Development of biomimetic materials : synthesis, helical structures, self-assembling morphologies, and biological activities of amphiphilic polyacetylenes containing biocompatible building blocks

Polymerization of a series of novel acetylenic monomers bearing different hydrophilic entities such as ethylene glycols (1), amino acids (2), saccharides (3), and nucleosides (4) has been studied. The monomers can be successfully polymerized by Rh-based complexes, but not by metathesis catalysts such as WCl6 and MoCl5. 1H NMR spectroscopy revealed strong signals characteristic of cis olefinic protons of the polymers, indicative of their high stereoregularities. Such signals, however, became unclear (broader) for the amino acid-containing polyacetylenes when less polar solvents like chloroform, dichloromethane, were used, demonstrating that their polymer chains were folded and their molecular motions were hence restricted by internal hydrogen bonding. Polyacetylene is a symmetric chain of conjugated macromolecule, whose chain symmetry can, however, be broken by external and internal perturbations, generating spirally rotating molecular wires. The external approach involves the use of asymmetric force field and interactive complexing agents, while the internal one involves the covalent attachment of stereogenic pendants to the conjugated backbone at the molecular level. Incorporation of above naturally occurring building blocks (2-4) into the polyacetylene structure therefore results in the formation of not only amphiphilic but also optically active polymers, whose chain helicity can be continuously and reversibly tuned by simple external stimuli such as solvent, pH, temperature, and additive. Such tuned behaviors have been fully imaged in their corresponding circular dichroism activities and optical rotations, illustrating that stabilization of their single-handed helical conformations relies on intra- and inter-chain hydrogen bonds and is affected by bulkiness of the pendant groups of the macromolecular chains. The amphiphilic polyacetylenes, in response to the changes in their environments, self-associate into robust organizational morphologies reminiscent of natural hierarchical structures such as helix, sphere, twisted cable, twisted ribbon, vesicle, tubule, twisted tubule, hairpin loop, extended fibril, coiling ribbon, honeycomb pattern, and mollusk shape. Some of the structures have been rarely reported in scientific literature. AFM and TEM observations revealed co-existence of the vesicular and nanotubular structures, strongly supporting that formation of the tubules resembles the biological architectural process, as resulted from coalescence of the vesicles. Further variation of the tubular structure promoted the formation of multi-stranded helical tubes by winding up the single-stranded nanotubules. The ethylene glycol-containing amphiphiles are capable of forming micellar structures, driving crystal growth of carbazole molecules to form unique intriguing morphologies depending on the structure of the pendant groups. Biological studies revealed that the amino acid-containing polyacetylenes are generally biocompatible with HeLa cells without serious cytotoxicity effects up to a dosage of 22.24 μg/cm2. Growth of the HeLa cells can be stimulated by the addition of a small amount of a monosaccharide-containing polyacetylene