Biochimica et Biophysica Acta, 1028 (1990) 43-48 43 Elsevier BBAMEM 74970 Effect of glycerol on the interfacial properties of dipalmitoylphosphatidylcholine liposomes as measured with merocyanine 540 A.C. Biondi 2 and E.A. Disalvo 1 t Area of Biomembranes, Instituto de Investigaclones Fisicoqulmicas Tdoricas y Aplicadas (INIFTA) (UNLP), La Plata, and 2 Facultad de Qulmica, Bioqulrnica y Farmacia, Universidad Naeional de Tueuman, Tucuman (Argentina) (Received 3 October 1989) (Revised manuscript received 1 June 1990) Key words: Dipalrnitoylphosphatidylcholine; Liposome;Merocyanine 540; Glycerol;Permeability Liposomes of dipalmitoylphosphatidylcholine (DPPC) prepared in increasing glycerol/glucose ratios show an increase in the absorbance at 570 nm of merocyanine spectra at temperatures below the phase transition. Since this effect is not observed when liposomes are prepared in solutions containing solely glucose, it is attributed to specific interactions of glycerol with the membrane phase. The increase in the 570 nm absorbance is ascribed to a partial fluidification of the membrane interface and is dependent on the distribution of the dye between the inner and the outer compartments of the liposomes and on their osmotic state. The greatest differences in the absorbance ratio are obtained when merocyanine is added to the external media. In consequence, the changes in the spectra of MC are dependant on the surface state of the liposomes which can be modified by an increase of glycerol or glucose in the external media. The present results are examined in the light of the perturbations that glycerol can induce on the barrier properties of the bilayer. Introduction It is well known that glycerol permeates natural and artificial lipid membranes. The mechanism of its penetration would involve a dehydration step of the molecule before entering the membrane phase in rela- tion to the number of hydrogen bonds that the molecule can concert with the aqueous phase [1,2]. Physicochemical factors such as temperature, osmotic pressure and chemical action of solutes present in the aqueous solutions can affect lipid-lipid and lipid-water interactions [4,5]. In this context, little attention has been paid to the changes that permeant molecules can produce on the membrane structure and how these changes affect the membrane permeability. Although there are evidences that glycerol, in particular, affects the phase state and the interdigitation of the lipid bilayer [6-8], permeability studies usually consider the membrane as a non-perturbed structure [4]. Abbreviations: DPPC, dipalmitoyiphosphatidylcholine; MC540, merocyanine540. Correspondence: E.A. Disalvo, INIFTA (UNLP), CC 16 Suc 4, 1900 La Plata, Argentina. Classical formalisms used to calculate permeability consider the lipid membrane as a phase interposed between two aqueous compartments thick enough to make the interfaces negligible. Obviously, this is not the case in a lipid membrane and consequently lipid mem- brane and aqueous environment respond as a single thermodynamic entity to physicochemical factors [5]. This is to say that the overall properties of the mem- brane are due to the organization of the lipids in the bilayer and the lipid-water association at the interfaces. The high glycerol concentration to built the gradient may affect the bilayer structure both by the osmotic volume contraction or by direct interactions of the glycerol with the lipid components. In this direction subtle changes have been found by X-ray diffraction, calorimetric measurements and spin paramagnetic resonance when glycerol substitutes for water [7]. The exchange of water by glycerol affects the polar head group conformation as measured by infrared spectroscopy [9,10]. Therefore, it is expected that the conditions to which liposomes are exposed in a permea- bility assay, namely changes in temperature and high glycerol concentrations at one or both siaes of the bilayer, may cause perturbations in the membrane re- gions which are relevant to determine the permeation kinetics. 0005-2736/90/$03.50 © 1990 Elsevier SciencePublishers B.V. (BiomedicalDivision)