Figure 4.4 (a) Lipophilicity profile of an ampholyte at two values of background salt and (b) log-log speciation plot at 0.15 M KCl. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]

Figures 4.2b, 4.3b, and 4.4b are log-log speciation plots, indicating the concentrations of species in units of the total aqueous sample concentration. (Similar plots were described by Scherrer [280].) The uppermost curve in Fig. 4.2b shows the concentration of the uncharged species in octanol, as a function of pH. If only uncharged species permeate across lipid membranes, as the pH-partition hypothesis suggests, then this curve deserves attention, perhaps more so than the log D curve (unless the active site is in the apical membrane outer leaflet of the epithelial cell surface, where permeation of the membrane by the charged species is not necessary). That curve is like that of the log D curve, but with the ion-pair component removed.


The octanol-water partitioning behavior of orally active quaternary ammonium drugs (which are always charged in the physiological pH range), such as propantheline, trantheline, homidium, and neostigmine, was reported by Takacs-Novák and Szasz [291]. Propanetheline has 10% oral absorption, whereas neostigmine is very poorly absorbed from the GIT [370]. Consistent with this, the octanol-water logP of the bromide salts range from —1.1 to <—3 [291]. However, in the presence of a 50-fold excess of the bile salt deoxycholate, the homidium apparent partition coefficient, logP, elevates to +2.18. Similarly heightened numbers were seen when the quaternary drugs were combined with prostaglandin anions, suggesting a possible role of endogenous lipophilic counterions in the GI absorption of the quaternary ammonium drugs.


Ion pair partitioning effects with simple salts should no longer be surprising, given the examples presented above. Partitioning of multiprotic molecules, however, warrants additional consideration. The partitioning behavior of charged molecules, including zwitterions (peptide and other kinds) and ordinary ampholytes, has been intriguing [229,276,278,282,283,285-289,371]. These molecules are sometimes charged over the physiological pH range. Scherrer proposed a classification system for ampholytes based on their pKa-pKoct relationships [276]. It is an important topic to understand, since the oral absorption of such molecules can be poor, and methods to overcome it are the focus of many efforts.

When the log D/pH measurement of a peptide is performed by the shake-flask or the partition chromatography method (using hydrophilic buffers to control pH), usually the shape of the curve is that of a parabola (see Ref. 371 and Fig. 1 in Ref. 282), where the maximum log D value corresponds to the pH at the isoelectric point (near pH 5-6). Surprisingly, when the potentiometric method is used to characterize the same peptide [275], the curve produced is a step function, as indicated by the thick line in Fig. 4.5 for dipeptide Trp-Phe.

Both results (parabola vs. step) are correct, even though there is a big difference in the profiles. The explanation for the difference lies in charged-species partitioning: the counterion (from background salt or buffer) plays an ineluctable role. In the potentiometric method, pH is controlled by adding HCl or KOH, to a solution that has a 0.15 M physiological level of salt (KCl or NaCl). Thus, the partitioning

Lipophilicity Profile of Zwitterionic Species trpyptophanylphenylalanine 0.0 M KCl

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