Photoinduced processes in vitamin B6 compounds.

[eng] In this work, we applied multi-wavelength stopped-flow spectroscopy (MSFS) to study the chemical equilibria between tautomeric or hydrated forms of various vitamin B6 compounds and the Schiff base formed by ε-aminocaproic acid (=6-aminohexanoic acid) with pyridoxal 5′-phosphate at 25° and variable pH. Since some of these compounds are photosensitive, we analyzed the possible occurrence of any secondary photo-induced processes under the conditions of irradiation in the MSFS equipment (continuous irradiation with light from a 75-W Xe lamp spanning the wavelength range of 200700 nm). To determine the tautomeric composition of these compounds, the electronic absorption spectra were analyzed by means of log-normal curves. Continuous irradiation of pyridoxamine and pyridoxal 5′-phosphate over the wavelength range of 200700 nm displaces the chemical equilibrium between the tautomeric or hydrated forms of these compounds. However, the Schiff base of ε-aminocaproic acid with pyridoxal 5′-phosphate is insensitive to the radiation used. The photo-induced processes detected in pyridoxamine and pyridoxal 5′-phosphate should be taken into account in examining vitamers by MSFS. In fact, these additional processes should be considered in studying the mechanism of action of vitamin B6-dependent enzymes by the MSFS technique, whenever some free vitamer may be present in solution