FIGURE 2.
Demonstration of tie-line for SPM/DOPC/Chol. (A) The ternary (compositional) phase diagram of SPM/DOPC/Chol lipid mixtures at 22°C (22). It contains an elliptical region of coexisting liquid-ordered (Lo) and liquid-disordered (Ld) phases enclosed by a PB (the elliptical solid line). The short dotted-dashed section at the far left of the PB indicates roughly where there is a possible critical point. The dashed section of the PB to the lower right indicates a region of estimated transition between Lo and Ld phases. The large dots along the PB (1 in Ld phase and 11 in Lo phase) are the compositions at the endpoints of the trial tie-lines and the compositions from which PB spectra were obtained. The solid line with diamonds is the trial tie-line that was determined to be the best estimate to the true tie-line by the method of Chiang et al. (25); this line connects the Ld phase with a mol fraction composition of 0.30:0.63:0.07 (Sm/DOPC/Chol) to the Lo phase with composition 0.24:0.42:0.34 (see respective ESR spectra in Fig. 2 B). The five diamonds give the compositions on that tie-line whose spectra (spectra 2–6 in Fig. 2 C) were fit by a linear combination of the Ld and Lo PB spectra from the end of the tie-line. The other dashed lines are the 10 other trial tie-lines. (B) Demonstrates the differences in the ESR spectra of 16PC at these two boundaries (spectra normalized to a common integrated intensity): dotted line is for Lo, dashed line is for Ld. (C) The solid spectra shown and labeled 2–6 are obtained in the two-phase Lo and Ld coexistence region along the tie-line connecting the two boundary spectra, which are again shown as a dotted line for Lo and a dashed line for Ld. Superimposed on the solid spectra 2–6 are dash-dotted ones that represent the appropriate linear combination of Lo and Ld boundary spectra in accordance with the lever rule (25) as the compositions are gradually changed from 100% Lo to 100% Ld (from top to bottom), as given by the diamonds along the true tie-line shown in Fig. 2 A.