On Biomimetics
340
Fig. 5.5. SEM images of Poly L-phenylalanine microspheres of different polymerization
conditions. (a) Hollow structure (b) Porous particles (c) Rough surface particles.^145
different from that of the corresponding racemic compound owing to their different
crystalline structures. The XRD pattern of valine crystals crystallized on chiral microspheres
shows a typical set of diffraction peaks corresponding to D, L-valine, as well as two
additional diffractions that match the diffraction from the (0,0,3) and (1,1,-3) crystal planes
of the pure enantiomer of valine. The observation that the XRD patterns of D, L-valine
crystals crystallized on the chiral microspheres display diffraction peaks corresponding to
the pure enantiomer of valine clearly demonstrates that enantioselective crystallization has
been achieved on the chiral microspheres. In addition, the thermal behaviour of the
racemates and pure enantiomers is different and, accordingly, traces of enantiomers can be
examined by DSC. Further evidence for the generation of pure enantiomeric crystals on the
chiral microspheres is obtained from DSC measurements showing a shift of the melting
point to lower temperatures (T=244^0 C), indicative of the presence of pure enantiomeric
valine crystals. Although XRD and DSC clearly reveal enantioselective crystallization onto
the chiral microspheres, it is difficult to obtain quantitative information about chiral
separation from these techniques. However, optical rotation measurements during
crystallization show an enantiomeric excess of ca. 25% for crystallization onto chiral
microspheres.
Hybrid organic inorganic nanoparticles have been reported recently.^153 Chen et al. have
reported on a novel approach for the synthesis of hybrid particles based on a chiral
polymeric core of poly polyacetylene with a silica shell. The particles were prepared in a
one-pot, two-step polymerization reaction. In the first step, acetylene monomers, N-
propargyl-( 1S)-camphor-10-sulfamide [(S)-SA], and N propargyl (1R)-camphor-10-
a bc