Figure 5.

Steps used in the discrimination of membrane phases using O₂ as a probe molecule. (a) Schematic drawings of membrane phases formed above (~25°C) and below (~20°C) the main phase transition temperature of the pure DMPC bilayer at different Chol contents in the Chol/DMPC mixture. Three basic bilayer phases are recognized: the solid-ordered (s_o) phase, the liquid-disordered (l_d) phase, and the liquid-ordered (l_o) phase (indicated by in grey). At 20°C, the s_o and the l_o phases coexist with a Chol/DMPC mixing ratio between ~5 and ~30 mol% Chol. At 25°C, the l_d and the l_o phases coexist with a Chol/DMPC mixing ratio between ~5 and ~30 mol% Chol. (b) Plots of the OTPs obtained with 5-PC and 14-PC as a function of the Chol mixing ratio in Chol/DMPC membranes allowed indicate Chol contents at which a single s_o (between 0 and ~5 mol%), a single l_d (between 0 and ~5 mol%), and a single l_o phase exists (between ~30 and ~50 mol%) and Chol contents at which s_o and l_o phases as well as l_d and l_o phases coexist (between ~5 and ~30 mol%). Data are for 20°C and 25°C. Symbols are explained in the figures. (c) Profiles of OTP obtained at 20°C and 25°C across DMPC membranes without Chol, containing 15 mol% Chol, and containing 50 mol% Chol. Symbols used are (Δ) for the s_o phase, (○) for the l_d phase, and (●,▲) for the l_o phase. Arrows indicate approximate locations of nitroxide moieties of spin labels. T indicates T-PC. The symbol × indicates OTP in the aqueous phase. It does not change significantly because the temperature dependences of O₂ diffusion and concentration in water are opposite. As shown, these profiles were obtained in single and coexisting domains and characterize their physical properties without physical separation of domains. Fig. 5b and Fig. 5c are reproduced from Ref. [29]. Copyright 2022, with permission from Elsevier.