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

Summary Attenuated total reflection infrared spectroscopy has been used to determine the equilibrium distribution of the peptide antibiotic alamethicinR_F30 between dipalmitoyl phosphatidylcholine bilayers and the aqueous environment. The distribution coefficientK=c_eq^W/c_eq^M turned out to be concentration dependent, pointing to alamethicin association in the membrane with increasing concentration in the aqueous phase (c_eq^W). This concentration was varied within 28 and 310nm, i.e., in a range typical for black film experiments. Furthermore, diffusion coefficients of alamethicin in the hydrophobic phase of the membrane (D_M) and across the membrane/water interface (D_I) have been estimated from the time course of the equilibration process. It was found that the diffusion rate of the uncharged analogueR_F50 is about 10 times higher than that of theR_F30 component, exhibiting one negative charge at theC-terminus. The time constants for transmembrane diffusion of alamethicinR_F30 varied between 2.2 hr at low concentration and 3.2 hr at higher concentration. The corresponding low concentration value of theR_F50 component was found to be 0.25 hr.