A fast and reliable spectroscopic method for the determination of membrane-water partition coefficients of organic compounds

Partition coefficients (K-p) between Egg yolk phosphatidylcholine multilamellar vesicles and water were determined for two nonsteroidal anti-inflammatory drugs (indomethacin and acemetacin) using two independent methodologies: derivative spectrophotometry and variation of the experimental acidity constant in the presence of increasing vesicle concentration. Second-derivative spectrophotometry allowed for total elimination of background signal effects arising from lipid vesicles, without the need for separation techniques that may disturb equilibrium states. By using a model based on a simple partition, the values of K-p(T) can be obtained directly; furthermore, by performing determinations at two different pH values it is possible to calculate partition coefficients for the the neutral and negatively charged forms of the drugs (K-p(AH) and K-p(A)). In the other methodology, values of apparent acidity constants (K-app) were determined by spectrophotometry at different pH and different lipid concentrations, and an increase in K-app with decreasing lipid concentration was observed for both drugs, and from this dependence it was possible to calculate K-p(AH) and K-p(A-) for each drug. These values were used as a check those obtained by derivative spectroscopy, which has proven to be a reliable and more expeditious method to obtain K-p(AH) and K-p(A-).