11B NMR studies on complexation of borate with linear and crosslinked polysaccharides

Abstract

¹¹ B NMR spectroscopy has been employed to elucidate the nature of the complexation of borate with crosslinked Dextran–containing glucopyranoside residues, and with linear Dextran and the monomer derivative, α-methyl-D-glucopyranoside. Well resolved ¹¹B NMR spectra for Sephadex gel as well as for a solution of Dextran or α-methyl-D-glucopyranoside were observed. Four borate complexes with α,β-diols and α,γ-diols of glucopyranoside residues were present. The characteristic ¹¹B chemical shift values were at δ–13 to –14 for the 1:1 complex with α,β-diol and at δca.–10 for the 1:2 complex with α,β-diols, and at δca.–18 for the 1:1 and 1:2 complexes with α,γ-diols, irrespective of the linear or crosslinked structure of the polysaccharide. The 1:1 complexes with the α,β-diols of Dextran and Sephadex gels have higher stability compared with those with α,γ-diols, whereas the complex with α,γ-diols is predominant for the α-methyl-D-glucopyranoside system. This indicates that the glucopyranoside residues of Dextran and Sephadex gels prefer the C1 conformation and the interconversion between the C1 and 1C conformers is restricted owing to the hydrodynamic and steric hindrance of the linear or crosslinked polymer in aqueous media. The formation of 1:1 complexes with α,γ-diol of the Dextran and Sephadex gels induces the conformational changes of the glucopyranoside residues from the C1 to 1C conformation in a part of the Dextran strands to create a domain of high local concentration of the α,γ-diol moiety, so that the α,γ-diols are located at a suitable position for 1:2 complexation.