- Iron Chelators of the Pyridoxal 2-Pyridyl Hydrazone Class. Part 4. pKa Values of the Chelators and their Relevance to Biological Properties
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The proton binding constants (pKa) and species distribution over pH range 1.5-12.0 of two types of biologically active iron chelators (a) pyridoxal type (Lx)-pyridoxal 2-pyridyl hydrazone (PPH) and pyridoxal isonicotinoyl hydrazone (PIH); (b) pyridoxal-betaine type (Ly)-1--2-hydrazine iodide (MPH) and 1--2-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 = 4.63 (PPH)-4.54 (PIH), to pyridoxylidenium protonation; pK=7.96 (PPH)-7.44 (PIH), to phenolate protonation, and pK=9.96 (PIH)-9.84 (PPH) to amine-hydrazone protonation.At pH2+x, H3Ly2+ and H3L+x) and at pH>11, in the fully deprotonated forms (L2+x, 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, H2Lx form.At higher pH (7.2), PPH and PIH, are present as mixtures of the neutral and the negatively charged monodeprotonated forms (IV).
- Doungdee, Prayong,Sarel, Shalom,Wongvisetsirikul, Nipon,Avramovici-Grisaru, Shelly
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p. 319 - 324
(2007/10/02)
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