Chemical modification of functionalized polyhydroxyalkanoates via "Click" chemistry: A proof of concept

A novel approach to the post-biosynthetic chemical modification of bromo and alkyne functionalized poly(3-hydroxyalkanoates), (PHAs), via copper-catalyzed azide-alkyne cycloaddition (CuAAC) and strain promoted azide alkyne cycloaddition (SPAAC) is reported. Optimum conditions for the biosynthesis of the PHA copolymers, poly(3-hydroxynonanoate-co-3-hydroxy-11-bromoundecanoate) (PHNUBr) and poly(3-hydroxynonanoate-co-3-hydroxy-10-undecynoate) (PHNUD), using Pseudomonas oleovorans as cell factories were 20h of fermentation time and a total carbon substrate concentration of 40mM. Percent incorporation of brominated repeat units and alkyne repeat units were 38.5% and 50% respectively, as determined by ¹H NMR. PHNUBr was converted into an azido-terminated precursor for the polymer analogous "click" reactions via S_N2 nucleophilic substitution reaction using sodium azide with a yield of 96.7% and analyzed by FTIR and ¹H NMR. CuAAC reactions were used to attach propargyl benzoate and methyl-2-azidoacetate to the PHAs with terminal azido and alkyne functional side groups respectively, with yields of 78.2% and 65.4% respectively. FTIR analysis of the products showed the disappearance of the azide peak at 2093.5cm^-1 and the alkynyl CCH stretch at 3292cm^-1. ¹H NMR analysis confirmed the formation of the expected triazole linkage; showing the expected proton chemical shift corresponding to the triazole proton at 7.73 and 7.47ppm respectively. A strain promoted azide-alkyne reaction was used to attach (1R,8S,9s)-Bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) to the azido-terminated PHA with an average yield of 94.5%. Products were analyzed by FTIR and ¹H NMR.