(649h) Molecular Insight into Boric Acid Mediated Cross-Linking of the Complex Polyscaccharide - Rhamnogalacturonan-II

Pectins are hetero-polysaccharide bio-polymer groups that are an integral part of the plant primary cell wall. Rhamnogalacturonan II (RG-II), a significant component of pectin, is the most complex polysaccharide unit known in nature. The structure of RG-II consists of 13 unique sugar moieties assembled via 21 glycosidic linkages and is conserved across all vascular plants. In the plant cell wall RG-II exists as a dimer and boric acid - an essential micronutrient for plant growth - is considered to be involved in the dimerization of RG-II. Boric acid is known to readily form ester linkages with hydroxyl groups in sugars. The RG-II dimer is observed to be borate cross-linked at the apiose glycosyl unit. Here, we propose a reaction mechanism consisting of four sequential esterification reactions leading to the formation of the final crosslinked apio-borate product. In this study, we use ab initio quantum mechanical calculations coupled with transition state theory to evaluate the thermodynamics and kinetics of the cross-linking process. We identify and characterize four unique reaction pathways for the cross-linking process and outline the rate-limiting steps, energy barriers and thermodynamics for each pathway. The reactant and product complexes reveal important interactions that stabilize the transition state for each step of the mechanism. Our simulations reveal that the final cross-linked product is chiral at the boron center and formation of both stereo-isomers is energetically feasible which has significant implications for the overall structure of the cross-linked RG-II dimer assembly.