| Abstract: | Metachromatic color change induced in a suitable dye by the polyanionic chromotrope involves (i) the formation of electrostatic bonds between the dye cations and polyanions and (ii) effective aggregation of the dye cations due to their occupying the adjacent sites on the polyanion. Parallel studies are made on the effects of reagents like ethanol, dioxane and urea on the destruction of metachromasia and the binding of the dye cations on the polyanion. Conductometric titrations of chondroitin sulfate with methylene blue indicate that the dye binding on the chromotropes is less susceptible to these reagents than the metachromasia of the dye as studied spectrophotometrically. This indicates that it is not the electrostatic bond that is responsible for the metachromatic spectral shift of the dye but rather the aggregation of dye ions; and these reagents preferably attack the bonds involved in the dye-dye interaction rather than the dye-polyanion electrostatic bond. Comparison of the equivalent weights of heparin as estimated from conductometric titrations with metachromatic dyes and estimated analytically indicates that all ionic sites in heparin, unlike chondroitin sulfate, are not occupied by dye ions; this may be due to the high charge density of heparin. |
