Observations on N.M.R. spectra of starches in dimethyl sulfoxide, iodine-complexing, and solvation in water-di-methyl sulfoxide

Abstract

N.m.r. spectra ( 1H- and 13C-) in dimethyl sulfoxide- d 6, together with complementary 13C- T 1 and -n.O.e. data, are described for a variety of starches, ranging in composition from high-amylopectin to high-amylose types. The spectra contain groups of minor 13C, 1H, or O 1H signals attributable to glycosyl end-groups and other residues associated with branching in amylopectin molecules. The relative intensities of these signals, and of the resonances due to the corresponding nuclei of main-chain residues, are evaluated in terms of the incidence of end groups, or branches, in the starches. They are also compared with spectrophotometric measurements on the iodine-complexing capacity of the starches, using a modified procedure. Complex-formation between amylose or amylopectin and iodine in water is suppressed by the introduction of dimethyl sulfoxide, due probably to competitive solvation of the starch molecules. This appears to be related to other observations, showing that the composition of water-dimethyl sulfoxide mixtures determines the rate of hydrolysis by beta amylase, optical activity, and 13C-chemical shifts. All of these observations concur in suggesting that, in admixture, more than 4–5 moles of water per mole of dimethyl sulfoxide are necessary for amylose or amylopectin to be effectively solvated by the water molecules present.