The essential reaction involved in boron bridging of RG-II.
Part of the neutral apiose residue of side-chain A (in each of two monomeric RG-II molecules) is shown. The charges of its four near-neighbouring anionic sugar residues are rendered as red circles (two α-GalA residues in the RG-II backbone plus an α-GalA and a β-GalA residue attached to the rhamnose adjacent to the apiose). Monomeric RG-II has a total of ∼11 additional anionic sugar residues, giving it a net charge of ∼−14 at the pH (4.8) used in our experiments. It is evident that the close approach of two RG-II monomers for boron bridging requires the overcoming of considerable electrostatic repulsion, and dimerisation introduces an additional negative charge on the previously neutral boron atom.