NEW FLUORESCENT OCTADECAPENTAENOIC ACIDS AS PROBES OF LIPID-MEMBRANESAND PROTEIN-LIPID INTERACTIONS

The chemical and spectroscopic properties of the new fluorescent acids all(E)-8,10,12,14,16-octadecapentaenoic acid (t-COPA) and its (8Z)-is omer (c-COPA) have been characterized in solvents of different polarit y, synthetic lipid bilayers, and lipid/protein systems. These compound s are reasonably photostable in solution, present an intense UV absorp tion band (epsilon(350 nm) approximate to 10(5) M(-1) cm(-1)) strongly overlapped by tryptophan fluorescence, and their emission, centered a t 470 nm, is strongly polarized (r(0) = 0.385 +/- 0.005) and decays wi th a major component (85%) of lifetime 23 ns and a faster minor one of lifetime 2 ns (D,L-alpha-dimyristoylphosphatidylcholine (DMPC), 15 de grees C), Both COPA isomers incorporate readily into vesicles and memb ranes (K-p approximate to 10(6)) and align parallel to the lipids. t-C OPA distributes homogeneously between gel and fluid lipid domains and the changes in polarization accurately reflect the lipid T-m values. F rom the decay of the fluorescence anisotropy in spherical bilayers of DMPC and POPC it is shown that t-COPA also correctly reflects the lipi d order parameters, determined by H-2 NMR techniques. Resonance energy transfer from tryptophan to the bound pentaenoic acid in serum albumi n in solution, and from the tryptophan residues of gramicidin in lipid bilayers also containing the pentaenoic acid, show that this probe is a useful acceptor of protein tryptophan excitation, with R(0) values of 30-34 Angstrom.