Guanidinylated Nanochitins: Guanidinylated Chitin Nanocrystal Is Dispersible at Neutral pH

Although nanochitins demonstrate favorable biological effects, their colloidal stability, derived from the amino group's positive charge, is limited to acidic pH, which differs from physiological environments. Here we present guanidinylated chitin nanocrystals (GCNCs) that are dispersible at neutral pH. GCNCs were synthesized by guanidinylating partially deacetylated chitin nanocrystals (CNCs) using 1-amidinopyrazole hydrochloride. The degrees of guanidinylation and acetylation of the GCNCs were 4.6% and 75.7%, respectively. A 1.0% GCNC dispersion was prepared in 0.5% acetic acid and subjected to sonication. Although slight white turbidity from light scattering was observed, no visible macroscopic precipitates were detected. Dynamic light scattering analysis indicated that the average GCNC diameter was 327.2 nm. Upon neutralization with 0.1 M NaOH, the GCNC dispersion reduced transmittance due to aggregation. Nonetheless, the dispersion remained more stable than that of CNCs, suggesting that the guanidino group’s strong basicity and positive charge helped mitigate aggregation. Interestingly, GCNCs also dispersed uniformly in 0.1 M HEPES buffer (pH 7.4) up to 0.5% after sonication, although the average diameter increased to 1115.1 nm, 3.4 times greater than at pH 3.0. In contrast, guanidinylated chitin nanofibers did not exhibit the same dispersibility despite the presence of guanidino groups. This result indicates that guanidinylation enhances the dispersion of low-aspect-ratio nanochitins such as CNCs. Moreover, we demonstrate that the dispersibility of GCNCs at neutral pH can be utilized in material development, where a gelatin‒GCNC composite hydrogel displaying enhanced mechanical properties is successfully prepared by adding 10% (w/w) of GCNC.