Photocontrol of polyion complex formation between polylysine and chondroitin sulfate in the presence of pararosaniline leucohydroxide

Abstract.

The photocontrol of polyion complex formation between polylysine bromide (PlysBr, M _w =21,800) and sodium chondroitin sulfate (NaChs, M _w =24,700) in aqueous solution was examined in the presence of a photochromic dye, pararosaniline leucohydroxide (leuco(OH)), with special reference to its pH-dependence. Under pH conditions examined (6< pH <12), NaChs was almost fully dissociated, while dissociation of PlysBr reduced with increasing pH, resulting in a random coil-to-α-helix transition at around pH 9.4. Thus, the complex formation clearly showed pH-dependence. On the other hand, leuco(OH) undergoes photodissociation to increase the solution pH. By coupling these processes, we aimed to control the complex formation by photoirradiation. Turbidimetry, dynamic light scattering and electrophoretic light scattering measurements showed that the optimum condition for the complex formation was at approximately pH 10.5, where the net charge of complex was nearly zero, for solutions of [NaChs]=1.0 mM, [PlysBr]=1.5 mM (in molalities of dissociable groups), at a salt concentration of 1.5 mM. In the presence of 0.5 mM leuco(OH), the variation of complex size was clearly observed by photoirradiation. The hydrodynamic radius of the complex stayed almost constant at pH <9 and pH> 11, increased at 9< pH <10, and decreased at 10< pH <11. The observed trend could be reasonably explained in terms of the pH change due to photodissociation of leuco(OH). A photoinduced coil-to-α-helix transition of PlysBr in the presence of leuco(OH) was also reported.