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Potentiometric Determination of Acid Dissociation Constants (pK a) for an Anticancer Pyrrole-Imidazole Polyamide.

To optimize the pharmacological properties of an anticancer pyrrole-imidazole (Py-Im) polyamide (PIP-1), we characterized the acid dissociation constants of PIP-1, three other structurally related hairpin-shaped polyamides, and a cyclic polyamide bearing the same core sequence as PIP-1 via potentiometric titration. The acidities of the carboxylic acid at the C-terminus and the tertiary amine in the triamine linker remained very similar among the polyamides tested, whereas the pK a of the N-methylimidazole (Im) moieties varied with the peptide sequence and molecular architecture. A nearly 0.2 pH unit pK a shift of terminal Im toward the neutral state compared to internal Im was observed. Furthermore, according to the dissociation constants, a speciation diagram of PIP-1 as a function of pH is presented, which allows an assessment of the net charge and distribution of protonated species in the range of physiological pH.