Figure 3.

(a) Representative SR signal from 5-PC in the DMPC bilayer containing 20 mol% Chol obtained at 30°C for the sample equilibrated with 50% air. In the deoxygenated sample, a single exponential signal is observed with a time constant of 5.10 μs (data not shown). In the presence of oxygen, fitting the search to a single exponential mode is unsatisfactory as shown by the residual (upper panel). The fit, using the double-exponential mode (time constants of 1.73 and 0.84 μs), is excellent (lower panel). The double-exponential fit is consistent with two immiscible domains (phases) with different OTPs that are present at these conditions. (We assigned them to the l_d phase and l_o phases.) (b) Plot of $T_{1^{-1}}$ for 5-PC in the l_o and l_d phases in a DMPC membrane containing 20 mol% Chol as a function of air fraction in the equilibrating gas mixture. Experimental points show a linear dependence up to 50% air, and extrapolation to 100% air is performed as a way of calculating OTPs in the l_o and l_d phases.