Organization of lipid mixtures containing a pyrene amphiphile in liposome and Langmuir monolayers: evidence of superlattice arrangement

Phase properties and lateral distribution of components in lipid membranes are frequently investigated by fluorescence of pyrene-labeled lipids. In particular conditions, at specific component molar fractions, lipid bilayers can present regions organized in regular arrangements, hexagonal lattices, where the acyl chain form regular patterns around the pyrene tagged chain of the labeled component. Liposomes composed of a pyrenyl twin type amphiphile (1) bearing a pyrrolidinium headgroup and either 1,2-dimyristoyl-sn-glycero-phosphocholine or 1,2-dioleoyl-sn-glycero-phosphocholine were investigated by fluorescence spectroscopy and Langmuir compression isotherm techniques in a restricted range of molar fraction, X1, (0.005-0.15) to evaluate the influence of composition, temperature and curvature radius on lipid organization and to assess the eventual organization in super lattice arrangements. Amphiphile 1 presents different characteristics with respect to the Pyr-PC usually used in similar investigations, both in terms of charge and conformational freedom, and its presence has a significant effect on membrane organization. Our results put in evidence that both temperature and the presence of unsaturation strongly influence the presence of hexagonal lattice in lipid bilayers.