Hydrophobicity and sorption of chlorophenolates to lipid membranes

We have studied sorption of ionized species of chlorophenols and pentahalophenols to lipid membranes using egg-phosphatidylcholine (egg-PC) vesicles and measuring their zeta-potential as a function of aqueous concentration of the phenolates. The zeta-potential isotherms can be understood in terms of a sorption model that is a combination of the Gouy-Chapman model of the electrical double layer at the membrane-water interface and the Langmuir model for sorption. Two intrinsic sorption parameters were determined: the linear partition coefficient, βm , which relates the membrane surface density of the phenolates to their aqueous concentration and the area of the adsorption site, Ps. The linear partition coefficient is the measure of the affinity of phenolates to the lipid membrane. It depends strongly on the molecular structure: 2,6-dichlorophenolate βim=(0.45±0.08)×10−7m; 3,5-dichlorophenolate βm=(0.22±0.02)×10 m; 2,4,6-trichlorophenolate βm=(0.63±0.06)×10−6 m; 2,4,5-trichlorophenolate βm=(0.11±0.01)×10−5 m; 2,3,5,6-tetrachlorophenolate βm=(0.56±0.07)×10−5 m; 2,3,4,5-tetrachlorophenolate βm=(0.55±0.06)×10−5 m; pentachlorophenolate βm=0.34±0.05)×10−4 m; pentachorophenolate βm=(1.00±0.13)×10−7 m and pentabromophenolate βm=(0.19±0.04)×10−3 m. Ps was found to be independent of phenolate structure, Ps= 3.3±0.1 nm2. The membrane affinity of chlorophenolates was compared with the octanol-water partition coefficients of un-ionized chlorophenols. It was shown that the free energy of transfer of chlorophenolates from water into the lipid membrane can be divided into non-electrostatic and electrostatic contributions. The no-nelectrostatic contribution corresponds to the hydrophobicity parameter α=−3.94±0.0.08 kcal per nm2 of molecular surface area. The electrostatic contribution contains a term inversely proportional to the molecular radius of the phenolate ion which has the physical meaning of the work of transfer of the phenolate ion from water into the membrane. The polarity of the sorption region of egg-PC membranes is given in terms of the dielectric constant and was estimated to be 12.4 (range 10.5–13.4).