Carbon-13 nmr spectra of vitamin B6 compounds: Aqueous solution equilibria and determination of microscopic acid dissociation constants

The aqueous solution equilibria and solute structure of vitamin B₆ compounds and several model compounds have been investigated using ¹³C-nmr spectroscopy. The unsubstituted α-carbon of these compounds is a very good probe for data which permits assignment of the ionization steps to indicidual groups. While the ionizations of the pyridinium and phenolic groups take place simultaneously in 3-hydroxypyridine, they take place in well-separated steps in pyridoxamine (PM), pyridoxamine phosphate (PMP), and pyridoxal phosphate. It has been established that the ionization with a pK_a value of 3.7 is predominantly phenolic in origin in PM and PMP. A zwitterionic structure consistent with the earlier spectroscopic investigations is proposed for the vitamin B₆ compounds in neutral aqueous solution.