Photo-degradation of bacteriochlorophyll c in intact cells and extracts from Chlorobium tepidum

The photo-stability of bacteriochlorophyll c (Bchlc) in an acetone–methanol–water solution extracted from Chlorobium tepidum and in the chlorosomes of intact Chlorobium tepidum cells is studied. The photo-stability of Bchlc in the chlorosomes is high (quantum yield of photo-degradation φD≈8×10−7). In air-saturated acetone–methanol–water solution the photo-degradation of Bchlc is caused by chemical reaction with the dissolved oxygen, where triplet Bchlc produces singlet oxygen, and singlet oxygen reacts with Bchlc to form stable oxidized linear tetrapyrroles as well as intermediate oxidized bacteriochlorophylls. The intermediate photoproducts degrade to linear tetrapyrroles by the catalytic action of triplet oxygen. The initial quantum yield of photo-degradation is direct proportional to the concentration of Bchlc giving φD,0≈0.011 for a 7×10−6 M solution at an excitation wavelength of 672 nm. In nitrogen-bubbled solution the photo-degradation is dominated by direct photoproduct formation in the triplet state of Bchlc. Long-wavelength absorbing intermediate photoproducts degrade slowly in the dark to short-wavelength absorbing stable photoproducts. The quantum yield of photo-degradation of a de-oxygenated solution was found to be φD,0≈0.0012 for excitation at 672 nm.