Rates and equilibria of aldimine formation between pyridoxal 5′-phosphate and N-hexylamine

Abstract

The rate and equilibrium constants of the reaction between n-hexylamine and pyridoxal 5′-phosphate have been studied in aqueous solution as a function of pH at 25 ± 0.1 °C and at an ionic strength of 0.1. The pH profiles of the observed constants have been explained by protonation of the pyridoxal 5′-phosphate and its Schiff's base. This study shows that there is an intramolecular general acid catalysis of the Schiff's base formation, which can explain why the aldimine is formed at a reasonably high rate at neutral pH, in spite of the low amount of unprotonated reacting amine at this pH.