Carbon-13 Nuclear Magnetic Resonance Studies of Compounds of the Vitamin B6 Group and Related Pyridine Derivatives

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Abstract
The I3C chemical shifts and 13C–31P spin–spin couplings (where applicable) are reported for pyridoxine, pyridoxamine, pyridoxal, pyridoxamine phosphate, and pyridoxal phosphate. Resonance assignments are made by consideration of substituent chemical shift effects, as tested on an analogous series of pyridine derivatives. Increased shielding of the C4′ and C5′ methylene groups of the vitamin B6 compounds and the methyl groups of 3,4-dimethylpyridine relative to the predicted values are attributed to steric compression. In aqueous solution pyridoxal exists as the hemiacetal form, although at high pH it is in rapid equilibrium with a significant amount of the aldehyde form. Pyridoxal phosphate exists as the aldehyde at high pH, as the hydrated aldehyde at low pH, and in a slow equilibrium between detectable amounts of both species at pH 4. The 13C–31P couplings through two bonds lie in the range 4.5 ± 0.5 Hz found for a variety of other organic phosphates. The couplings through three bonds indicate a preference for a trans arrangement of the phosphorus and ring carbon-5.

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