Theoretical pattern of the selective hydration and tautomerization effects on pyridoxal and pyridoxamine 5′-phosphate UV-visible spectra: a tool to band assignment

Abstract

The geometries of pyridoxamine 5′-phosphate (PMP) tautomers and pyridoxal 5′-phosphate (PLP) Schiff's bases with glycine were optimized at the AMl level. The SCF molecular orbitals and the monoelectronic transitions were calculated using the CNDOL hamiltonian. The most probable transitions in each structure were selected by taking into account the value of their molar absortivity, obtaining a similarity with respect to the experimental data. The influence of the solvent in the electronic spectra was studied by the supermolecular approach, placing water molecules on the most relevant positions. A relation has been found between the band shifts of the spectra and the quantity and position of water molecules in the studied systems. It was determined that the carboxylate and phosphate groups do not have a significant participation in the studied region of the spectrum. The spectrum bands were assigned to possible electronic transitions in various tautomers. The appearance of a band near the 415nm was attributed to those structures that have protonated the imine group, and a hidden band around the 265nm appears when the pyridine nitrogen is protonated.