Photo‐Induced Processes in Vitamin B₆ Compounds

Abstract

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 epsilon-aminocaproic acid (= 6-aminohexanoic acid) with pyridoxal 5'-phosphate at 25 degrees 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 200-700 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 200-700 nm displaces the chemical equilibrium between the tautomeric or hydrated forms of these compounds. However, the Schiff base of epsilon-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.

Impact of the ionic forms on the UV–Vis spectra 2‐hydroxybenzylamine. A TD‐DFT study

“…Bands II and IV for the ionic form HB, which can be assigned to the tautomer HB(0), are virtually identical with bands I and II for H 2 B þ ; consequently, the ionization status of N1 00 has little influence on molecular E exc values. These conclusions are consistent with others previously drawn for aromatic amines [24,31,33].…”

“…number of bands and their position) were taken to be the theoretical results provided by TD-DFT calculations. As in previous work, the number of variables to be optimized was minimized by using W ¼ 3.5 Â 10 3 cm À1 and q ¼ 1.3 for all bands with E exc > 5.1 eV [4,24,25,31,37,57]. Table III gives the excitation energy and the band width, asymmetry, area, and intensity (e) obtained from the fitting.…”

“…Band II, centered at 4.83 eV, was a weak, submerged band used for improved fitting. Submerged bands, which have been widely used to facilitate the fitting of valleys in spectral deconvolution, are rarely associated to specific electronic transitions [4,24,25,31], so they have been omitted in Table III. As can be seen from this table and Figure 4(B), the B3LYP excitation energies for the anionic species were similar to their experimental counterparts.…”

“…Despite its significance, little quantitative information about its tautomeric equilibria and the electronic properties of its minor tautomers is still available. The energy of its first p-p* transition has been determined from changes with the polarity of the medium [24] and photo-induced processes [31].…”

Phenol Group in Pyridoxamine Acts as a Stabilizing Element for Its Carbinolamines and Schiff Bases

“…The difference of 0.5 -0.6 units from the values obtained in H 2 O can be ascribed to the isotopic effect of D on ionization constants as previously found by other authors in weak acids and bases [23] [24]. In aqueous solutions, the major tautomers for H 2 PM þ and HPM are the zwitterionic forms (a and d in Scheme 1) [25]. It should be noted that NMR data did not allow micro-pK a to be calculated.…”