ON THE MECHANISM OF GELATION OF HELIX-FORMING BIOPOLYMERS

The theory of double-helix formation has been applied to experimental coil-helix transition curves published by Rochas et al. (Carbohydr. Polym. 1990, 12, 2551) on kappa-carrageenan samples covering a large range of molecular weights. Within the uncertainty due to sample polydispersity, the theory satisfactorily reproduces the experimental data, yielding values for the microscopic parameters (the association and initiation parameters) that govern the cooperativity of the transition. These parameters were used in a detailed theoretical analysis of the belix formation of kappa-carrageenan, which indicates that (1) an all-or-none transition is a poor description of the process, (2) the concentration dependence of the transition is significant only for the short chains, and (3) the double-helical regions are always too few to generate an infinite network by helix formation alone. The analysis implies that the kappa-carrageenan gel network is not created on the helical level, by each chain forming double-helical junctions with, on the average, two or more partners. The latter conclusion was supported by a viscosimetric experiment, performed under nonaggregating conditions (in 0.2 M LiI), which showed that the relative viscosity of a kappa-carrageenan solution was a monotonic function of the helical content in the system. A further experiment showed that a nongelling solution of helical rods of kappa-carrageenan could be reversibly transformed into a gel by dialysis against an appropriate salt solution under such conditions that the carrageenan molecules never pass the coil state. It is concluded that the kappa-carrageenan gel network is created essentially on the superhelical level, by helix aggregation.