Preferential solvent interactions and the dissolution of the B-type crystalline polymorph of starch in aqueous solutions

The preferential interactions of starch in aqueous solutions of low-molecular-weight solutes were probed using densimetric techniques which have previously been applied to proteins and nucleic acids. In d-glucitol solutions, the starch was preferentially hydrated, whereas in solutions of urea and guanidinium thiocyanate there was a preferred interaction of the solute with the starch chain. In solutions of d-glucose and glycerol no preferential interactions were observed. The effect of the same solutes on the dissolution temperature, Tm, of the crystalline B-type polymorph of starch was examined by calorimetry. The addition of the hydroxy compounds glycerol, d-glucose and d-glucitol elevated Tm, while the addition of urea and guanidinium thiocyanate caused a depression. For starch–urea–water mixtures, Flory theory predictions of the melting temperature were in good agreement with experiment, but were poor for starch–d-glucitol–water and starch–glucose–water mixtures. The neglect of non-configurational contributions to the entropy of mixing were identified as the principal reason for this lack of agreement.