Development of Fluorescent Poly-L-Tyrosine for Bio-medical Applications

Abstract

Synthetic polypeptides, with their ability to form distinct secondary structures, serve as interesting protein-mimetic biomaterials. Further, their biocompatibility coupled with their facile tunability to form a myriad of nanoarchitectures upon self-assembly endow them with pivotal roles in applications spanning from materials science to the bio-medical industry. This thesis investigates the possibility of synthesising an inherently fluorescent non-toxic polypeptide by post-polymerisation chemical modification onto the amino acid side chain. Here, PEG5k-b-Tyrosine amphiphilic homopolymers were formylated following the Duff reaction protocol to directly obtain the salicylaldehyde fluorophore moiety on the polypeptide backbone. This modification transformed a previously non-fluorescent polypeptide into a fluorescent one via the process of ESIPT, occurring specifically in aqueous environments. We saw that this amphiphilic system exhibited a dual emission; blue in its solvated state in DMSO (enol emission) and green upon self-assembly into nano-micelles in water (keto emission). This gives one of the first investigations into the ingenious design of a self-fluorescent polypeptide without the covalent attaching of any fluorescent moiety or loading of a fluorescent dye. Upon forming nano-micelles in water, this fluorescent polypeptide was also shown to encapsulate dyes and drugs such as Rhodamine B, IR780 and Doxorubicin. Further, resulting from considerable overlap between the polymer’s keto emission and the absorption profile of Rhodamine B, the phenomenon of micelle-assisted-ESIPT-FRET occurring in this system was also investigated with data up to the cellular level, obtained from the MCF-7 cell line. This versatile self-reporting fluorescent polypeptide nano-vehicle opens doors to future opportunities in therapeutics, diagnostics and advanced bio-imaging.