The spatial organization of lipids in biological membranes plays an important role in diverse cellular processes. Of particular interest are membrane rafts which are considered to enable cellular signal-ing and transport. Such membrane rafts are currently mimicked by liquid ordered (Lo) domains, observed in several lipid-only mixtures. The physical properties of such Lo domains and the coex-isting liquid disordered (Ld) phase are presently not well known. We developed a global small-angle x ray scattering data analysis for multilamellar vesicles that allows to determine membrane structural parameters and bending fluctuations of coexisting lipid domains [1,2]. Different scattering length density profiles were used for modeling the lipid bilayers with increasing details and com-plexity. The technique was applied to different ternary and quarternary phospholipid mixtures of DOPC/DPPC/Chol, DOPC/DSPC/Chol and DOPC/DSPC/POPC/Chol. Of particular interest are four component lipid mixtures entailing the variation of lipid domain size from micron regime down to a few nanometers [3]. We have studied changes of membrane thickness, lateral lipid packing and bending fluctuations for coexisting Lo/Ld phases in the four component lipid mixtures along this domain-size trajectory, including the melting of Lo domains as a function of temperature. Bending fluctuations for coexisting Lo domains were found to be significantly lower than for single Lo phas-es at the boundary of the Lo+Ld regime. In turn, little variation was observed when domains ex-ceeded sizes of 160 nm. Further, we found that the melting of Lo domains as a function of tem-perature is controlled by thickness differences between Lo and Ld and the associated domain line tension. 

[1] P. Heftberger et al., J. Appl. Crystallogr. 47: 173-180 (2013) 

[2] P. Heftberger et al., Biophys. J. 108: 854-862 (2015) 

[3] F.A. Heberle et al., J. Am. Chem. Soc. 135 (18), 6853-6859 (2013)