Enzymatic synthesis of cellulose in space: gravity is a crucial factor for building cellulose II gel structure

We previously reported in vitro synthesis of highly ordered crystalline cellulose II by reverse reaction of cellodextrin phosphorylase from the cellulolytic bacterium Clostridium ( Hungateiclostridium ) thermocellum ( Ct CDP), but the formation mechanism of the cellulose crystals and highly ordered structure has long been unclear. Considering the specific density of cellulose versus water, the formation of crystalline and highly ordered structure in an aqueous solution should be affected by gravity. Thus, we synthesized cellulose with Ct CDP stable variant at the International Space Station, where sedimentation and convection due to gravity are negligible. Optical microscopic observation suggested that cellulose in space has a gel-like appearance without apparent aggregation, in contrast to cellulose synthesized on the ground. Small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) indicated that cellulose synthesized in space has a more uniform particle distribution in the ~ 100 nm scale region than cellulose synthesized on the ground. Scanning electron microscopy (SEM) showed that both celluloses have a micrometer scale network structure, whereas a fine fiber network was constructed only under microgravity. These results indicate that gravity plays a role in cellulose II crystal sedimentation and the building of network structure, and synthesis in space could play a role in designing unique materials. Graphical abstract