On the structure, conformation and reactivity of β-1,4-linked plant cell wall glycans: why are xylan polysaccharides or furanosyl substituents easier to hydrolyze than cellulose?

Abstract

Plants have been essential to human technological development since the beginning of time. Today, due to their structural diversity and adaptability, they continue to hold a great potential for addressing modern energy and material challenges. Plant glycans, as central components of the plant cell wall, play a crucial role in defining many of the wall’s unique mechanical and chemical characteristics. A deep understanding of the structure and chemical properties of these biopolymers can help optimize the use of plant resources. Here, we discuss fundamental aspects of the primary structure, conformation, and reactivity of plant glycans, focusing on the ubiquitous β-1,4-linked plant glycans (cellulose, xylans, glucomannans, xyloglucans) and the glycosyl residues that constitute their backbones: glucosyl, xylosyl, and mannosyl residues. In the discussion, the higher rate of acidic hydrolysis in aqueous solution observed for xylans in comparison to cellulose is attributed to the lower electron deficiency and greater conformational freedom of xylosyl rings, with both factors resulting from the absence of the hydroxymethyl (CH₂OH) group in these rings. In furanosides, the higher rate of acidic hydrolysis when compared to their pyranosyl counterparts is explained by the greater similarity between the conformations of furanosides in the ground state and those in the oxocarbenium ion-like transition state upon glycosidic bond cleavage. These phenomena, alongside other factors such as steric interactions, offer an effective explanation for the rates of acidic hydrolysis in solution observed for plant glycans.