Departments of Basic Chemistry, Organic Chemistry, and Medical Chemistry, University of Helsinki, Helsinki, Finland Received May 15, 1984 abstract: We have synthesized l-palmitoyl-2-pyrenedecanoyl-jn-glycero derivatives of 3-phosphatidylcholine, 3-phosphatidylethanolamine, 3-phosphatidylserine, 3-phosphatidylglycerol, 3-phosphatidylinositol, and 3-phosphatidic acid and investigated their behavior in monolayers and in neat and mixed bilayers. Fluorescence spectroscopy of neat pyrene phospholipid dispersions revealed a well-defined thermotropic transition at 13.5-19 C depending on the polar head group. An endotherm coinciding with this transition was observed with differential scanning calorimetry, indicating it to be due to the melting of the lipid acyl chains. For pyrenephosphatidylethanolamine, the endotherm was observed at a much higher temperature (70 C). Compression isotherms obtained at an argon/water interface revealed that the pyrene moiety somewhat increases the mean molecular area of a phospholipid molecule but does not prevent the expression of head-group-dependent packing behavior. Partition of the pyrene lipids between coexisting fluid and solid phases was investigated with fluorometry and calorimetry. Both techniques indicate that these lipids prefer the fluid phase and that thispreference is independent of the head group. The rates and apparent activation energies of lateral diffusion in fluid bilayers were found to be similar for most pyrene lipids, suggesting that the lateral movement of phospholipids is not critically dependent on interactions at the head-group level. Lateral distribution of the pyrene lipids in gel and fluid phosphatidylcholine bilayers was studied with the excimer technique and calorimetry. In gel-state dipalmitoylphosphatidylcholine bilayers, the pyrene lipids form clusters. These clusters, however, do not consist of pure pyrene lipid but of aggregates (compounds) of the labeled and unlabeled lipid. The stoichiometry of these aggregates seems to depend on the head group of the labeled lipid. In fluid bilayers, the pyrene lipids tend to adapt a regular rather than random or clustered distribution. This is suggested by the excimer to monomer intensity ratio vs. pyrene lipid mole fraction plots, which consist of several linear regions separated by kinks. The data for different pyrene lipids are alike, indicating that the lateral distribution of these lipids is governed by the pyrene moiety. The tendency of the pyrene lipids to distribute regularly is proposed to result from the presence of long-range order in liquid-crystalline bilayers.

The last few years have seen an exponential increase of membrane research employing fluorescent probes. The pop-ularity of the fluorescence technique is due to its sensitivity as well as the relative simplicity of basic methodology and instrumentation. Furthermore, recent development of the theoretical aspects of fluorescence spectroscopy of membranes [for reviews, see Lakowitz (1981) and Yegurabide & Foster (1981)] allows one to obtain more detailed information than was possible before.