Effects of conformational ordering on protein/polyelectrolyte electrostatic complexation: ionic binding and chain stiffening

Coupling of electrostatic complexation with conformational transition is rather general in protein/polyelectrolyte interaction and has important implications in many biological processes and practical applications. This work studied the electrostatic complexation between kappa-carrageenan (kappa-car) and type B gelatin, and analyzed the effects of the conformational ordering of kappa-car induced upon cooling in the presence of potassium chloride (KCl) or tetramethylammonium iodide (Me4NI). Experimental results showed that the effects of conformational ordering on protein/polyelectrolyte electrostatic complexation can be decomposed into ionic binding and chain stiffening. At the initial stage of conformational ordering, electrostatic complexation can be either suppressed or enhanced due to the ionic bindings of K+ and I- ions, which significantly alter the charge density of kappa-car or occupy the binding sites of gelatin. Beyond a certain stage of conformational ordering, i.e., helix content theta > 0.30, the effect of chain stiffening, accompanied with a rapid increase in helix length zeta, becomes dominant and tends to dissociate the electrostatic complexation. The effect of chain stiffening can be theoretically interpreted in terms of double helix association.