Upon the addition of small amounts of sodium dodecyl sulfate (SDS), the helicity of human serum albumin (HSA), lost in the urea denaturation, was mostly recovered. The profile of the recovery differed depending on the urea concentration. Then the urea concentrations were divided into three ranges: [1] a range below 3 M where the helicity only decreased as in the absence of urea (the helicity decreased down to 49% in the SDS solution); [2] a range between 4 and 8 M where the helicity initially increased up to 66% (this was the same as in the native state) and then sharply decreased; [3] a range above 9 M where the helicity only increased with an increase in added SDS concentration. When SDS was added prior to the urea denaturation, the same helicity was obtained at each surfactant concentration. Thus the SDS denaturation finally predominates over the urea denaturation. In the middle range, profiles of the structural change were rather complicated. The increase and decrease of helicity were accomplished below 3 mM SDS. It is worth noting that the helicity is recovered upon the addition of SDS less than 300 µM in the second range except for 8 M urea ([HSA] ) 10 µM). The helicity-recovery profile of HSA differs from that of a homologous protein, bovine serum albumin, the helicity of which recovers to some degree, but not completely. This difference might be attributed to the fact that the C-terminal helical-rod of HSA is amphiphilic, while that of bovine serum albumin is hydrophobic as a whole.

The interaction between sodium dextran sulfate (DxS) and alkyltrimethylammonium bromides was studied by means of potentiometric titration and electrophoretic light scattering methods in the phosphate buffer of pH 7.0 and ionic strength 0.014 at 25 °C. The bindings of dodecyltrimethylammonium ion and tetradecyltrimethylammonium ion to DxS occurred in two stages. In the first stage, the steep rise of the binding number is characteristic of a strong cooperative binding. The profile of hexadecyltrimethylammonium ion binding was similar to those of the others, as a whole. However, the concentration region, where the binding proceeded, became lower with an elongation of the hydrocarbon chain of the surfactant. The mobility of DxS was −3.7 × 10-4 cm2 s-1 V-1 in the absence of the surfactant. In the first binding stage, the negative mobility of DxS decreased with an increase in each surfactant concentration. It crossed zero mobility when the binding degree of the surfactant (moles of bound surfactant ion/mole of sulfate group of DxS) was 1. In the second stage where the binding degree exceeds 1 and approaches 2, a further binding of the surfactant ions appears to form micelle-like aggregates on DxS. The mobility of DxS became positive because of excessive positive charges introduced on the polymer. The mechanism of the complex formation is considered to consist of not only the binding process of the surfactant but also a change of shape of the complex itself.