Lipid bilayers in the gel phase become saturated by triton X-100 at lower surfactant concentrations than those in the fluid phase

Abstract

It has been repeatedly observed that lipid bilayers in the gel phase are solubilized by lower concentrations of Triton X-100, at least within certain temperature ranges, or other nonionic detergents than bilayers in the fluid phase. In a previous study, we showed that detergent partition coefficients into the lipid bilayer were the same for the gel and the fluid phases. In this contribution, turbidity, calorimetry, and 31P-NMR concur in showing that bilayers in the gel state (at least down to 13-20°C below the gel-fluid transition temperature) become saturated with detergent at lower detergent concentrations than those in the fluid state, irrespective of temperature. The different saturation may explain the observed differences in solubilization.

Figure 1

Solubilization of SM vesicles. (A) Solubilization at 20°C, characterized by the decrease in turbidity as a function of Triton X-100 concentration. SM concentration was 1 mM. (D_on) is the total detergent concentration required for the onset of lipid solubilization. (B) Solubilization at 50°C, monitored by MC540 absorption spectra as a function of Triton X-100 concentration. SM concentration was 1 mM. (C) Comparison of detergent concentrations required for the onset (D_on) of solubilization obtained by the turbidity assay (○) or from MC 540 absorption spectra (●). D_on is plotted as a function of SM concentration at 20°C. Data are average values (n = 3). SD is the size of the symbols, or smaller.

Figure 2

Computation of R_e^sat for SM and Triton X-100. The detergent concentrations required for the onset (D_on) of solubilization are plotted as a function of SM concentration at 20°C (○) and 50°C (●). Measurements at 20 and 50°C were carried out with the turbidity and the MC540 methods, respectively. Data are average values (n = 3). SD is the size of the symbols, or smaller. Regression coefficients for the straight lines are 0.93 (20°C), 0.99 (50°C).

Figure 3

³¹P-NMR spectra of SM vesicles in the presence of Triton X-100 at 20°C (lower spectrum) and 50°C (upper spectrum). SM concentration was 20 mM and effective detergent/lipid molar ratio was 0.42.

Figure 4

Solubilization of various PC bilayers at different temperatures. The detergent concentrations required for the onset (D_on) of solubilization are plotted as a function of phospholipid concentration at 20°C (○) and 50°C (●). (A) DPPC; (B) DMPC; (C) egg PC. Data are average values (n = 3). SD is the size of the symbols, or smaller. Regression coefficients: (A) 20°C, 0.97; 50°C, 0.99; (B) 10°C, 0.97; 30°C, 0.98; (C) 20°C, 0.99; 50°C, 0.99.