Preparation And Self-assembly Of Amphiphilic DNA Hybrid Copolymer

In the last few decades, the assembly of amphiphilic DNA hybrid copolymers has become an attractive strategy to construct nanoscopic structures. The amphiphilic DNA hybrid copolymers are usually composed of two segments. One of the segments is the hydrophobic synthetic polymer that can be predesigned to have active functional groups. The self-assembling process is usually attributed to this segment driven by the hydrophobic effect. The other segment is DNA molecule, whose unique programmable molecular recognition ability adds structural diversities and biological activities to the hybrid copolymers. The self-assembling of this amphiphilic DNA hybrid copolymer follows a bottom-up strategy, i.e. the molecular structure of the hybrid copolymer is well-defined, the self-assembling is generated from molecular levels and can be modulated by molecule structures. The self-assemblies of this kind of amphiphilic DNA hybrid copolymers are expected to have broad applications in drug carriers, biomedicine, intelligent hydrogel, energy transfer and supramolecular electronics. In this paper, the poly (propargyl methacrylate) (PPMA) was synthesized to act as an hydrophobic multi-alkynyl backbone polymer to which azide modified DNA chains were grafted via click chemistry. Furthermore, the self-assembling behaviours, molecular recognition ability and the capability to encapsulate hydrophobic drugs were studied in detail.First, the PPMA back bone was synthesized by the solution polymerization of propargyl methacrylate (PMA) monomers. The FT-IR spectrum and H’NMR spectrum verified that the clickable alkynyl groups were arranged along the chain. The molecular weight measured by gel permeation chromatography (GPC) was Mn=7K, MW=13K with PDI=1.9. The structure of the polymer was characterized by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF).Then, the azide modified DNA molecules were grafted onto PPMA backbone to generate PPMA-g-DNA via click chemistry. The click reaction took place effectively by using Cu+as the catalyzer and pentamethyldiethylenetriamine (PMDETA) as the stabilizer. The influences of solvents, temperature and time upon the reaction yields were studied. The results showed that the click reaction could be carried out in both aqueous solution and organic solvent. The increasing of temperature improved the yields dramatically. And the click reaction was almost completed by 3 h. The optimized condition of click reaction was in a mixed solvent of water and THF and the temperature was set at 55℃ for 3h. The grafting density of DNA segments was 11% according to agarose gel electrophoresis.Finally, the self-assembling behaviours, molecular recognition ability and the capability to encapsulate hydrophobic drugs of the amphipathic PPMA-g-DNA graft copolymer were studied. In aqueous solution, PPMA-g-DNA self assembled into nanofibers that orientated in averaged directions and wrapped around each another. The nanofiber was in micrometers long with an average diameter about 10 nm. In 20% THF solution, the above nanofibers unwound and appeared in single without interactions. When 40% THF was introduced, the single nanofibers began to wrap around in the same direction to form compact multi-strand helices with a screw pitch around 36 nm according to atomic force microscope (AFM) measurement. The molecular recognition ability of DNA segments was verified by the hybridization experiments with the complementary DNA labeled gold nanoparticles (AuNPs). The PPMA-g-DNA assembled on the surface of AuNPs to form an organic layer. The hydrophobic dye Nile red mimicking drug molecules was encapsulated in the core of PPMA-g-DNA nanofibers, which showed vivid colour image in fluorescence microscope. It demonstrated that the PPMA-g-DNA nanofibers had good drug loading ability.