Conformational change of poly(L-lysine) by formation of complexes with poly(styrene sulfonate)

Circular dichroic studies were made on the conformational change of poly(L -lysine) (PLL) by complex formation with sulfonated polystyrenes (PSS) as functions of pH, temperature and polymer compositions in comparison with the complexes of PLL with sulfated poly(vinyl alcohol) (PVS). The coil-to-helix transition of PLL by complex formation at neutral and acidic pHs, which took place in the PLL/PVS (with high degrees of sulfation) complexes, was prevented in the PLL/PSS complexes due to the less chain flexibility of PSS than PVS. Above pH 11, the α-helix of PLL was partially broken into random coil by complex formation with PSS. The drastic decrease in the solution viscosity by the complex formation revealed a large contraction of PSS chain and subsequently a large strain of the α-helical PLL chain so as to give the breakdown of the α-helix. This is denoted a strain-induced conformational change of PLL. It was found that the α-helix of PLL was thermally stabilized by the complex formation with PSS as in the case of PLL/PVS complexes. A β-to-α transition of PLL by the complex formation with PSS was also found, while the β-form was maintained in the PLL/PVS (with high degrees of sulfation) complexes.