Methionine-based cationic polypeptide/polypeptide block copolymer with triple-stimuli responsiveness: DNA polyplexation and phototriggered release

Abstract

This study demonstrates the synthesis of positively charged 2-nitrobenzyl functionalized polymethionine (P[MetNB][Br]) and its block copolymer of pseudopeptidic poly(2-ethyl-2-oxazoline) (PEtOx-b-P[MetNB][Br]) by firstly employing ring-opening polymerization (ROP), followed by post-functionalization through nucleophilic substitution. The as-prepared cationic P[MetNB][Br] and PEtOx-b-P[MetNB][Br] are readily soluble in water, exhibiting responsiveness towards multiple stimuli such as ions, temperature and light. The aqueous solutions of both of the cationic polypeptides become cloudy in the presence of anions like I⁻, ClO₄⁻ and SCN⁻ due to the formation of insoluble anionically cross-linked colloidal aggregates of cationic polypeptides. These cloudy solutions of the polypeptide and polypeptide block copolymer both show reversible upper critical solution temperature (UCST)-type phase transitions due to the dissolution of the aggregates upon heating. The parameters, polypeptide concentrations and the nature of the anion and its concentration affect the cloud points in aqueous solutions. Upon irradiation with UV light (λ = 350 nm), the pendent nitrobenzyl sulfonium moiety of both of the cationic polypeptides undergoes photocleavage to a neutral polypeptide/polypeptide block copolymer. This study further demonstrates the binding of a cationic polypeptide/polypeptide block copolymer with calf thymus DNA (ctDNA) through electrostatic complexation and its phototriggered release as monitored by fluorescence spectroscopy and gel electrophoresis.