Twisting Fibers Formation of Cyanobacterial Supra-Polysaccharide through Self-Assembly
Authors: Budpud Kulisara , Okeyoshi Kosuke , Okajima Maiko K , Kaneko Tatsuo 
Published: February 2020

Biopolymers such as polysaccharides, cytoskeletal proteins, and DNAs show helical structures in molecular scale and self-orientation in larger scale. Sacran is one of fascinating bacterial polysaccharides which could naturally form incredibly giant fiber in micrometer scale. It is extracted from cyanobacteria, Aphanothece sacrum, which has attached our interest due to the extremely high molecular weight (> 1.6 × 107 g/mol)1 and the self-orienting giant micro-fibers (~1 μm of outer diameter and > 20 μm length)2 during the drying process which cannot encounter in any other soluble polysaccharides. From our previous researches, we found that in drying process of the sacran solution, the rigid polysaccharides exhibit self-orientation and they gather to build a rod-like microdomain in micrometer scale. Moreover, in some conditions, it shows clear twisting structure in micrometer scale. Here, we study the fiber formation using polarized optical microscope and scanning electron microscope to clarify the effect of polymer’s initial concentration, evaporation speed and ionic effect on the fiber formation. This unique formation is significant not only to understand basic fiber natural formation but also to develop for material field applications.

The formation mechanism was investigated in micrometer scale from drying process using polarized optical microscope (POM) and scanning electron microscope (SEM). The samples were prepared by drying the 1 μL droplet on a glass substrate at room temperature under control of the initial polymer concentration. From the drying records of a solution (5 × 10-1 wt%), we could confirm three main regions with different morphologies. Around the record of the droplet edge, fibers which oriented apparel to the contact line were clearly observed. In the middle of the record, the twisting fibers in random direction were shown. In the center, dendrite crystal structures could be observed. These results suggest that the sacran forms variety of geometric structures under drying environment and mild condition.

Focusing on the fibers at the edge and middle area of sacran droplet, the deposition from 5 × 10-3 wt% solutions showed not only particles but also short fibrous with ~1 μm diameter as main structures. In contrast, deposition from a solution at higher concentration of 5 × 10-2 wt%, giant fibers in micrometer scale were observed. It is clearly seen that self-assembly of loosely-twisting fibers led to form larger structures through snaking or bundling. Considering the different diameter of the micro-rods and the twisting fibers as shown in Fig.1, the loosely-twisting fibers could be observed and they showed a semi-rigid fiber characteristic which causes low orientation around the droplet edge. As for the deposition from 5 × 10-1 wt% solutions, rigid fibers were observed with high orientation around the droplet edge. This might be owing to tightly-twisting fibers at high concentration.

The effect of evaporation speed is one of the main factors which effects to the polymer morphology. At low evaporation speed, straight sacran fibers were observed. However, when evaporation speed was increased, the straight fiber transform to snaking or twisting form.

The effect on assembly and disassembly of the fibers were also clarified by mixing various kinds of ionic (HCl, NaCl and CaCl2) with various concentrations (1, 5, 10, 60 mM). Sacran fibers were disassembled in all concentration of HCl and NaCl due to carboxylic groups were interacted by monovalent cations. In case of CaCl2, calcium divalent cations could bind with the carboxylate groups as crosslinker. Therefore, it remained fibers assembly of sacran. This unique formation is not only important to understand basic helical natural fiber formation which still being an ambiguous issue but also to develop new environmental friendly material in several fields such as electro optical film, bio-mimic templating and components of medicines in near future.

1) Okajima, M. K., et al., Macromolecules
2009, 42, 3057.

2) Okeyoshi, K., et al., Biomacromolecules
2016, 17, 2096.

Figure 1