Phloretin and phloretin analogs: Mode of action in planar lipid bilayers and monolayers

Phloretin and other neutral phloretin-like molecules are able to decrease the electrostatic potential within neutral lipid bilayers and monolayers. The relationship between the change in the dipole potential and the aqueous concentration of the molecule is well described by a Langmuir isotherm. From the Langmuir isotherm, the apparent dissociation constants (K_D^A) and the maximum dipole potential change (ΔΔψ_max) are obtained for the different phloretin-like molecules tested. Considering the phloretin analogs as derivatives of acetophenone containing two kinds of substituents, one on the benzene ring and another on the carbon chain, it is found that (a)K_D^A is related to the hydrophobicity of the compound and is also a function of the position of the hydroxyl substituent in the ring; (b) from the dependence of K_D^A on the length of the acyl chain, it is estimated that the free-energy change is ∼650 cal/mole CH₂; (c)ΔΔψ_max is not a simple function of the dipole moment of the molecule but depends on the substituent on the carbon chain and on the position and number of hydroxyl groups on the benzene ring; (d) phloretin adsorption parameters are a function of membrane lipid composition. The results are discussed in terms of the effect of these compounds on chloride transport in red blood cells.