Influence of cosolutes upon the conformation of carbohydrates in aqueous solutions. Part III. Effect of inorganic ions upon the binding of iodine by starch and derivatives of amylose

Abstract

For solutions of starch, diethylaminoethylamylose, amylose sulphate, and carboxymethylamylose in aqueous inorganic acids or salts at 20°, plots of the optical density at 680 nm against the concentration of added potassium tri-iodide (KI₃) showed maxima whose values were highly dependent upon the concentration and identity of the ions present. The absorption spectra in the different media were also different, and similar changes in the absorption spectra could not be brought about by varying the concentration of I₃^– in the system.

With polyvinyl alcohol, there was no tendency to develop different maxima in different media, and the absorption spectrum was independent of the medium. It thus appeared that the acids and salts were bringing about a conformational change in the glucans, leading to an increase or decrease in the number of sites that were able to bind I₃^– strongly.

By working with a very large excess of I₃^–, it was possible to avoid complications arising from the competitive binding of species other than I₃^–, or salt effects upon the activity coefficient of I₃^–, and to study the effect of different media upon the conformational change alone. These media included aqueous sulphuric, phosphoric, hydrochloric, and hydrobromic acids, alkali-metal halides, and various other salts of lithium, sodium, and potassium.

The effect of the acids and salts upon the maximal optical density at 680 nm could not be correlated with their effect upon the volume of gels prepared by cross-linking starch with epichlorohydrin, or with their salting-out parameters for non-polar compounds. This was interpreted as evidence that the conformational changes were not primarily associated with changes in the stability of intramolecular hydrogen bonding, or of hydrophobic interactions.

On the other hand, there was a close correlation between the maximal optical density in the four acids named above, and their previously demonstrated capacity to change the magnitude of the anomeric effect in the methyl glucopyranosides and in cellobiose and maltose. With an apparent exception that was noted for fluorides, the effect of all the other salts upon the optical density correlated well with their expected capacity to modify the magnitude of the anomeric effect, based upon the idea that the ring and glycoside-oxygen atoms are selectively desolvated by salts whose anion is the stronger electrostrictor, and solvated by salts whose cation is the stronger electrostrictor.

It was therefore concluded that the conformational changes were mainly due to changes in the magnitude of the exo-anomeric effect, which would be expected to modify the relative stabilities of the rotamers generated about the C(1)–O(1) bonds, and, hence, the ease with which the linear segments of the glucan chains are able to take up the helical, V-conformation in solution. It is pointed out that similar changes can be expected in any flexible molecule, whose net dipole moment in different conformational states can be expected to vary.