Fluorescence monitoring of the hydrophobic surface of dextrin using p-toluidinonaphthalenesulfonate

Abstract

The effect of the linear oligosaccharides dextrin (α-1a,4e-D-glucopyranose) and dextran (α-1,6-D-glucopyranose) on the emission properties of 2-(p-toluidino)-6-naphthalenesulfonate (TNS) has been studied in aqueous solution. TNS exhibits a 30-fold fluorescence enhancement on binding to dextrin, while with dextran there is only a marginal increase in the emission intensity. The polarity values of the binding sites are estimated by comparing the nonradiative rates of TNS with those in aqueous dioxane-water mixtures. The results indicate that the binding surface of dextrin is markedly less polar than that of dextran. This difference is attributed to the differing stereochemical constraints imposed on the two chains. The binding constants of TNS with the two polysaccharides in pure water have been determined in the presence and absence of several salting-in denaturants and salting-out reagents. Salting-in agents cause a decrease in the number of TNS molecules bound to the dextrin surface and an increase in the local polarity, while the salting-out agent LiCl increases the number of TNS molecules bound to dextrin. The interaction of TNS with dextran, however, is found to be very weak even in the presence of LiCl. Energy minimization studies reveal a hydrophobic surface for dextrin while no such amphiphilicity is observed in dextran.