A DFT-based investigation of chitin-to-chitosan transition: effects of N-acetylation on structure and reactivity

The degree and pattern of deacetylation in chitin-derived polymers critically determine their physicochemical properties and functional potential. In this study, a comprehensive theoretical analysis is performed of decameric chitin-like chains with systematically varied degrees of deacetylation (DDA), using density functional theory (DFT), electrostatic surface mapping, noncovalent interaction (NCI) analysis, and global reactivity descriptors. Structural optimizations revealed that partial deacetylation induces significant torsional rearrangements and enhanced intra-chain hydrogen bonding, leading to increased conformational flexibility. Molecular electrostatic potential (MEP) surfaces demonstrated a transition from neutral, acetyl-dominated topologies to highly polarized and reactive amine-rich domains. NCI analysis confirmed the emergence of cooperative hydrogen bonding and van der Waals networks in mid-range DDA structures. Furthermore, HOMO-LUMO analysis and TAFF-derived descriptors identified 20% ([[GlcNac]₄- GlcN]₂)-60% ([[GlcN]₃-[GlcNac]₂]₂) DDA chains as electronically soft, highly polarizable, and capable of dual electron donation and acceptance. These findings suggest that partially deacetylated chitosan chains exhibit a unique combination of flexibility, reactivity, and internal cohesion, providing a molecular rationale for their superior performance in biomedical and functional materials applications.