Poly(gamma-glutamic acid) esters with reactive functional groups suitable for orthogonal conjugation strategies

Abstract

We report on a series of novel poly(c-glutamic acid) (PGGA) esters, in which the chemical structure and composition, and the molecular weight are systematically changed. Modification of PGGA of microbial origin, used either as the sodium salt or in the free acid form, by means of alkylation with highly reactive bromides under SN2 conditions, affords copolymers with an essentially random microstructure. These reaction conditions are applied iteratively to achieve full esterification, obtaining allyl or propargyl ester functionalities within the polymer backbone, diluted with inert functional groups, such as benzyl, ethyl, or hexyl ester functionalities. The copolymers have been characterized regarding their chemical structure and thermal and bulk properties using nuclear magnetic resonance, thermogravimetry, differential scanning calorimetry, and X-ray diffraction techniques. We demonstrate that allyl and propargyl ester groups can be efficiently transformed using click chemistries, such as thiol-ene or copper(I)- catalyzed azide–alkyne cycloaddition reactions; such efficient conjugation strategies will be required to transformthe native bacterial biopolymer into a material with tailored properties for bulk scale or biomedical applications.