Is Mg the Sole Central Metal of Chlorophylls ?

Except for metal-free chlorophylls (pheophytins) functioning as primary electron acceptors in purple bacteria and in photosystem II of higher plants, all the naturally occurring chlorophylls have been believed to be magnesium complexes. To clarify the reason for the choice of Mg as the central metal of chlorophylls, we have systematically studied the absorption, fluorescence and redox properties of metallo-substituted chlorophylls, and concluded that Zn-substituted chlorophylls may act as both antenna and primary electron donor of photosystems. Quite recently, we discovered novel photosynthesis using Zn-containing bacteriochlorophyll a in an acidophilic bacterium Acidiphilium rubrum. This finding indicates an unexpectedly wide variability of photosynthesis.