In vitro synthesis of linear α-1,3-glucan and chemical modification to ester derivatives exhibiting outstanding thermal properties

Abstract

Bio-based polymer is considered as one of potentially renewable materials to reduce the consumption of petroleum resources. We report herein on the one-pot synthesis and development of unnatural-type bio-based polysaccharide, α-1,3-glucan. The synthesis can be achieved by in vitro enzymatic polymerization with GtfJ enzyme, one type of glucosyltransferase, cloned from Streptococcus salivarius ATCC 25975 utilizing sucrose, a renewable feedstock, as a glucose monomer source, via environmentally friendly one-pot water-based reaction. The structure of α-1,3-glucan is completely linear without branches with weight-average molecular weight (Mw) of 700 kDa. Furthermore, acetate and propionate esters of α-1,3-glucan were synthesized and characterized. Interestingly, α-1,3-glucan acetate showed a comparatively high melting temperature at 339 °C, higher than that of commercially available thermoplastics such as PET (265 °C) and Nylon 6 (220 °C). Thus, the discovery of crystalline α-1,3-glucan esters without branches with high thermal stability and melting temperature opens the gate for further researches in the application of thermoplastic materials.

Figure 1. Synthesis of α-1,3-glucan.

(a) The source of GtfJ enzyme. IPTG is isopropyl β-D-1-thiogalactopyranoside, a protein inducer. (b) Scheme of enzymatic polymerization of α-1,3-glucan. (c) Pictures of enzymatic polymerization of α-1,3-glucan produced at 30 °C. The number indicates the polymerization day. (d) Effect of pH on GtfJ activity at 30 °C. The enzyme activity of GtfJ is defined as the amount of released fructose per minute (μmole/min, U) per one mL of GtfJ at the initial state of reaction.

Figure 2. One- and two- dimensional NMR spectra of α-1,3-glucan.

(a) ¹H-NMR spectrum of α-1,3-glucan. (b) ¹³C-NMR spectrum of α-1,3-glucan. (c) HSQC NMR spectrum of α-1,3-glucan. (d) DQF-COSY NMR spectrum of α-1,3-glucan. All samples were prepared in DMSO-d6.

Figure 3. Effect of reaction temperature on molecular weight and molecular weight distribution of α-1,3-glucan.

M_w is the weight-average molecular weight. PDI is polydispersity index calculated as the weight-average molecular weight (M_w) divided by the number-average molecular weight (M_n).

Figure 4. Thermal characterizations of α-1,3-glucan esters.

(a) TGA curves of α-1,3-glucan (grey line), α-1,3-glucan acetate (dark red line) and α-1,3-glucan propionate (blue line). (b) DSC curves of α-1,3-glucan acetate (dark red line) and α-1,3-glucan propionate (blue line). All DSC curves are from 2^nd run.

Figure 5. Comparison of glass transition and melting temperature between those of α-1,3-glucan acetate and propionate, esters of other polysaccharides and commercially available polymers.

T_g and T_m are glass transition and melting temperature respectively. PE, PP and PET are polyethylene, polypropylene and polyethylene terephthalate respectively.