On Biomimetics
334
ascorbate (vitamin C), (S)-proline, or (R)-gluconate were attached to the PEI amino branches.
The different optically-active groups attached are summarized in Figure 5.1. Chiral moieties
were attached to PEI throughout two main routes. In the case of amino-acid peptide-coupling
methods and other chiral moieties, a nucleophilic attack by the PEI amine groups on the
desired molecules was preformed. The authors demonstrated the chiral crystallization for
conglomerate systems of sodium ammonium tartrate (NaNH 4 T) and also for racemic systems
of calcium tartrate tetrahydrate (CaT). In the crystallization of CaT, it was found that
appropriate chiral DHBCs can slow down the formation of the thermodynamically most stable
racemic crystals as well as the formation of one of the pure enantiomeric crystals. Therefore,
chiral separation by crystallization occurs even when racemic crystals are thermodynamically
favored. However, chiral separation was only observed (though still with low chiral purity) at
the early stages of the crystallization reaction. At longer crystallization times, the enantiomeric
separation decreased again in favor of the racemate crystal formation. The presence of DHBCs
can also modify the crystal morphology and create unusual morphologies of higher
complexity, reflecting the polymer crystal interactions.
Fig. 5.1. Chiral copolymers used in the crystallization of racemic CaT and conglomerate
NaNH 4 T. (N=number of optically-active units). Crystal morphology of: a) conglomerate
NaNH 4 T formed in the presence of 4 mgmL-1 PEG-b-PEI ± (S)-ascorbate; b) racemic CaT
formed in the presence of 15 mgmL-1 PEG-b-PEI ± (S)-proline (scale bar = 5 μm).^139
Based on Mastai's findings, Menahem et al.^140 studied extensively the crystallization of
racemic and conglomerate systems in the presence of soluble chiral polymers based on N-
acrylic amino acid monomers. In Menahem’s study, a series of poly-N-acrylic amino acid
were synthesized by a method that involves the radical polymerization of N-acryl amino-
ab