Steady‐state and time‐resolved fluorescence anisotropy study of phospholipid molecular motion in the gel phase using 1‐palmitoyl‐2[9‐(2‐anthryl)‐nonanoyl]‐sn‐glycero‐3‐phosphocholine as probe

1‐Palmitoyl‐2‐[9‐(2‐anthryl)‐nonanoyl]‐sn‐glycero‐3‐phosphocholine (Anthr‐PC), a non‐perturbing phospholipid probe [de Bony, J. and Tocane, J. F. (1983) Chem. Phys. Lipids 32, 105–121], has been designed in order to obtain insight into the membrane lipid organization at a ‘microscopic’ level, in terms of lateral distribution both in model and in natural membranes [de Bony, J. et al. (1984) Eur. J. Biochem. 143, 373–379; FEBS Lett. 174, 1–6]. In the present study, the molecular motions of this new fluorescent probe embedded in a lipid matrix have been investigated by fluorescence anisotropy techniques in steady‐state and time‐resolved modes. The results indicate that long axis rotation, monitored by the out‐of‐plane mode of rotation of the fluorophore, is fast even in the phospholipid gel state. It is moderately sensitive to the phase transition. The data suggest that this rotation is anisotropic. Cholesterol exhibits little effect on this rotation. The rotation of the long axis itself is sensitive to the transition. It is hindered as inferred from measurements at wavelength where both the in‐plane and out‐of‐plane motions contribute to the depolarization of the emitted fluorescence light. Cholesterol restricts this motion. The behaviour of the free 9‐(2‐anthryl)‐nonanoic acid is not significantly different from that of Anthr‐PC. These results are discussed with respect to the influence of orientational constraints on the photodimerization process when this lipid probe is used to monitor phospholipid lateral distribution.