A laser-T-jump study of the adsorption of dipolar molecules to planar lipid membranes. II. Phloretin and phloretin analogues.

Phloretin and structurally related neutral molecules adsorb to the interface of lipid membranes and modify the electric dipole potential of the membrane/water interface. The adsorption process has been studied using a laser-T-jump relaxation technique in combination with an analysis of nonactin mediated potassium transport (see part I, Awiszus and Stark 1988). Deviations from the Langmuir isotherm were observed for most of the substances. The discrepancies were most pronounced at large surface densities, whereas good agreement was found at low concentrations in many cases. The partition coefficient in the limit of low concentrations was compared with that of octanol/water bulk phases. No correlation was found. The individual values of the two partition coefficients differed by more than three orders of magnitude. The contribution, b, of a single adsorbed molecule to the dipole potential could not be predicted from the dipole moment, microL, of the molecule measured in the bulk phase. Different values of b were found at identical values of microL. The study shows the limitations of the use of bulk phase data to predict molecular properties in lipid membranes.