Conformational phase diagram of poly(l-lysine) in sodium alkanesulfonate aqueous solutions at various temperatures

Abstract

The conformation of poly(l-lysine) (PLL) was investigated in sodium alkanesulfonate CnSO3Na (n=8, 7) at various temperatures by circular dichroism spectrum measurements. C8SO3Na induced a double-step conformational change from a coil, to a β-sheet, and then to an α-helix, in which C7SO3Na induced a single-step coil-to-helix conformational change. Binding isotherms of C8SO3Na by PLL were constructed from the potentiometry of equilibrium concentration of the surfactant using a surfactant ion-selective electrode. The curves indicated the cooperative binding characteristic and were analyzed by a linear lattice model using the Bethe approximation. The thermodynamic parameters obtained from the model revealed that the binding of C8SO3Na by PLL was an entropy-driven process. The conformational change was observed at nearly full binding, presumably due to the surfactant clustering of the ordered conformation.