SynopsisThe binding isotherms of sodium decyl sulfate to poly(L-ornithine), poly(D,L-ornithine), and poly(1,-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 @-structure of (L-LYS), even at a very low binding ratio of surfactant to polypeptide residue, whereas the onset of the helical structure for (L-Orn), begins after about 7 surfactant ions are bound to two turns of the helix. The CD results are consistent with this explanation.

The conductometric determination was made of the association constants of α-cyclodextrin (αCD) with ionic surfactants and their homologs. For 1-alkanesulfonate ions with carbon atoms (n) of 5–12, the association constant was found to increase regularly with n and become abruptly constant at n=10. The association constants for sodium 1-dodecanesulfonate, dodecylammonium chloride, dodecyltrimethylammonium chloride, and 1-dodecylpyridinium chloride were all similar in their magnitude. The effect of complexation on the micellar properties of sodium dodecyl sulfate and dodecyltrimethylammonium chloride were also studied potentiometrically by determining the counterion activities. In both cases, the critical micelle concentration increased regularly, while the degree of counterion association to the micelle decreased almost linearly with increasing αCD concentrations. It was suggested that the αCD-surfactant ion complex is solubilized in the hydrophilic surface region of surfactant micelle to give rise to a decrease in the surface charge density.

The association constants (K) of β-cyclodextrin with 13 amphiphilic ions were determined. For a homologous series 1-alkanesulfonate ions, the saturation phenomenon of K occurred at the number of carbon atoms of 10. The values of K for amphiphilic ions with dodecyl chain were similar to one another.

Dodecyl sulfates of bivalent nickel, cobalt, copper and magnesium were synthesized, and studies were carried out on the CMC values of these detergents, the effects of the added electrolytes on these CMC values, on micellar molecular weights, and on micellar charges. The CMC values of the detergents were independent of the kind of metal, and the CMC value of 1.2×10−3 mol./l. was obtained. The micellar aggregation numbers of bivalent metal dodecyl sulfates were of the order of 100, which is about twice that of sodium dodecyl sulfate. The dissociation degrees of the micelles were also evaluated from the conductivity data. The α values obtained were 0.66 for cobalt, 0.60 for copper and 0.42 for magnesium. The dissociation degree of the copper dodecyl sulfate micelle was also estimated to be 0.52 from the depression of the CMC value by the added electrolyte; this figure is approximately the same as the value of 0.60 obtained from the conductivity data. These results suggest that the fundamental factor responsible for determining the surface activities of bivalent metal dodecyl sulfates is the valency of the gegenion and that the kind of metal has little or no detectable effect on them.

Conformations of poly(L-lysine) (PLL) and poly(L-ornithine) (PLO) were examined in aqueous solutions of sodium alkanesulfontates (CnSO3Na, n=9,10,11,12) in the presence of 0.02 M NaC1 by circular dichroisrn (CD) spectroscopy. These surfactants induce the ~-structure for PLL and the s-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 CgSO3Na and starts immediately on binding of surfactant in the case of C1 2SO3Na. 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 CloSO3Na 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.