Synthesis, Chain Helicity, Assembling Structure, and Biological Compatibility of Poly(phenylacetylene)s Containingl-Alanine Moieties

A functional phenylacetylene monomer containing a naturally occurring building block of l-alanine, namely 4-ethynylbenzoyl-l-alanine methyl ester (PA-Ae), was synthesized. The monomer was polymerized by organorhodium catalysts, giving the corresponding “polyester” (PPA-Ae) with high molecular weights (Mw up to 1.2 × 106) and stereoregularities (Z content up to 97%) in high yields (up to ∼91%). The polyene backbone of PPA-Ae undergoes irreversible Z-to-E isomerization at ∼180−250 °C. The ester groups in the pendants of the polymer are selectively deprotected by the base-catalyzed hydrolysis, producing a “polyacid” with “free” l-alanine pendants (PPA-A). While PA-Ae monomer is CD-inactive at wavelengths longer than 300 nm, both PPA-Ae and PPA-A polymers exhibit strong Cotton effects in the long wavelength region where their polyene backbones absorb, indicating that the chiral pendants have induced the polymer chain to take a helical conformation with an excess in one-handedness. Upon natural evaporation of its solutions, the amphiphilic chains of PPA-Ae self-associate in a cooperative fashion, furnishing a variety of organizational morphologies including twisting cables, spiral ribbons, spherical vesicles, and helical nanotubes. The polymers are biocompatible: the living cells are all survived after they have been subcultured in the presence of the polymers.