Iron chelators of the pyridoxal 2-pyridyl hydrazone class. Part 4. pKa Values of the chelators and their relevance to biological properties

Abstract

The proton binding constants (pK_a) and species distribution over pH range 1.5–12.0 of two types of biologically active iron chelators (a) pyridoxal type (L_x)—pyridoxal 2-pyridyl hydrazone (PPH) and pyridoxal isonicotinoyl hydrazone (PIH); (b) pyridoxal-betaine type (L_y)—1-[N-methylpyridoxylidenium]-2-[2′-pyridyl] hydrazine iodide (M PH) and 1-[N-ethoxy-carbonylmethylpyridoxylidenium]-2-[2′-pyridyl] hydrazine bromide (EPH) have been determined by glass electrode potentiometry. The lowest pK value in type (a), in the range 2.62 (PPH)–2.45 (PIH) was assigned to pyridinium protonation; the following ionization constants, pK_a2= 4.63 (PPH)–4.54 (PIH), to pyridoxylidenium protonation; pK_a3= 7.96 (PPH)–7.44 (PIH), to phenolate protonation, and pK_a4= 9.96 (PIH)-9.84 (PPH) to amine-hydrazone protonation. At pH < 2, all ligands exist in the respective protonated forms (H₄L_x²⁺, H₃L_y²⁺ and H₃L_x⁺) and at pH > 11, in the fully deprotonated forms (L_x²⁺, and Ly^–). At pH ca. 5.0, the pyridoxal-betaines, MPH and EPH, exist predominantly as zwitterions, whereas PPH and PIH are present at that pH predominantly in the neutral, non-zwitterionic, H₂L_x form. At higher pH (7.2), PPH and PIH, are present as mixtures of the neutral and the negatively charged monodeprotonated forms (IV).