Biochimica et Biophysica Acta vol:1104 issue:1 pages:63-72
The interaction of the tetramisole derivative (+/-)-5,6-dihydro-6-phenyl-imidazo[2,1-b]thiazole and a number of its 2-n-alkyl homologues (-ethyl through -n-pentyl and -n-heptyl) with large unilamellar phosphatidylcholine/phosphatidylethanolamine/dipalmitoylphosphatidic acid (2:1:0.06, w/w) vesicles was studied by means of steady-state fluorescence quenching using 8-(2-anthryl)octanoic acid as membrane probe. Linear Stern-Volmer plots were obtained for each derivative, indicating dynamic quenching. The slopes of the plots decreased with increasing liposomal concentration. For four short-chain homologues (-H, -ethyl, -n-propyl and -n-butyl), the respective membrane partition coefficients K(p) and bimolecular quenching rate constants k(q) were determined from the plots of the reciprocal of the apparent quenching rate constant (k(q)app)-1 against the lipid volume fraction alpha(L) of the liposomes. The partition coefficients increased with increasing chain-length of the tetramisoles. A linear relationship was found between the free energy of partitioning and the number of methylene units of the homologues (-DELTA-G-degrees per methylene group = 1.6 +/- 0.1 kJ mol-1). For the n-pentyl and n-heptyl derivatives, the fluorescence quenching technique did not allow one to determine their membrane partition coefficients. Analysis of the fluorescence intensity measurements with Scatchard plots gave further evidence for the partitioning nature of the tetramisole derivatives' association with the liposomal membranes.