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The liquid-liquid (Ld + Lo) coexistence region within a distearoyl-phosphatidylcholine/dioleoyl-phosphatidylcholine/palmitoyl-oleoyl-phosphatidylcholine/cholesterol (DSPC/DOPC/POPC/CHOL) mixture displays a nanoscopic-to-macroscopic transition of phase domains as POPC is replaced by DOPC. Previously, we showed that the transition goes through a modulated phase regime during this replacement, in which patterned liquid phase morphologies are observed on giant unilamellar vesicles (GUVs). Here, we describe a more detailed investigation of the modulated phase regime along two different thermodynamic tielines within the Ld + Lo region of this four-component mixture. Using fluorescence microscopy of GUVs, we found that the modulated phase regime occurs at relatively narrow DOPC/(DOPC+POPC) ratios. This modulated phase window shifts to higher values of DOPC/(DOPC+POPC) when CHOL concentration is increased, and coexisting phases become closer in properties. Monte Carlo simulations reproduced the patterns observed on GUVs, using a competing interactions model of line tension and curvature energies. Sufficiently low line tension and high bending moduli are required to generate stable modulated phases. Altogether, our studies indicate that by tuning the lipid composition, both the domain size and morphology can be altered drastically within a narrow composition space. This lends insight into a possible mechanism whereby cells can reorganize plasma membrane compartmentalization simply by tuning the local membrane composition or line tension., (Copyright © 2013 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

Phase diagrams of ternary lipid mixtures containing cholesterol have provided valuable insight into cell membrane behaviors, especially by describing regions of coexisting liquid-disordered (Ld) and liquid-ordered (Lo) phases. Fluorescence microscopy imaging of giant unilamellar vesicles has greatly assisted the determination of phase behavior in these systems. However, the requirement for optically resolved Ld + Lo domains can lead to the incorrect inference that in lipid-only mixtures, Ld + Lo domain coexistence generally shows macroscopic domains. Here we show this inference is incorrect for the low melting temperature phosphatidylcholines abundant in mammalian plasma membranes. By use of high compositional resolution Förster resonance energy transfer measurements, together with electron spin resonance data and spectral simulation, we find that ternary mixtures of DSPC and cholesterol together with either POPC or SOPC, do indeed have regions of Ld + Lo coexistence. However, phase domains are much smaller than the optical resolution limit, likely on the order of the Förster distance for energy transfer (R(0), ∼2-8 nm)., (Copyright © 2010 Biophysical Society. Published by Elsevier Inc. All rights reserved.)