Gelatin/iota-carrageenan interactions in non-gelling conditions

The negatively charged polysaccharide, iota-carrageenan has been mixed with two types of gelatin (PS and LH) which differ by their pI, about 9 and 4.5, respectively. At pH 7, gelatin PS chains have a net positive charge and gelatin LH chains have a net negative charge. Phase diagrams in non-gelling conditions (60°C) and pH 7 have been established in both 25 and 100 mM NaCl. The iota-carrageenan/gelatin PS phase diagrams are typical of systems involving attractive interactions leading to the formation of complexes: both polymers are mostly in the lower phase and increasing salt concentration leads to a reduction of the phase separated domain. The iota-carrageenan/gelatin LH mixture shows a very different behavior. No macroscopic bulk phase separation is observed. However, an increase in the turbidity measured by spectrophotometry is observed for certain compositions which would indicate microscopic phase separation. From the limit between the turbid and transparent concentration domains, the binodal may be drawn. Considering the negative net charge of gelatin LH chains at pH 7, repulsive interactions were expected to govern the phase separation. However, in this case too, by increasing salt concentration the phase-separated domain is reduced, where an increase would have been expected for thermodynamic incompatibility. By following the evolution of the absorption spectrum of a methylene blue/iota-carrageenan/gelatin LH mixture as a function of the salt concentration, it has been shown that gelatin LH chains associate with carrageenan ones in low salt conditions up to 200 mM NaCl.