanine anilide
resolved amino acid derivatives with different side chains or amide substituents [17].
Anilides of all aromatic amino acids were here resolved as well as β-naphthylamides
and p-nitroanilides of leucine and alanine (Table 6-3). Furthermore, in aqueous
mobile phases, the free amino acid phenylalanine could also be base line resolved on
an L-PA-imprinted polymer [32]. Apparently, substitution of groups that are not
involved in potential binding interactions only leads to a small loss in enantioselec-
tivity. Also it was noted that the dipeptide, D,L-phenylalanylglycine anilide was
resolved, while glycyl-D,L-phenylalanine anilide was not. This observation empha-
sizes the importance of the spatial relationship between the functional groups at the
sites, and indicates that substitutions made at some distance away from the center of
chirality are allowed.

6.3.3 Studies of the Monomer–Template Solution Structures

To what extent do the solution complexes formed between the monomer and the
template in solution reflect the architecture of the polymeric binding sites ? This
question is important, since a thorough characterization of the monomer template
assemblies may assist in deducing the structure of the binding sites in the polymer
1
and thus have a predictive value. H-NMR spectroscopy and chromatography were
used to study the association between MAA and the template L-PA in solution as a
1
mimic of the pre-polymerization mixture [15]. The H-NMR chemical shifts of
either the template or the monomer versus the amount of added MAA as well as the
chromatographic retention of D,L-PA versus the amount of acid in the mobile phase,
varied in accordance with the formation of multimolecular complexes between the
template and the monomer in the mobile phase. A 1:2 template–monomer complex
was proposed to exist prior to polymerization based on the modeled complex distri-
bution curves. Based on these results, hydrogen bond theory, and the assumption that
the solution structure was essentially fixed by the polymerization, a structure of the
template bound to the site was proposed (Fig. 6-5). Since these initial studies, a num-
ber of other examples support this model, i.e. the recognition is due to functional
group complementarity and a correct positioning of the functional groups in the sites
as well as steric fit in the complementary cavity [33-36]. Rebinding to sites formed
of residual nonextracted template have also been proposed as a contributing factor to
the observed recognition [37]. In most imprinted systems however, rebinding selec-
tivity or catalytic efficiency increase with increasing recovery of the template [38]
and the Langmuir-type adsorption indicates a true receptor behavior [39].