Using the decay of incorporated photoexcited triplet probes to study unilamellar phospholipid bilayer membranes

The decay kinetics of triplet excited probes (meso-tetraphenylporphyrin or anthracene) in unilamellar phospholipid bilayer membranes was studied using nanosecond laser induced transient absorption spectroscopy. Two decay modes, quenching by molecular oxygen and triplet-triplet annihilation, were investigated as a function of temperature. The rates of both processes were sensitive to the phase of the membrane and underwent a significant change at the temperature corresponding to the main phase transition. The analysis of triplet annihilation data obtained in dimyristoylphosphatidylcholine or dipalmitoylphosphatidylcholine unilamellar vesicles, and mixtures thereof, demonstrates that the phase transition can be analyzed in these systems with an accuracy matching that of other techniques, including calorimetry; this accuracy applies to the determination of critical temperatures as well as to the estimation of transition cooperativity. Triplet quenching by molecular oxygen is shown to be largely determined by diffusion of oxygen from the bulk medium to the membrane. Triplet decay is thus a suitable probe of fluidity as well as of diffusional processes in unilamellar bilayers.