Synthesis, characterization, fluorescence labeling and cellular internalization of novel amine-functionalized poly(ethylene glycol)-block-poly(ε-caprolactone) amphiphilic block copolymers

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Abstract
We report in this paper a facile way to prepare novel amine-functionalized monomethoxy-poly(ethylene glycol)-b-poly(ε-caprolactone) (mPEG-b-PCL) amphiphilic block copolymers, which are subsequently fluorescently labeled. In our synthetic route, monomethoxy-poly(ethylene glycol)-b-poly[ε-caprolactone-co-γ-(carbamic acid benzyl ester)-ε-caprolactone] [mPEG-b-P(CL-co-CABCL)] copolymers were synthesized viaring-opening polymerization (ROP) of ε-caprolactone (CL) and a newly developed monomer, γ-(carbamic acid benzyl ester)-ε-caprolactone (CABCL) at varied ratios using mPEG as macroinitiator and Sn(Oct)2 as catalyst. Subsequent deprotection upon removal of carbobenzoxy (Cbz) group yielded monomethoxy-poly(ethylene glycol)-b-poly(ε-caprolactone-co-γ-amino-ε-caprolactone) [mPEG-b-P(CL-co-ACL)] copolymers bearing primary amine functional groups on the PCL block. The structures of polymers were characterized with NMR, FT-IR and GPC techniques. These amphiphilic block copolymers self-assembled into micelles in aqueous solution and the critical micelle concentration (CMC) was dependent on the compositions of the copolymers. In addition, the particle size and morphology of micelles were studied with DLS and TEM, respectively. In vitro study demonstrated that the micelles were nontoxic to human fibroblasts based on MTT and live/dead assays. Furthermore, a proof-of-concept usage of amino groups for bioconjugation was illustrated by tagging the copolymer with a fluorophore, fluorescein isothiocyanate (FITC). Internalization of FITC-labeled micelles by fibroblast cells was observed under fluorescence microscopy. Through facile conjugation of chemical moieties such as drugs, peptides, proteins or fluorophores, micelles prepared with these amine-functionalized mPEG-b-PCL copolymers hold great promise in biomedical applications.

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