The surface tensions of the systems of hexa(oxyethylene) dodecyl ether (C12E6) vs. lysozyme (Lyz) or bovine serum albumin (BSA) were measured for constructing the binding isotherms, since the surface tension method is more convenient for the systems of nonionic detergents and proteins than is equilibrium dialysis. The obtained binding isotherms depend on the protein concentration; this effect is much more remarkable for Lyz–C12E6 systems than for BSA–C12E6 ones. The binding isotherms of BSA–C12E6 systems obey the Scatchard equation, and the maximum number and estimated free energy for C12E6 binding to BSA are comparable to those for the system of Triton X-100 and BSA determined by equilibrium dialysis. The agreement suggests that the surface tension method is sound and has advantages over equilibrium dialysis for the estimation of the number of detergent molecules bound to protein. In the systems of Lyz–C12E6, the binding isotherms follow the Freundlich-type equation rather than the Scatchard equation. The remarkable protein concentration dependence of the binding isotherms and this result suggest the occurrence of protein aggregation.

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The binding mode of sodium dodecyl sulfate to lysozyme and the accompanying structural change of lysozyme by binding have been investigated by means of the binding isotherm, the precipitation curve, and the CD spectra in pure water, NaCl, and borate buffer solutions. The precipitation phenomena could be explained in terms of the neutralization of the net charge of lysozyme due to dodecyl sulfate-ion binding. The analysis of the binding isotherms by the use of the BET equation gave the site number of the first layer corresponding to the positively charged residues at the pH studied. The conformational change from the β-structure to the α-helix has been observed in the second-layer binding. The environmental change of the side-chain residues has also been observed.

In order to divide the inorganic salt effect on the micelle formation of nonionic surfactants into the effects on the hydrocarbon and on the hydrophilic moieties of the surfactant, the critical micelle concentrations(CMC) of nonionic surfactant homologs (8,10,12 methylene and 6 oxyethylene groups) were determined in aqueous salt solutions. The salt-effect parameters of methylene and hexa(oxyethylene) groups were calculated from the CMC data. The orders of both the parameters with respect to the anion obeyed the Hofmeister series. The variation in the extent of the parameters with respect to the cation was much less than that with respect to the anion. These phenomena were discussed in terms of the direct and indirect effects of ions on the water structure around the hydrocarbon and hydrophilic moieties of the surfactants. In addition, the salt effect on the cloud point (CP) and the amount of solubilization toward the Yellow OB dyestuff in aqueous solutions were discussed in connection with the salt effect on the hydrophilic moiety.

The variation of critical micelle concentrations (CMC) of polyoxyethylene lauryl ethers having various ethylene oxide chain lengths \barn(\barn=6, 11, 20, and 31) on addition of short chain alcohols to the surfactant aqueous solutions was determined at 20 °C. Thermodynamic quantities of micelle formation in methanol– and ethanol–water mixtures and in pure water were obtained in connection with the heat of micelle formation determined by direct calorimetry. Methanol and ethanol showed a CMC-increasing effect only, whereas the addition of n-propanol, n-butanol, and n-pentanol showed a CMC-decreasing effect. The overall CMC-increasing effect is interpreted as being mainly due to the fact that methanol and ethanol weaken the hydrophobic bond. However, the effect also has a secondary effect of lowering the CMC slightly on the polyoxyethylene portion of a surfactant. Several factors are suggested for explaining this secondary effect. The CMC-decreasing effect is explained by a decrease in the free energy of mixing resulting from the solubilization of alcohol molecules into the micelle.

KC1, as shown in Figure 6, although the slope (0.41) was slightly smaller than those of typical ionic surfactants: 0.55-0.7.26 like to thank Professor S. Ikeda and Dr. T. Tomiyama (Nagoya University) for the light-scattering measurements. This work was supported in part by a Grant-in-Aid from the Ministry of Education (No. 61470084).

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