Thermoplasticization of euglenoid β-1,3-glucans by mixed esterification
Introduction
The use of microalgae to produce useful chemicals has recently attracted attention because they can transform carbon dioxide and water into carbohydrates by photosynthesis (Chisti, 2007). Attention has particularly focused on production of biofuels as part of the effort to reduce dependence on carbon-based fuels (Banerjee et al., 2002, Kaya et al., 2011, Mata et al., 2010, Tucci et al., 2010, Yamane et al., 2013). By contrast, less attention has been directed at the use of microalgae to create other useful difficult-to-synthesize carbohydrates. Our efforts in this area have focused on Euglena-derived storage polysaccharide (β-1,3-glucan), which is generally referred to as paramylon (Shibakami et al., 2012, Shibakami et al., 2013).
Since paramylon has unique helical structures due to its β-1,3-bonds, it is likely that materials made from this polysaccharide will exhibit intriguing thermal and mechanical properties that differ from those of materials made from other polysaccharides. One physical property that is useful for preparing materials of various shapes is thermoplasticity. Paramylon, however, does not inherently have this property. It has been reported that mixed cellulose esters having at least two types of substituents exhibit thermoplasticity (Peydecastaing et al., 2011, Teramoto et al., 2002). Wax esters, i.e., esters of long-chain fatty acid and higher alcohol, are another major euglenoid product in addition to paramylon. Of particular interest is that hydrolysis of wax esters releases the fatty acid and higher alcohol. We hypothesized that if wax ester-derived fatty acids are introduced into paramylon, we will likely obtain thermoplastic mixed paramylon esters with a high euglenoid constituent ratio. Toward this goal, we have started a program aimed at synthesizing mixed paramylon esters containing long-chain fatty acids.
Here we report the creation of thermoplastic paramylon derivatives. To the best of our knowledge, this is the first creation of Euglena-based thermoplastics despite the long study of Euglena since the establishment of genus Euglena by Ehrenberg in 1830 (Gojdics, 1953). The primary objective of the work described here was twofold: (i) to establish a mix-esterification method that provides thermoplastic paramylon derivatives and (ii) to investigate the relationship between the chemical structures of the derivatives and their thermal and mechanical properties.
Section snippets
General methods
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE 500 spectrometer. Quantitative 13C NMR spectra were obtained by means of the inverse gated decoupling method. Fourier transform infrared (FT-IR) spectra were recorded using a JASCO FT/IR-480ST spectrophotometer equipped with an attenuated total reflectance accessory (ATR Pro 400-S, ZnSe prism, JASCO) with a resolution of 4 cm−1. Melting behavior was observed using a Yanako MP-500D melting-point apparatus.
Synthesis of mixed paramylon esters
There have been three main strategies for preparing thermoplastic polysaccharides: addition of external plasticizers, formation of polymer blends, and chemical modification or grafting of saccharide backbones (Warth, Mulhaupt, & Schatzle, 1997). In the work reported here, we examined the feasibility of a modification method that utilizes long-chain alkyl groups as an “internal plasticizer” for paramylon. This modification is advantageous in terms of obtaining a high euglenoid constituent ratio
Conclusion
We have demonstrated the synthesis and clarified the properties of mixed paramylon esters that exhibit thermoplasticity. The results fundamentally validated the idea of introducing long alkyl chains and acetyl groups into the glucose to weaken the polymer chain interaction, resulting in sufficient thermoplasticity. An additional benefit of using paramylon as a starting material for functional materials is carbon neutrality. Other benefits include the ease of extracting paramylon from Euglena
Acknowledgments
This research was partly supported by the Advanced Low Carbon Technology Research and Development Program of the Japan Science and Technology Agency. We are grateful to Dr. Masatoshi Iji and Ms. Ai Meguro (NEC) for their helpful discussions and technical assistance with the synthesis and measurements. We are also grateful to Ms. Tomomi Miyata (Shoko Scientific, Co., Ltd.) for technical assistance with the SEC-MALLS measurements.
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