Local chain mobility of gellan in aqueous systems studied by fluorescence depolarization

The local chain mobility of a gellan, an electrolyte polysaccharide, in aqueous systems was examined with respect to the effect of the temperature, the concentration of gellan (c(G)), and the concentration of added salt (c(S)). The relaxation time of local motion was estimated for fluorescein isothiocyanate (FITC)-labeled-gellan by the fluorescence depolarization technique, and the chain mobility was discussed. The relaxation time increased with decreasing temperature, in particular when accompanying the coil-helix transition due to the great difference in chain mobility between the coil and the helical conformations. The effect of c(G) was observed for gellan solutions even below the critical concentration of chain entanglement (2 wt.-%) for well-expanded nonelectrolyte polymers with size similar to that of the gellan. This suggests that the actual excluded volume of gellan is larger than that of nonelectrolyte polymers due to the electrostatic repulsion between segments. The relaxation time for 0.2 wt.-% systems of gellan in coil conformation is independent of c(S), whereas a c(S) dependence of the relaxation time is clearly observed for 0.5 wt.-% systems. The degree of expansion of the gellan chain is independent of the shielding effect of cations on the electrostatic repulsion between gellan segments due to the stiffness of gellan chain. On the other hand, the c(G) as well as the c(S) dependence of the chain mobility is clearly observed for gellan in the helical conformation, examined over the concentration range, probably due to the partial aggregation of helices induced by the attractive interaction between gellan segments.