Figure 4.5 Potentiometrically determined [162] lipophilicity profiles of a dipeptide, showing the effect of background salt concentrations. The unfilled symbols [282] and the filled symbols [371] are based on shake-flask measurements. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]

medium always has at least 0.15 M K+ and Cl— with which to associate into ion pairs. The effect of buffers in shake-flask or HPLC assays is not always taken into account in discussions of results. We can see in Figs. 4.2a and 4.3a, that the log D profiles take on different values when the background salt is reduced from 0.15 to 0.001-0.01 M. In Fig. 4.5, we indicate what happens to the log D curve when three different levels of salt are used. Very good match to the ''anomalous'' values, indicated by open and closed symbols, is found [282,371]. The upward turns in the dashed curve in Fig. 4.5 for pH >11.5 and <2.5 are due to the common-ion effect of the salt introduced by the titrant: K+ (from KOH) and Cl— (from HCl), respectively.

In studies of the salt dependence of peptides, an attempt was made to look for evidence of ion triplet formation [162], as suggested by the work of Tomlinson and Davis [278]. Phe-Phe-Phe was used as a test tripeptide, and it was reasoned that by performing the octanol-water partitioning in an aqueous solution containing different levels of salt (0.02-0.50 M KCl), one might see the zwitterion log P show the salt dependence that is to be expected of an ion triplet formation. None was evident (other than for the cation at low pH and the anion at high pH, as expected of simple ion-extraction reactions) [162]. An interesting explanation was suggested Dr. Miloñ Tichy [1995, unpublished], based on conformational analysis of the structure of the tripeptide in water, that Phe-Phe-Phe can form a cyclic structure, with an intramolecular (internally-compensated) electrostatic bond, (— CO2—.. .+NH3—), formed between the two ends of the molecule. A highly stabilized ring structure may be more stable than a K+ .. —O2C )—(NH3+.. Cl— ion triplet.

0 0

Post a comment