Synthesis of AIE polyethylene glycol-block-polypeptide bioconjugates and cell uptake assessments of their self-assembled nanoparticles

A synthetic polypeptide-based bioconjugate bearing tetraphenylethylene (TPE) at the polypeptide side chains, named mPEG-PPLGgTPE, was synthesized by ring-opening polymerization (ROP) of α-amino acid-N-carboxyanhydride (NCA) along with post-polymerization modification method. Interestingly, the bioconjugate enabled both the aggregation induced emission (AIE) effect at 469 nm and the aggregation caused quenching (ACQ) effect around 365 nm depending on water fraction in the tetrahydrofuran/water (THF/H2O) mixed solution, and this feature was very different from the commonly used TPE-based AIE polymers. The strong emission at the 365 nm was a rarely seen monomeric emission for the TPE units. Further studies on the fluorescence spectra indicated that the monomeric emission was not related with the secondary structure of polypeptides, but caused by the TPE unit itself. The mPEG-PPLGgTPE could self-assemble to AIE nanoparticles (NPs) in water in a mean size of 253 nm with polyethylene glycol (PEG) as the shells in surface. In addition, HeLa cell uptake level of the NPs was unsatisfactory according to the cell imaging results, which was independent of the polypeptides conformation, the PEG length and the size of NPs.