Chemistry and Physics o f Lipids, 62 (1992) 39-43 39 Elsevier Scientific Publishers Ireland Ltd. Activation energy and entropy for intramolecular excimer formation in a dipyrenylphosphatidylcholine probe in lamellar and hexagonal lipid phases Peter Butko and Kwan Hon Cheng Department of Physics, Texas Tech University, Lubbock, TX 79409 (USA) (Received December 31st, 1991; revision received March 16th, 1992; accepted March 31st, 19921 lntramolecular excimer formation in pyrene-labeled phosphatidylcholine was used as a tool to determine thermodynamic characteristics of the lamellar to hexagonal phase transitions in a binary lipid system dilinoleoylphosphatidylethanolamine (DLPE)/palmitoyloleoylphosphatidylcholine (POPC). Upon an LdHn phase transition, the activation energy Ea for excimer forma- tion increased from 5.6 ± 0.2 kcal/mol to 6.3 ± 0.2 kcal/mol, while the activation entropy AS¢ decreased from --40.0 ± 0.8 cal/K.mol to -38.4 ± 0.8 cal/K.mol. The results are consistent with the idea of molecular splaying of the acyl chains in the hexagonal phase. It is estimated that the molecular area at the terminal carbon of the lipid acyl chains increases by a factor of 2.2 upon the I.aHn transition in DLPE/POPC. Key words: pyrene-labeled lipid; excimer; phase transition Introduction Di-(1 '-pyrenemyristoyl)-phosphatidylcholine (di- pyPC) is a fluorescent phospholipid in which two fluorophores (pyrenes) are conjugated to the ter- minal carbons of both acyl chains. When one of the fluorophores is excited, there is a probability that, during the fluorescence lifetime, the excita- tion will be shared with the other fluorophore, creating an excited-state dimer, an excimer. Intermolecular excimer formation, where the excimer is formed by two fluorophores in two dif- ferent molecules [1], has been used to study the dynamic and structural properties of lipid mem- branes [2-4]. The rate of intermolecular excimer formation depends on the fluorescent probe con- centration and its diffusibility in the lipid bilayer, which is determined by temperature and the mem- brane fluidity. Correspondence to: Peter Butko, Institute for Biological Sciences, M-54, National Research Council, Ottawa, Canada KIA 0R6. Intramolecular excimer formation, where the excimer is formed by two fluorophores within the same molecule, was introduced into membrane studies only recently. Vauhkonen et al. [5] used dipyrenylphosphatidylcholine as a membrane fluidity probe and Cheng et al, [6] showed that dipyPC fluorescence responds to the lamellar to hexagonal phase transition of the matrix lipid. The rate of intramolecular excimer formation is concentration-independent, but is a function of temperature and the geometry of lipid packing (the free volume of a membrane, i.e. the fractional volume not occupied by the matrix molecules). In this study we determined activation energies and entropies for intramolecular excimer formation in dipyPC in a binary system consisting of dilinole- oylphosphatidylethanolamine (DLPE) and l-pal- mitoyl-2-oleoylphosphatidylcholine (POPC). We also presented a simple theory based on basic ther- modynamics that allowed us to estimate the geometric change in lipid packing upon a phase transition from lamellar L~ to inverted hexagonal HII phase. 0009-3084/92/$05.00 © 1992 Elsevier Scientific Publishers Ireland Ltd. Printed and Published in Ireland