Spectroscopic Properties of 4‐Pyridoxic Acid as a Function of pH and Solvent

Abstract

The photophysical properties and acid/base equilibria of 4-pyridoxic acid ( 3-hydroxy-5-(hydroxymethyl)-2-methylpyridine-4-carboxylic acid), the final product of the catabolism of vitamin B 6 , have been studied in aqueous solutions. The ground state of 4-pyridoxic acid exhibits the different protonated forms A ± D in the range of H 0 À 6 to pH 11.5. HMQC-and HMBC-NMR Studies allowed the pH-dependent assignment of the different C-atoms, and the evaluation of the deprotonation sequence. The 3-OH group in the ground state has a −pK a × of H 0 À 0.64, which is much lower than that found for other vitamin B 6 related compounds. The pK a value of the 4-COOH group is 5.4. Fluorescence studies showed that the same species exist at the lowest excited singlet state, but in different pH ranges. The 3-OH group is four pH units more acidic in the lowest excited singlet state than in the ground state. Excitation spectra and emission decays in the pH range of 8 to 11.5 indicate that the pyridine N-atom is more basic in the excited singlet state than in the ground state. The emission spectra are red-shifted in protic solvents, in agreement with an intramolecular H-bond between the ionized 3-OH group and the nonionized 4-COOH group.

Photophysical Properties of the Prefluorescent Nitroxide Probes QT and C343T

“…The pK a values of 3.7 and 7.4 were assigned to the deprotonation of the phenol moiety in form (A) and the pyridinium nitrogen in form (C), respectively. These pK a values are of similar magnitude to those determined for the structurally related compounds pyridoxamine and 4-pyridoxic acid (37,38).…”

Untargeted Metabolomics Identifies Enterobiome Metabolites and Putative Uremic Toxins as Substrates of Organic Anion Transporter 1 (Oat1)

“…Most of the metabolites are clearly organic anions whereas others, such as 4-pyridoxic acid, are likely to be so at physiological pH. 35 Several of these metabolites have been shown to directly interact with the Oat1 transporter by in vivo and in vitro assays. The identified known and novel compounds, from the untargeted metabolomic analysis have allowed us to design Oat1 metabolite-specific pharmacophores using computational modeling methods and predicted further compounds with the pharmacophore-directed search of the NCI database.…”

Photophysics and Photochemical Studies of the Vitamin B6 Group and Related Derivatives

“…Pyridoxamine showed that in solvents of intermediate proton‐donor power like alcohols and methylformamide both IIa and IIb species coexist with comparable proportion (24). Pyridoxic acid does not undergo tautomerization (25). In this case, the lack of tautomerization is due to the stabilization of the phenolic oxygen by intramolecular hydrogen bonding with the carboxylic group.…”