Interaction of sodium decyl sulfate with poly(L-ornithine) and poly(L-lysine) in aqueous solution

The binding isotherms of sodium decyl sulfate to poly(L‐ornithine), poly(D,L‐ornithine), and poly(L‐lysine) at neutral pH were determined potentiometrically. The nature of a highly cooperative binding in all three cases suggests a micelle‐like clustering of the surfactant ions onto the polypeptide side groups. The hydrophobic interaction between the nonpolar groups overshadows the coulombic interaction between the charged groups. The titration curves can be interpreted well by the Zimm–Bragg theory. The average cluster size of bound surfactant ions is sufficiently large to promote the β‐structure of (L‐Lys)n even at a very low binding ratio of surfactant to polypeptide residue, whereas the onset of the helical structure for (L‐Orn)n begins after about 7 surfactant ions are bound to two turns of the helix. The CD results are consistent with this explanation.