Band-shape analysis and resolution of electronic spectra of pyridoxal 5′-phosphate with amino acids

Abstract

Formation equilibria of Schiff bases of pyridoxal 5′-phosphate (PLP) and different amino acids have been studied over a wide range of pH. The acid dissociation constant and absorption spectrum of every ionic species have been calculated. The latter has been resolved into components with log-normal curves, to provide a precise description of the band shapes, the peak positions and the area under the curves, which allowed the estimation of tautomerization constants and microscopic acid dissociation constants. Because of the acidity of the α-hydrogen of amino acids, α-amino acid Schiff bases are in equilibrium with quinonoid forms which are hydrolysed to yield pyridoxamine 5′-phosphate (PMP). This process could explain the bathochromic shifts of the peak position in α-amino acid Schiff bases to the corresponding band in non-α-amino acid systems.