Tryptophan-based in vivo coloring of polyhydroxybutyrate through co-production with indigo and effect on the properties of polymer

With the continuous growth of the demand for eco-friendly alternatives to petroleum-based plastics, polyhydroxybutyrate (PHB) has emerged as a promising alternative because of its biodegradability. However, certain characteristics, such as its color, rigidity, and brittleness, have limited its range of applications. In this study, we investigated the in vivo coloring of PHB with indigo by combining the biosynthesis operons of PHB and indigo, which involved the use of tryptophan as the substrate for the biosynthesis of indigo. The heterologous and homologous tryptophanase genes converted tryptophan to indole, which was further oxidized to indoxyl by flavin monooxygenase (FMO), followed by spontaneous dimerization to indigo in the presence of oxygen. Different indigo-PHB films were obtained by controlling the concentration of tryptophan. Additionally, we investigated the mechanical properties of the indigo-PHB films using UTM analysis and found that their mechanical properties varied according to the amount of indigo incorporated into PHB. Notably, elongation at break in indigo-PHB film, which was produced using 10 mM of tryptophan resulting in a 1.1 mg indigo/g PHB content, significantly improved to 70 % compared to the 7.2 % observed in normal PHB films. Furthermore, indigo-PHB films were subjected to FE-SEM, GPC, and DSC analyses to identify differences, and it was observed that there was a decrease in Tc and Tg values, more obvious in Tg value, in indigo PHB film with 1.1 mg indigo/g PHB content. Considering many approaches to improving PHB properties, our study showed the possibility of different changes in color and improved flexibility via the co-production of indigo and PHB.