METHODS FOR THE DETERMINATION OF PARTITION-COEFFICIENTS BASED ON THE EFFECT OF SOLUTES UPON MEMBRANE-STRUCTURE

General procedures are presented for the determination of membrane-water partition coefficients (Kp) based on the measurement of effects that depend on the extent of partitioning. Two cases are deal with: (1) the general case when the effect is an unknown function of the total number of moles of partitioning solute (nT) and (2) the particular case when the effect is a linear function of nT. The procedure involves the evaluation of the effect as a function of nT for a variety of membrane volumes (VM), at constant volume of aqueous phase (VH), generating a family of curves. It is shown that when a given set of nT, VM produces the same effect, which indicates that the concentration of the solute in the membrane (nM/VM) is the same for the whole set, a plot of nT versus VM yields a straight line that allows the calculation of KP. In the particular case when the effect is linearly proportional to nT, the higher VM, the smaller the slope. In this case, these slopes can be used to calculate KP by plotting their inverse as a function of VM. These procedures were applied to the partitioning of a series of compounds, some of them with known KP, in egg phosphatidylcholine (EPC) membranes. The interaction of those compounds with the membrane was monitored either by fluorescence spectroscopy (by measuring either the excimer/monomer ratio or the quenching of intensity of an intercalated probe) or by ESR making use of a lipid spin probe. Compounds with known KP yielded values in good agreement with previously found ones. In principle, the procedures are applicable whenever it is possible to measure a property that depends on the extent of partitioning, allowing the determination of KP from raw data. In particular, it is very adequate for the use of spectroscopic techniques, one main advantage being that no separation of membrane and aqueous phases is required. © 1990.