The 5-dimethylaminonapththalene-1 -sulfonyl (dansyl) chromophore attached to the polar head groups of lipids has been used as a fluorescent lipoid pH indicator to evaluate the interfacial pH in lipid-water lamellar systems prepared from negatively charged lipids. The pH in the vicinity of the charged lipid bilayers is different from the pH of the bulk aqueous phase and the difference is a function of the electrolyte concentration in the aqueous phase and of the lipid packing in the bilayer. At a fixed electrolyte concentration in the aqueous phase, the observed interfacial pH is 0.6 to 0.7 pH units lower above the thermal phase transition of the lipid than it is below this temperature. A quantitative interpretation of the results is given on the basis of the Gouy-Chapman theory. The results indicate that the dansyl chromophore is located in front of the charged surface and its distance from this surface increases with a decrease in lipid packing.The properties of charged lipid membranes and their role in biological systems has been the subject of intensive investigation in the recent literature [l -91. The membrane charges attract counterions from the surrounding electrolyte giving rise to a local enrichment of them in front of the membrane. The strength of this effect depends upon the surface charge density, i.e. on the lateral packing of the lipid molecules. So it is evident that membrane surface charges play an important role in mediating between the membrane structure and the properties of the electrolyte environment. At the lipid phase transition the membrane expands abruptly and at the same time ions are released into the medium [lo].Methods which would allow the direct measurement of the interfacial ionic strength or pH would be useful in obtaining a better understanding of the properties of charged lipid bilayers. pH-sensitive chromophores have been applied by other workers to study the interfacial potential in energy-transducing membranes, artificial lipid bilayers and micellar systems [11 -151. In this paper we wish to report the use of the fluorescent lipid derivative, dansyl-cephalin, as a lipoid pH indicator incorporated into charged bilayers prepared from the monovalent negatively charged lipid, methyl phosphatidic acid.This lipid has been the subject of intensive investigation in this laboratory [2,4,10] and is ideally Abbreviation. Dansyl, 5-dimethylaminonaphthalene-1 -sulfonyl.suited for the study of the electrostatic properties of charged lipid bilayers since it has only one ionizable group per lipid molecule. Dansyl-cephalin is also an ideal lipoid pH indicator since the dansyl group is located near the polar head-group region of the lipids when this lipid derivative is incorporated into bilayers V61.
MATERIALS AND METHODSDimyristoyl-methyl phosphatidic acid was a gift from Dr H. Eibl of this institute. Dimyristoyl-dansylcephalin was prepared according to the method described by Waggoner and Stryer [16]. Chromatographically pure dansylamide was obtained from Sigma Chemical Company (St Louis, U...
Aqueous dispersions of synthetic lecithins with different fatty acids in positions 1 and 2 of the glycerol molecule were studied by calorimetry. The data show that variation of the acyl chains in different positions of the glycerol backbone either have no influence upon or contribute 0.5 kcal/mol per CH2 segment to the phase transition enthalpy. Different molecular ordering of the mixed acyl chain lecithins in the bilayer is discussed in light of the results.
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