Probing phospholipid main phase transition by fluorescence spectroscopy and a surface redox reaction

Headgroup and acyl chain NBD-labeled phospholipid derivatives, viz. 1,2-dip almitoyl-sn-glycero-3-phospho-[N-(4-nitrobenz-2-oxa-1,3-diazole)-ethanolami ne] (DPPN) and 1-acyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodeca noyl]-sn-glycero-3-phosphocholine (NBD-PC), were employed to study lipid co nformations and organization in the course of the main phase transition of dipalmitoylglycerophosphocholine (DPPQ liposomes. The differential sensitiv ity of the 335 and 470 nm absorption bands to environment polarity was used to explore the ground-state NBD environment. Anisotropy measurements with excitation at those bands revealed the presence of two NBD-PC populations b elow T-m. The fluorescence maximum emission intensity, the ratio of emissio n intensities with excitation at 470 and 335 run, and the rate coefficient k(1) for the reduction of NBD by dithionite to nonfluorescent derivatives u ndergo complex changes in the course of the main phase transition of DPPC. For DPPN, the alterations in k(1) and intensity ratio are likely to origina te from a redistribution of this fluorophore at the transition and suggest a maximum in the bilayer transverse compressibility. For NBD-PC, the change s in these properties appear to be related to a shift in the fluorophore di stribution at the percolation threshold for the two-phase region and self-q uenching of the NBD moiety.