Biochimica et Biophysica Acta, 1146 (1993) 169-177 169 © 1993 Elsevier Science Publishers B.V. All rights reserved 0005-2736/93/$06.00 BBAMEM 75856 Gel state surface properties of phosphatidylcholine liposomes as measured with merocyanine 540 Delia L. Bernik and E.A. Disalvo Area de Biomembranas, lnstituto de Investigaciones Fisicoqufmicas Te6ricas y Aplicadas (INIFTA), Universidad Nacional de La Plata; CC 16 Suc 4 1900 La Plata (Argentina) (Received 15 May 1992) (Revised manuscript received 7 September 1992) Key words: Saturated phosphatidylcholine; Phosphatidylcholine; Cholesterol; Lipid bilayer; Gel state; Merocyanine 540; Visible absorption spectrum; Fluorescence band; Surface property The surface properties of liposomes composed by saturated phosphatidylcholines and their mixtures with cholesterol in the gel state have been studied using merocyanine 540 as a fluorescent and optical probe. A new absorption peak at 450 nm and a new fluorescent band at 630 nm were observed when the dye was added to suspensions of DMPC multilamellar liposomes in the gel state. These peaks were also observed in membranes with different lipid compositions in conditions in which the P~, and the Le, phases were present. The increase of temperature above the main transition temperature of DMPC or the incorporation of 35% cholesterol into DMPC bilayers at 13°C caused the disappearance of these peaks. The changes in the absorption and fluorescent spectra upon addition of cholesterol resembles very well the phase diagrams reported by Mortensen et al. ((1988) Biochim. Biophys. Acta 945, 221-245) indicating that the corrugated structures characteristic of the L~, and the P~, phases have different surface properties related to the partitioning of amphiphilic dies. Introduction The gel state between the pre and the main phase transition temperature (the Pa, phase) shows topologi- cal characteristics different than that observed below the pretransition (the L~, phase). This region is char- acterized by the presence of rippled structures, whose periodicity and organization is a function of tempera- ture and membrane composition, in particular by the presence of cholesterol in the bilayer [1-7]. The analy- sis of Mortensen et al. indicates that the sides of the ripples primarily contain gel phase phosphatidyl- choline, whereas the apices are enriched in cholesterol and are liquid crystalline. The boundaries separating the L~, and P~, phases show undulations and defects in Correspondence to: E.A. Disalvo, Area de Biomembranas, Instituto de Investigaciones Fisicoqu~micas Te6ricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CC 16 Suc 4, 1900 La Plata, Argentina. Abbreviations: DMPC, dimyristoylphosphatidylcholine; di-15:0-PC, dipentadecanoylphosphatidylcholine; DPPC, dipalmitoylphospha- tidylcholine; 14: 0/18 : 0-PC, 1-myristoyl-2-stearoylphosphatidylcho- line; di-17: 0i-PC, diisomargaroylphosphatidylcholine; MC 540, mero- cyanine 540. the structure which seems to propagate into both phases in contact [5]. A number of experimental methods, giving informa- tion on different length and time scales have attempt to understand the structure and dynamics of phospho- lipid bilayers around the phase boundaries and with the incorporation of cholesterol. The resulting data show still some controversies related to the formation of domains that may determine local properties at different depths in the plane of the membrane with different affinities for amphiphile dies. Among the dyes used in membrane spectroscopy, merocyanine 540 is able to detect changes in the lipid phase state and in the electrical properties of the surface. This probe has been used as a sensor of molecular events either in model membrane systems or in biological membranes. In addition, merocyanine have different affinities for cholesterol-free and cholestrol- rich regions [8]. Therefore, changes in the MC spectra can be ex- pected in relation to the different phase state of the regions of the membrane in which the dye has the highest probability for partitioning. In addition, the partition of the probe may change the dimer/mono- mer ratio affecting the fluorescence spectra [15].