Nickel(II) interactions with chlorophylls in solution: impact to degradation induced by UV-irradiation

Abstract

Chlorophylls and metal substituted complexes of chlorophylls with known photosensitizing abilities can be used in many types of biological and solar energy applications; thus, it is important to analyze their basic properties when they are treated by visible light and/or UV radiation. Nickel(II) can replace central magnesium from chlorophyll (Chl) molecule to form a metal substituted, “central” Ni-Chl complex. So, the possible formation of nickel-chlorophyll complexes in 95% ethanol at 40 °C was studied by UHPLC-DAD chromatography (UltraHigh Performance Liquid Chromatography–Diode Array Detector). Nickel(II)-chlorophylls interactions in the reaction solutions at 40 °C lead not only to the formation of the Ni-related Chls, Ni-Chl complexes but several oxidation and demetalation products of chlorophylls. On the other hand, the influence of nickel(II) – chlorophyll interactions on chlorophylls solutions stability under continual UV-A, -B, and -C irradiation in 95% ethanol, was followed by UV-Vis spectrophotometry. Chlorophylls as well as their reaction solutions with nickel(II) undergo photochemical degradation obeying first- order kinetics. In general, the degradation is “energy-dependent”, i.e. proportional to the UV photons energy input. Chlorophylls are less stable than their reaction solutions with nickel(II), probably due to the higher stability of Ni-chlorophylls complexes, to all three subranges of UV- irradiation.