CHLOROPHYLL PHEOPHYTINIZATION UNDER THE INFLUENCE OF METAL CHELATORS OF THE PYRIDINE SERIES

Metal chelators of the pyridine series - 1, 10-phenanthroline (Ph) and 2,2'-dipyridyl - are shown to be capable of removing magnesium from the porphyrin ring of the Chl molecule in 9% aqueous acetone in the dark. The process transpires most efficiently under the influence of Ph. When Chl is used in a concentration of 2.5-9. 0 nmoles.ml-1, only Chl a (not Chl b) is pheophytinized under the influence of 30 mM Ph. Pheophytin is also capable of pheophytinizing Chl a in bean leaves shaded after treatment with the chelator. It is hypothesized that photodynamic injury to plants exposed to light 17 h after treatment with Ph results from dark accumulation under its influence of both Chl precursors -protoporphyrin IX, Mg-protoporphyrin IX, and its monomethyl ester, which are active sensitizers of photodestructive processes - and the product of Chl decomposition-pheophytin molecules, which apparently can also sensitize formation of singlet oxygen in vivo.