Structural and conformational effects on the complexation of calcium by 2,3-dicarboxy derivatives of β-cyclodextrin (cyclomaltoheptaose), amylose, and cellulose

Abstract

The conformational effects upon complexation of calcium (II) and lanthanide (III) ions by 2,3-dicarboxy-β-cyclodextrin (DC-βCD) have been studied by 1H-, 13C-, and 17O-n.m.r. spectroscopy. The complexation of Ca 2+ by DC-βCD involves conformational transitions at metal/ligand ratios (ϱ) of 0.25 and 0.5 with predominant changes in the resonances of C-1,4,5. For the La 3-DC-βCD complex (up to ϱ 0.3), a structure involving two La 3+ per DC-βCD macrocycle, each coordinated to 8 oxygen atoms, is proposed. Optical rotation data indicated similar conformational behaviour on complexation of Ca 2+ with DC-βCD and dicarboxyamylose (DCA), whereas dicarboxycellulose (DCC) showed a substantially larger change in conformation. Static and dynamic light scattering indicated aggregation of DCA and DCC on complexation with Ca 2+, with a concomitant reduction of the hydrodynamic radius ( R h).