Gelatin/carrageenan interactions in coil and ordered conformations followed by a methylene blue spectrophotometric method

A negatively charged polysaccharide, iota-carrageenan (CI) has been mixed with a gelatin (G) sample having a pI of about 4.5. At pH 7, the net charge of the gelatin is negative. However, associative interactions may be formed locally when both biopolymers are in random coil conformation. In order to understand the evolution of these interactions during gelation, a methylene blue (MB) spectrophotometric method has been used. Methylene blue molecules interact with carrageenan sulphated groups giving rise to a modification of the optical density spectrum. As the CI/MB spectrum changes at decreasing temperatures, the effect of the carrageenan concentration on the CI/MB spectra has been studied at 60, 50, 40, 30 and 20 °C. The corresponding carrageenan concentration of saturation has been found. Its value does not change with temperature showing that the stoichiometry of binding is unchanged. Therefore, the change in spectrum with temperature has been attributed to an increase in carrageenan chains rigidity. Different concentrations of gelatin (in the range 0.1–0.75 wt%) have been added to the CI/MB solution. The optical density spectrum of CI/MB/G solution versus decreasing temperature is measured and compared with the one obtained for the CI/MB solution. When carrageenan chains are in ordered conformation, the modifications of the spectra cannot be simply interpreted by a competition between gelatin and MB for the carrageenan sulphated groups. From these results, the hypothesis of an increase in carrageenan chains rigidity, induced by local associative interactions between carrageenan chains and ordered gelatin chains is discussed.