The agarose gelling system was investigated with the fluorescence polarization method using fluorescent dyes, such as uranine (fluorescein sodium), as an extrinsic probe. The method detected three critical points, c1, c2, and c3, in the concentration change of agarose at room temperatare, corresponding to the concentrations at which the agarose domain occupies the whole solution, the junction zone is made to gel, and the network is presumably completed to hold gel elasticity, respectively. Three transition points, T1, T2, and T3, were also detected in the temperature range studied for these systems. T1 and T3 are well defined transition points of agarose, the gel point and the melting point. T2 is an unknown, new transition point. In order to examine the transition at T2, some other solution properties were determined with the fluorescence polarization method, and T2 was interpreted to be the coil-helix transition temperature of agarose. Thus the fluorescence polarization method provides useful information for the investigation of a polymer gelling system such as agarose.

The sol-gel transformation of agarose and other polysaccharide fractions from agar were investigated by the fluorescence polarization method in order to study the gelling mechanism of agarose. The results showed that the gelation in dilute systems was a cooperative process of a helix segment with a kink segment in the agarose. In the process, the role of the helix segment was interpreted on the basis of Flory's mechanism for the transition and phase equilibria of asymmetric polymers. That is, the helix segment folded into a rodlike helical chain at the helix point to form an isotropic solution. At the gel point, the isotropic solution was separated into the tactoidal concentrated solution phase (coacervate) and the isotropic more dilute solution phase. The coacervate droplets were joined to each other with the non-helical kink segment to form a three-dimensional network structure. The rodlike helical chain tended to aggregate into the crystalline phase, though the rate was fairly slow just below the helix point because of the nucleation controlled process. Some of the helical chains still tended to aggregate into the crystalline phase in the coacervate. The melting point of the gel corresponded to that of the crystalline phase, and was higher than the gel setting point. The hysteresis of the agarose sol-gel transformation was thus interpreted by the difference between the melting point and the setting point.