Mechanism and Reaction Energy Landscape for Apiose Cross-Linking by Boric Acid in Rhamnogalacturonan II

Rhamnogalacturonan
II (RG-II)the most complex polysaccharide
known in natureexists as a borate cross-linked dimer in the
plant primary cell wall. Boric acid facilitates the formation of this
cross-link on the apiosyl residues of RG-II’s side chain A.
Here, we detail the reaction mechanism for the cross-linking process
with ab initio calculations coupled with transition state theory.
We determine the formation of the first ester linkage to be the rate-limiting
step of the mechanism. Our findings demonstrate that the regio- and
stereospecific nature of subsequent steps in the reaction itinerary
presents four distinct energetically plausible reaction pathways.
This has significant implications for the overall structure of the
cross-linked RG-II dimer assembly. Our transition state and reaction
path analyses reveal key geometric insights that corroborate previous
experimental hypotheses on borate ester formation reactions.