Study of exotic chlorophylls in photosynthesis

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We found a novel Zn-containing bacteriochlorophyll in an aerobic bacterium Acidiphilium rubrum growing under acidic conditions. We have unambiguously determined the molecular structure of this pigment by HPLC, optical spectroscopy, circular dichroism, fast atom bombardment mass spectrometry and ィイD11ィエD1H-NMR measurements. The structure of the pigment was confirmed to be identical to that of bacteriochlorophyll a esterified with phytol, except that the central metal is zinc (Zn) instead of magnesium (Mg).The major pigments of both antenna systems and reaction centers (RCs) are chlorophylls (Chls) in higher plants and bacteriochlorophylls (BChls) in photosynthetic bacteria. Chls and BChls have been unquestionedly believed to be Mg complexes of substituted chlorin or bacteriochlorin macrocycles, with a sole exception of metal-free Chl a and BChl a, called Phe a and BPhe a, that act as primary electron acceptors in photosystem 2 of higher plants and in the RC of purple bacteria. However, we found a novel BChl a containing Zn as the central metal in an aerobic bacterium Acidiphilium rubrum growing under low pH. We discussed the reason for the choice of Zn as the central metal in A. rubrum, referring to such physicochemical behaviors of Zn-BChl a as the spectroscopic and redox properties.Zn-BChl a from Acidiphilium rubrum was compared with the normal BChl a from Rhodobacter sphaeroides for the pheophytinization (de-metalation) rate in aqueous acetone containing HCl. Zn-BChl a was pheophytinized roughly 10ィイD16ィエD1-fold more slowly than BChl a. The high resistance of Zn-BChl a to acid may be one of the reasons why A. rubrum, living in media of low pH, chose Zn instead of Mg as the central metal.