Chiral copoly(methacrylate)s carrying amino acid pendants in the side-chains.

Abstract

Main chain chiral poly(methacrylate)s have been synthesized by controlled reversible addition–fragmentation chain transfer (RAFT) copolymerization of chiral monomer Boc-l-alanine methacryloyloxyethyl ester (Boc-l-Ala-HEMA, A) with Boc-d-alanine methacryloyloxyethyl ester (Boc-d-Ala-HEMA, B) and achiral monomer Boc-2-methylalanine-methacryloyloxyethyl ester (Boc-Aib-HEMA, C), and B with C by changing comonomer feed compositions. We systematically examined the role of l-alanine, d-alanine and 2-aminoisobutyric acid (Aib) as folding nuclei in our designed hybrid polymer-amino acid conjugates. Specific optical rotation and circular dichroism (CD) spectroscopy studies reveal that poly(Boc-l-Ala-HEMA) and poly(Boc-d-Ala-HEMA) formed entirely opposite handed helical structure stabilized by intramolecular hydrogen bonding among the pendant chromophores of the chiral monomers. Poly(Boc-Aib-HEMA) formed random coil structures conformation (unstructured conformation), whereas the resultant copolymers of C with A and B showed an unusual linear relationship of optical activity against comonomer composition, in sharp contrast with Green’s “majority” and “sergeant-and-soldier” rule. After Boc-group expulsion in acidic environment from the polymer side-chains, backbone retained its helicity. Variable temperature chiroptical properties investigation was also performed to examine the thermal stability of the homo and copolymer’s helical conformations. Additionally, pH induced conformational transformation was observed for the homo and copolymers.