Neutron Total Scattering Investigation of the Dissolution Mechanism of Chitotriose in Alkali/Urea Aqueous Solutions

Despite the promise of alkali/urea aqueous solutions as green solvents, the atomistic-scale mechanism of chitin dissolution remains elusive. In contrast to the ability to dissolve cellulose, the order of alkali effectiveness for dissolving chitin is potassium hydroxide (KOH) as the best and lithium hydroxide (LiOH) as the worst. Employing chitotriose as a model compound and utilizing neutron total scattering coupled with Empirical Potential Structure Refinement (EPSR) as the principal analytical tools, we compared its KOH/urea and LiOH/urea aqueous solutions and elucidated the intrinsic mechanism of chitotriose dissolution via its all-atom structure in solution. The results show that, compared with Li+, K+ preferentially enriches around the oxygen atoms of the amide group and hydroxyl group in the periphery of chitotriose molecules, triggering the disruption of its intermolecular hydrogen-bond network in its crystal state. Then, urea molecules play an indirect role via cation bridging. This "cation-dominated, urea-assisted" mechanism elucidates the synergistic dissolution principle of the alkali/urea system. Taking into account that the dissolution of biomacromolecules requires prior swelling, we proposed that the dissolution of chitin in alkali-urea aqueous solution first requires the disruption of intermolecular hydrogen bonds in the chitin crystals during the swelling process, followed by dissolution through van der Waals interactions.