11B NMR investigation of the complexation behavior of borate with polysaccharides in aqueous solution

Abstract

The complexation of borate with linear dextrans consisting of pyranoside residues and their monomer derivatives (α-methyl- d-glucopyranoside and α-methyl- d-mannopyranoside) was investigated in detail by using 11B NMR spectroscopy. For the monomer systems five kinds of borate complexes with α,β-diols and α,γ-diols of the pyranosides were present: that is, (α,β)-monochelate, (α,γ)-monochelate, (α,β)(α,β)-bischelate, (α,γ)(α,γ)-bischelate and (α,β)(α,γ)-bischelate complexes. Borate reacts with dextran to form inter-chain (α,β)(α,β)- and (α,β)(α,γ)-bischelate complexes in the 1:2-complexation in addition to the (α,β)- and (α,γ)-monochelate complexes. The formation of intra-chain bischelate complexes can be negligible for a rather stiff polymer as dextran. Formation constants of the (α,γ)-monochelate complex are almost constant among dextrans with different chain lengths except Dextran 3000 which is the shortest polymer examined. On the other hand, the stability of (α,β)-monochelate complexes decreases with an increase in the chain length. The complexation behavior for polysaccharides with various chain lengths was discussed in correlation with hydrodynamic and steric hindrance of the polymer ligands.