Limited rotational motion of amphiphilic flavins in dipalmitoylphosphatidylcholine vesicles

The rotational motion of amphiphilic flavins in dipalmitoyl phospholipid bilayers was investigated with fluorescence anisotropy decay measurements. At temperatures between 10 and 50 degrees C the rotation proved to be anisotropic, which indicated composite motion of both the aliphatic side-chain and the isoalloxazine moiety of the octadecyllumiflavin derivatives. Above the phase transition temperature (crystalline leads to liquid-crystalline state) the depolarization is complete within the average flavin fluorescence lifetime, implicating unrestricted motion and resulting in a non-ordered microenvironment. In the gel or crystalline state the flavin motion can best be characterized as a limited rotation or librational motion. The fluorescence decay of the flavins is heterogeneous at temperatures between 10 and 50 degrees C, which is explained by assuming nanosecond relaxation of the polar phosphatidyl head-groups around the excited flavin. The lack of a significant cholesterol effect suggests that the isoalloxazine is located at the interphase of the bilayer and not in the hydrocarbon region. The microstructure is fluid-like, not in agreement with a preferred static localization of the flavins in the bilayer.