Distribution and diffusion of alamethicin in a lecithin/water model membrane system

Attenuated total reflection infrared spectroscopy has been used to determine the equilibrium distribution of the peptide antibiotic alamethicin RF30 between dipalmitoyl phosphatidylcholine bilayers and the aqueous environment. The distribution coefficient K = cWeq/cMeq turned out to be concentration dependent, pointing to alamethicin association in the membrane with increasing concentration in the aqueous phase (cWeq). This concentration was varied within 28 and 310 nM, i.e., in a range typical for black film experiments. Furthermore. diffusion coefficients of alamethicin in the hydrophobic phase of the membrane (DM) and across the membrane/water interface (DI) have been estimated from the time course of the equilibration process. It was found that the diffusion rate of the uncharged analogue RF50 is about 10 times higher than that of the RF30 component, exhibiting one negative charge at the C-terminus. The time constants for transmembrane diffusion of alamethicin RF30 varied between 2.2hr at low concentration and 3.2 hr at higher concentration. The corresponding low concentration value of the RF50 component was found to be 0.25 hr.