On Biomimetics
328
developed by David Avnir’s group. In a series of articles, Avnir et al. 111-112showed that
template molecules such as propranolol, 2,2,2-trifluoro-1-(9-anthryl) ethanol, DOPA, and
tyrosine can be used to prepare a chiral imprint sol-gel matrix. The shape of the chiral
matrix is maintained when the template molecule is extracted. Therefore, the porous
materials formed are enantiopure; i.e., the cavity left inside the sol-gel films can discriminate
between optical enantiomers.
Recently, we demonstrated^113 a new approach for chiral imprinting onto a solid support
based on the use of chiral block copolymers. In this method, silica is templated by the chiral
DHBCs of a poly (ethylene oxide) block and a chiral block of amino acids such as D-
phenylalanine [PEO-b-D-Phe]. In general, the synthesis of the chiral DHBCs is based on a
simple ring opening polymerization of protected amino acid N-carboxyanhydrides (NCAs).
This process results in the formation of chiral block copolymers based on PEOn-b-D- or L-
amino acids. The obtained chiral DHBCs were then suspended in an acidic solution and the
amino acid block found to have a helical structure, It's helically was determined by CD
measurements. The ability of chiral DHBCs to aggregate into well-defined micelles at the
reaction conditions was crucial to the formation of a well-defined template. The chiral
DHBCs aggregation behaviour was achieved by the change in the DHBC concentration in
the reaction solution. As a final step a silica processor was added to the reaction solutions
containing the chiral DHBC and the sol-gel process was accomplished. As a subsequent step
the chiral template was removed by solvent extraction, leaving chiral voids. Therefore, a
block co-polymer with a D-phenylalanine block composed of ten repeating units in average
was synthesized. The copolymers exhibit spherical micelles with a 10 nm diameter at the pH
(pH=2) reaction. The sol-gel condensation of a silica processor, TEOS, was used to form
silica in the presence of the chiral polymers in their Critical Micelle Concentration (CMC) to
give the chiral imprinted mesoporous silica. Consequently, after the chiral DHBC template
was removed, chiral hexagonal cavities of 5 nm diameter and a high surface area (700
gr/m^2 ) were obtained. [Figure 4.2]
(a) (b)
Fig. 4.2. TEM images: (a) high magnification (b) low magnification of chiral silica made from
PEO 113 - b-D-Phe 10 after chiral copolymer extraction. The light area corresponds to the pores,
while the dark area corresponds to the walls.