Mechanism of the Temperature-Dependent Self-Assembly and Polymorphism of Chitin

Chitin is the second most abundant natural biopolymer. Its crystalline structures have been extensively studied. However, the mechanism of chitin’s self-assembly is unknown. Here, we applied all-atom molecular dynamics to study chitin’s self-assembly process at different temperatures. Strikingly, at 278 K, an amorphous aggregate was formed, whereas at 300 K single-sheet and at 323 K both single-sheet and multisheet nanofibril regions were formed. The nanofibrils contain antiparallel, parallel, or mixed orientation chains, with antiparallel being slightly preferred, recapitulating chitin’s polymorphism observed in nature. The inverse temperature dependence is consistent with a recent experiment conducted in the aqueous KOH/urea solution. The analysis suggested that the multisheet nanofibrils are assembled by stacking the single nanofibril sheets, which are formed through two types of pathways in which hydrophobic collapse either precedes or is concomitant with the increasing number of interchain hydrogen bonds and solvent expulsion. Furthermore, the antiparallel and parallel chains are mediated by different interchain hydrogen bonds. The analysis also suggested that the inverse temperature dependence may be attributed to the hydrophobic effect reminiscent of the low critical solution temperature phase behavior. The present study provides a rich, atomic-level view of chitin’s polymorphic self-assembly process, paving the way for the rational design of chitin-derived novel materials.