Biochimica et Biophysica Acta 938 (1988) 135-142 135 Elsevier BBA73871 Specific volumes of lipids in fully hydrated bilayer dispersions M.C. Wiener, S. Tristram-Nagle, D. Allan Wilkinson *, L.E. Campbell and J.F. Nagle Departments of Physics and Biological Sciences, Carnegie Mellon University, Pittsburgh, PA (U.S.A.) (Received 28 October 1987) Key words: Lipid; Absolute specific volume; Bilayer; Molecular volume; Hydration; Specific volume The neutral buoyancy method of obtaining absolute specific volumes of lipid in multilamellar dispersions is critically investigated. Control experiments show that there is no preferential partitioning of 2H 2 ° vs. H20 into the liposomes, and several thermodynamic properties of the samples, such as the enthalpy change and the volume change of the main transition, are changed very little with deuteration of the solvent. The assumption that the molecular volume of the solvent in the interlamellar space is essentially the same as in bulk solution is discussed; and it is shown to introduce rather small corrections. Previous procedures have been modified to avoid possible kinetic limitations in phases with low water permeability. It is concluded that the molecular volume of lipid in bilayers can be obtained to an accuracy better than 0.002 nm 3 (2~,3) which is less than 0.2% of typical molecular volumes of lipids. * Present address: Medical Physics, Allegheny Singer Re- search Institute, Pittsburgh, PA 15212, U.S.A. ** Present address: Department of Physics, Hobart and Wil- liam Smith Colleges, Geneva, NY 14456, U.S.A. Abbreviations: DPPC, dipalmitoylphosphatidylcholine; DLPE, dilaurylphosphatidylethanolamine; DSC, differential scanning calorimetry; vM, the specific volume measured by the neutral buoyancy technique; G, the specific volume of the H 20/2H 2 ° solvent; VL, the volume of a single lipid molecule in the bilayer; V×, the volume of a single lipid plus its associated n w waters; V s, the volume/molecule of the excess solvent; Vw, the volume/molecule of the solvent between the bilayers; mL, the molecular weight of the lipid; ms, the molecular weight of the H20/2H20 excess solvent; mw, the molecular weight of the solvent between the bilayers; n w, the number of waters per lipid molecule between the bilayers; C phase, the L c, or crystalline subgel phase; G phase, the LB, or gel phase; R phase, the Pa, or ripple phase; F phase, the L,, or fluid chain-melted phase. Correspondence: J.F. Nagie, Departments of Physics and Bio- logical Sciences, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A. Introduction The volume occupied by lipid molecules is an important quantity for understanding and char- acterizing biomembranes. Comparing volumes for different systems is a way of organizing our under- standing of large classes of lipids and alkanes, as has recently been emphasized by Small [1]. In this laboratory volumetric data have been essential in order to obtain the Van der Waals cohesive inter- action energy, which is a major contribution to the enthalpy of the main chain-melting transition [2-4]. It is customary and most appropriate in physi- cal chemistry, when dealing with solutions, to report the partial specific volume which is defined as the incremental change in volume per g added solute [5]. The less fundamental alternative is to report the apparent specific volume which is just the total volume of the solution minus the volume 0005-2736/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)