Observation of the phase transition in phospholipid liposomes taking advantage of the particular optical properties of octa-α-butyloxy-H2phthalocyanines

In this work we report on the influence of the polarity of the environment on the optical properties of octa-alpha -butyloxy-H(2)phthalocyanine (H2PC(OBu)(8)) in organic solvents and liposomes using steady-state absorption as well as time correlated single photon counting (TCSPC). A change was found in Q-band absorption in various solvents. Taking into account that the absorption properties of H2PC(OBu)(8) are modulated by the polarity of the direct surrounding, polarities inside micro-heterogeneous systems can be probed. This was demonstrated for liposomes of dipalmitoylphosphatidylcholine (DPPC), with regard to the behavior during the phase transition between the gel and liquid-crystal state. Absorption spectra of H2PC(OBu)(8) embedded in DPPC-liposomes have been recorded in the temperature range from 15 to 70 degreesC, covering the phase transition region at about 36-39 degreesC. The Q-band absorption at temperatures below and above the phase transition resembles, those in polar and non-polar solvents, respectively. Measurements of fluorescence lifetime confirm this equivalence. We explain this behavior by a change of the localization of the dye molecules inside the membrane. For H2PC(OBu)(8) embedded in DPPC-liposomes we demonstrated by using optical methods that the position of the dye molecules strongly depends on the phase-state of the phospholipid. In addition, H2PC(OBu)(8) can be used as a probe for phase transition processes taking advantage of the specific behavior in surroundings with different polarity. Copyright (C) 2001 John Wiley & Sons, Ltd.