Synthesis of nanofiber-formable carboxymethylated Euglena-derived β-1,3-glucan

•Efficient carboxymethylation of paramylon, Euglena-derived β-1,3-glucan, using heterogeneous reactions was established.•Carboxymethylation using a heterogeneous system requires paramylon to be in an amorphous state.•Carboxymethyl paramylons with a high DScm form nanofibers in the aqueous phase.•Carboxymethyl paramylons with a low DScm form nanofibers in both the solid and aqueous phases.

Carboxymethylation of paramylon, a storage polysaccharide of Euglena, was explored using homogeneous and heterogeneous reaction systems. Homogeneous reactions yielded carboxymethyl paramylons with a low degree of substitution of carboxymethyl group (DScm) of ∼0.13. Heterogeneous reactions of paramylon with chloroacetic acid in a mixture of 2-propanol and a NaOH aqueous solution yielded carboxymethyl paramylons with a high DScm of up to ∼0.80. An essential step for successful carboxymethylation using the heterogeneous reaction systems devised in this study was the pretreatment of a paramylon particle to break its inherent highly crystalline state into an amorphous one. A high degree of substitution provided carboxymethyl paramylons with a nanofiber-forming ability in the aqueous phase while a low degree provided them with a fiber-forming ability in both the aqueous and solid phases.