Cooperative interaction of sodium and chlorine ions with β-cellobiose in aqueous solution from quantum mechanics and molecular dynamics

Abstract

Cellobiose is usually preferred as model compound of cellulose to explore its conversion mechanism, in which the first challenge is to acquire the molecular solvation mechanism. The interactions of β-cellobiose with the hydrated Na+ cation and Cl− anion in aqueous solution have been theoretically investigated, using quantum chemical calculations at LC-ωPBE/6-311++G(d,p), aug-cc-pVTZ level under a polarized continuum model (PCM-SMD), together with molecular dynamics stimulation. In aqueous solution, the most stable form of β-cellobiose is syn-ϕ/syn-φ (CB2), including the exocylcic hydroxyl groups and three intramolecular H-bonds. The favorite solvation structure of NaCl is concerned with [Na(H2O)3]+···Cl− ion pair. The Cl− anion interacts with β-cellobiose, favorably affording $$[\eta_{{\text{H}}6,{\text{H}}2^{\prime}}^2{-}{\text{CB}}2){\text{Cl}}{]^- }$$ and $$[\eta_{{\text{H}}3^{\prime},{\text{H}}4^{\prime}}^2{-}{\text{CB}}2){\text{Cl}}{]^- }$$ with a bridge of OH···Cl···HO interaction. Alternatively, the [Na(H2O)3]+ cation interacts with β-cellobiose, preferably yielding $$[\eta_{{\text{O}}3,{\text{O}}5^{\prime},{\text{O}}6^{\prime}}^3{-}{\text{CB}}2){\text{Na}}{]^+ }$$ . Additionally, the [Na(H2O)3]+···Cl− ion-pair interacts cooperatively with β-cellobiose, preferentially producing $$[\eta_{{\text{O}}3,{\text{O}}5^{\prime},{\text{O}}6^{\prime},{\text{H}}6,{\text{H}}2^{\prime}}^3{-}{\text{CB}}2){\text{Na}}{\left( {{{\text{H}}_2}{\text{O}}} \right)_1}]{\text{Cl}}$$ . For CB2, the reactive sites for the nucleophilic and electrophilic reactions locate at the O5 atom on the reducing end and the H6′ atom on the non-reducing end, respectively. Alternatively, for $$[\eta_{{\text{O}}3,{\text{O}}5^{\prime},{\text{O}}6^{\prime},{\text{H}}6,{\text{H}}2^{\prime}}^3{-}{\text{CB}}2){\text{Na}}{\left( {{{\text{H}}_2}{\text{O}}} \right)_1}]{\text{Cl}}$$ , the O5 and H6′ atoms should no longer be characteristic of nucleophilicity and electrophilicity. Moreover, [Na(H2O)3]+···Cl− ion-pair can break some intramolecular H-bonds of CB2, which may promote the solvation of β-cellobiose in aqueous solution. It also shields the glycosidic bond by steric effect, which make CB2 not readily hydrated at the glycosidic bond. These functions would alter the reaction pathway in the further degradation of β-cellobiose. This study may advance on the novel design for sustainable conversion system of cellulose into high-added value chemicals.