Copper Replacement of Magnesium in the Chlorophylls and Bacteriochlorophyll

Abstract

Copper chelates of chlorophylls “a” and “b” and an oxidized form of bacteriochlorophyll “a” were prepared and separated by an improved method of column and thin-layer chromatography, and their physical properties and thermodynamics involved in the primary metal replacement reaction were studied. In glacial acetic acid the Mg (II) ions of the photosynthetic pigments were replaced rapidly by Cu (II) ions at 40 — 100° and profound physical changes were noted in the chelation products. Copper chelates were not fluorescent while their parent pigments and pheophytins were. A general lowering of absorbance and a blue shift of absorption maxima were observed with the copper complexes. The molar absorptivity values of copper chelates were determined by the metallic microtitration method and the direct analysis of chelated copper by the oxalyldihydrazide (ODH) of copper method. In the present assay, the primary reaction of copper replacement of Mg (II) in the 3 photosynthetic pigments was the bimolecular SE2 type. The primary reaction lasted only a short time (1 — 5 min) at temperatures of 40 — 90°, and the higher the temperature, the larger the constants of the bimolecular reaction became. On longer treatment, the metal replacement reaction was complicated by the increasing content of pheophytin. The reaction rate constants became progressively smaller in the order of chlorophyll “a” — “b”-bacteriochlorophyll “a”. At 70° the half lives of 20 μΜ chlorophylls “a” and “b” and bacteriochlorophyll “a” for the copper replacement were 1.2, 15.2, and 117.4 minutes, respectively. Based on transition state theory, some thermodynamic constants relevant to this primary metal substitution reaction at various temperatures were calculated, and the possible mechanism involved were discussed.