CONFORMATIONAL CHANGE OF POLY(L-LYSINE) AND POLY(L-ORNITHINE) AND COOPERATIVE BINDING OF SODIUM ALKANESULFONATE SURFACTANTS WITH DIFFERENT CHAIN-LENGTH

Conformations of poly(L-lysine) (PLL) and poly(L-ornithine) (PLO) were examined in aqueous solutions of sodium alkanesulfontates (C(n)SO3Na, n = 9, 10, 11, 12) in the presence of 0.02 M NaCl by circular dichroism (CD) spectroscopy. These surfactants induce the beta-structure for PLL and the alpha-helix for PLO. The binding of surfactants on the polypeptides was measured potentiometrically with a surfactant ion electrode and was found to be highly cooperative. The cooperativity increases with increasing chain length of surfactant. The behavior accompanying the surfactant binding and the conformational change indicated that the conformational change requires a certain amount of bound surfactants in the case of C9SO3Na and starts immediately on binding of surfactant in the case of C12SO3Na. The clustering of bound surfactants due to the cooperative binding as well as neutralization of polypeptides contributes to their conformational change. A slow conformational change of PLO was found in the time scale of hours, sometimes days, for C9- and C10SO3Na at low concentrations, but the binding process reached the equilibrium quickly. This slow mode might occur due to the slow interaction between surfactant/polypeptide complexes.