Determination of orientational order parameters in oriented lipid membrane systems by angle-resolved fluorescence depolarization experiments

Angle-resolved fluorescence depolarization experiments have been performed on some lipid membrane systems in order to test the validity of a recently developed theory describing this type of experiments. Good agreement between theory and experiment was found. The experiments yield the order parameters 〈P₂〉 and 〈P₄〉 and a rotational correlation time τ_c. On applying an information-theoretic form of the orientational distribution function. 〈P₂〉 and 〈P₄〉 were used to estimate the degree of order in the systems studied. Knowledge of 〈P₄〉 proved useful to get more detailed information on the orientational order. Another finding was that the angle between the absorption and emission moment of the used fluorescent probe, diphenyl-hexatriene, depends on the membrane system in which it is incorporated. Finally, the experimental results point to the fact that for some membrane systems the molecular motion is inadequately described by the simple strong-collision model.