Divergent Pathways to Transverse Stiffness Modulation: Conformational Switching in α-Chitin vs Water-Assisted Hardening in β-Chitin

Chitin is the primary structural polysaccharide in arthropod exoskeletons and many other biological systems, yet the molecular origins of its anisotropic mechanical properties remain incompletely understood. Here, DFT-D3 calculations were used to estimate the elastic tensor of β-chitin dihydrate. Compared with anhydrous β-chitin, structural water in β-chitin dihydrate increases E22 from 10.0 to 31.7 GPa by reconstructing an extensive inter-sheet hydrogen bond network, while E33 decreases from 118.2 to 94.8 GPa, a reduction attributable to the ∼15% transverse chain dilution caused by inter-sheet water intercalation. Although α-chitin cannot form hydrate crystals, its inter-sheet hydrogen-bond network can vary through the variation of hydroxymethyl conformational between the gg and gt states, with the gt conformation enabling additional inter-sheet hydrogen bonding interaction, and thus increase the transverse stiffness (E22 from 22.5 to 38.1 GPa). Therefore, the two naturally occurring chitin polymorphs exhibit distinct molecular mechanisms for transverse stiffness modulation, in which water-mediated hydrogen bond network reconstruction in β-chitin and hydroxymethyl conformational switching in α-chitin show that different paths lead to a similar destination.