Amylose Conformational Transitions in Binary DMSO/Water Mixtures

Abstract

The effects of percentage water content in DMSO/water mixtures on amylose conformation were measured by following changes in specific optical rotation, limiting viscosity number, and 13C‐NMR chemical shifts. The temperature dependence of specific optical rotation showed differences in amylose conformation for four ratios of dimethyl suphoxide/water. The changes in limiting viscosity of amylose in different proportions of DMSO/water with increasing temperature, and the effect of tetramethylurea on the specific rotation of amylose, indicate that the number of intramolecular hydrogen bonds decreases with increased water content. An amylose conformational change was also deduced from 13C‐NMR chemical shift changes. Thirty‐three percent water appears to be a crossover concentration. above which the helical conformation is progressively lost with increased temperature. With further increase in water conent to 66.6%, transition to a random coil conformation is complete, and iodine complexation may take place. The transition of amylose conformation from helix to loose helix to random coil with increasing water content was deduced from the experimental results.

Impact of structural changes due to heat-moisture treatment at different temperatures on the susceptibility of normal and waxy potato starches towards hydrolysis by porcine pancreatic alpha amylase

“…A helix to coil transformation would decrease the number of helical turns required to form the typical deep blue color with iodine. It is highly unlikely that a coil → helix transition (Cheetham & Tao, 1998) could occur on the addition of iodine vapor to NP130 starch, since an aqueous medium would be required for such a transformation.…”

Viscometric Properties of Solutions of Amylose and Amylopectin in Aqueous Potassium Thiocyanate

“…Cheetham and Tao [5] have studied the effects of different concentrations of DMSO in water on the formation of the amylose-iodine complex. They suggested that the conformation of amylose in pure DMSO, in which solvent no complex is formed, is tightly helical and that addition of water makes some regions of the molecule become loosely helical, allowing entry of iodine (as I 5 -) to form the complex.…”

“…Its solutions are thus more stable than those of amylose, which tends to form hydrogen bonds with other amylose molecules and hence form aggregates which precipitate or cause instability. Amylose, by comparison with amylopectin, has a simpler structure, although various types of helical conformation have been reported [4,5]. DMSO is a useful solvent for studies of amylose, and produces clear solutions, especially when the mixtures are warmed to about 60 °C.…”

“…These solutions are quite stable [6], unlike those in water. Consequently, mixtures of DMSO and water have been used to study the molecular properties of amylose [5].These studies showed that DMSO affected the limiting viscosity of amylose solutions and the ability of the amylose to form the iodine inclusion complex was decreased, which was attributed to a tendency to form random coils.…”

Assessing the susceptibility of amylose–lysophosphatidylcholine complexes to amylase by the use of iodine

“…In order to remove this obstacle, water was subsequently added to the samples to reach a final concentration of 60% v/v. At this concentration, iodine complexation is complete [23].…”