Determination of lipophilicity by means of reversed-phase thin-layer chromatography III. Study of the TLC equations for a series of ionizable quinolone derivatives

The RM values of a series of antibacterial quinolones were measured at pH 9.0 and 1.2 using a reversed-phase TLC system with acetone, methanol or acetonitrile as the organic modifier of the mobile phase and silicone DC 200 as the impregnating agent of the silica gel layer. The data obtained provide a further contribution to the assessment of the basic aspects of the chromatographic determination of lipophilicity for ionizable compounds. The very good correlations between experimental and extrapolated RM values support the validity of the extrapolation technique. The overlapping of the extrapolated RM values from three different systems show that they are not dependent on the nature of the organic solvent. In a series of congeneric compounds there is a relationship between intercepts (a) and slopes (b) of the TLC equations. Factors affecting chromatographic congenerity are discussed. The slopes of the TLC equations and those of the equations correlating the parameters a and b are related to the solvent strength of the organic modifiers. © 1994.