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

B-11 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, alpha-methyl-D-glucopyranoside. Well resolved B-11 NMR spectra for Sephadex gel as well as for a solution of Dextran or alpha-methyl-D-glucopyranoside were observed. Four borate complexes with alpha,beta-diols and alpha,gamma-diols of glucopyranoside residues were present. The characteristic B-11 chemical shift values were at delta - 13 to - 14 for the 1:1 complex with alpha,beta-diol and at delta ca. - 10 for the 1:2 complex with alpha,beta-diols, and at delta ca. - 18 for the 1:1 and 1:2 complexes with alpha,gamma-diols, irrespective of the linear or crosslinked structure of the polysaccharide. The 1:1 complexes with the alpha,beta-diols of Dextran and Sephadex gels have higher stability compared with those with alpha,gamma-diols, whereas the complex with alpha,gamma-diols is predominant for the alpha-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 alpha,gamma-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 alpha,gamma-diol moiety, so that the alpha,gamma-diols are located at a suitable position for 1:2 complexation.