Colloid & Polymer Science Colloid Polym Sci 272:1289-1294 (1994) Effects of high electrolyte concentration on DPPC-multilayers: an ESR and DSC investigation P. Sapial), L. Coppola2), G. Ranieri2), and L. Sportelli 1) 1) Dipartimento di Fisica, Laboratorio di Biofisica Molecolare and Unifft INFM, and 2) Dipartimento di Chimica, Universifft della Calabria, Arcavacata di Rende, Italy Abstract: The effects of salinity on the lateral headgroup interactions of dipal- mitoylphosphatidylcholine (DPPC) molecules in fully hydrated multilayers have been investigated by spin label electron spin resonance (ESR) spectro- scopy and differential scanning calorimetry (DSC). By increasing the NaC1 concentration from 0 to 3 M in the multilayers' dispersion medium, the ESR measurements performed with the 5-stearic acid spin label and di-tert-butyl-nitroxide show an increase in the orientational degree of order of the lipid molecules, mainly in the gel phase, and a decrease of the membrane permeability. An upward shift from 31.5 ~ to 36.5 ~ and from 40.5 ~ to 41.9 ~ of the pre- and main DPPC phase transition temperatures, respectively, is observed with 5-SASL, while slightly higher values are detected with DTBN. Small effectsare evident on the properties of the liquid crystalline phase of the DPPC multilayers. The DSC measurements also reveal an upward shift of the pre- and main transition temperatures. The shifts, however, are more marked if compared to the ones observed with the ESR technique. The findings suggest an increase in the packing density of the DPPC molecules in the multilayers in presence of high salt concentration. Dehydration of the DPPC interfacial region with a variation of the lateral electrostatic interactions between phospholipid polar heads trigger the phenomena observed. Key words: Ionic strength - DPPC - multilayer - ESR -DSC Introduction The role of ions in determining the structure and function of biological membranes is well as- certained. Owing to this, the effects of ions on natural membranes as well as on their related model systems have been extensively investigated [1-8]. However, all studies performed with fluo- rescence, DSC, ESR, NMR, and x-ray diffraction techniques have been concerned with mono-, bi- and trivalent salts up to 1 M concentration in the dispersion medium. In addition, in these inves- tigations membrane model systems made with lipids carrying one or more charges per polar head giving rise to a net charge density on the membrane surface were used. The results of these investigations are quite well understood in the limit of the Gouy-Chapman-Stern lipid double layer theory [9]. On the other hand, very few works have appeared on the effects of salt con- centrations higher than 1 M and on the lateral headgroup interaction of neutral phospholipid molecules such as phosphatidylcholine and phos- phatidyl-ethanolamine [6, 10-19]. In this respect, we have investigated the effects on the orientational degree of order and fluidity of DPPC multilayers when the NaC1 concentration in the dispersion medium is increased up to 3 M. The upward shift of the pre- and main phase transition temperatures of DPPC multilayers ob- served either with the ESR spectroscopy or with the DSC technique suggests an overall increase in GL 393