The structure and dynamics of chitin nanofibrils in an aqueous environment revealed by molecular dynamics simulations

Chitin is one of the most abundant structural biomolecules in nature, where it occurs in the form of nanofibrils that are the smallest building blocks for many biological structural materials, such as the exoskeleton of Arthropoda. Despite this fact, little is known about the structural properties of these nanofibrils. Here, we present a theoretical study of a single chitin molecule and 10 alpha-chitin nanofibrils with different numbers of chains in an aqueous environment that mimics the conditions in natural systems during self-assembly. Our extensive analysis of the molecular dynamics trajectories, including free energy calculations for every model system, reveals not only the structural properties of the nanofibrils, but also provides insight into the principles of nanofibril formation. We identified the fundamental phenomena occurring in the chitin nanofibrils such as their hydrogen bonding pattern and resulting helical shape. With increasing size, the nanofibrils become increasingly stable and their structural properties approach those of crystalline alpha-chitin if they consist of more than 20 chains. Interestingly, this coincides with the typical size of chitin nanofibrils observed in natural systems, suggesting that their evolutionary success was at least partially driven by these specific structure-property relations.