Organization and dynamics of NBD-labeled lipids in lipid bilayer analyzed by FRET using the small membrane fluorescent probe AHBA as donor.

Fluorescent probes are employed to investigate natural and model membranes. It is important to know probe location and extent of perturbations they cause into the lipid bilayer. Förster Resonance Energy Transfer (FRET) is a useful tool to investigate phenomena involving plasma membranes, and reports in literature used relatively large fluorophores like 1,6-diphenylhexatriene, located at the center of the hydrophobic region, 4-aminophthalimide-based molecules located at lipid/water interfaces and BODIPY-labeled phosphatidylcholine. In this work we explored FRET process in 1,2-dimyristoyl-L-α-GPC large unilamellar vesicles, in gel and fluid phase, using as donor the very small group o-Abz bound to hexadecyl chain (2-amino-N-hexadecyl-benzamide - AHBA) and 7-nitro-2-1,3-benzoxadiazol-4-yl (NBD) labeled lipids as acceptor. From the intensity decay of donor in presence of acceptors, the FRET efficiency was calculated, and used to fit the model proposed by Fung and Stryer to that efficiency. Using lipid bilayer structural data, the procedure allowed the determination of Förster distance for each donor-acceptor pair in vesicles, without imposing any value for the orientational factor κ. From distance distributions between o-Abz in AHBA and NBD in lipid bilayer obtained using the program CONTIN, we obtained donor-acceptor populations having different separation distances. The populations reflect the occurrence of FRET involving probes in the same or in opposite leaflet. A dynamic picture emerged showing how relative position of the probes is dependent on the structural thermal phase of the DMPC bilayer. The results emphasize the need of careful analysis in order to understand processes involving fluorescent probes in model membranes.