Investigations on bacteriochlorophyll in organic solutions

Various spectral and photochemical properties of bacteriochlorophyll were studied. It was found that the spectral shift of the second absorption band of this pigment in polar solvents is nearly absent in the magnesium-free pigment. The quantum yield of irreversible bleaching was determined with respect to the type of solvent and the purity of the pigment. A difference of a factor of 100 was found to occur between the values in methanol and those in ether. The quantum-yield values were found to be slightly higher at lower temperatures. Addition of quinone resulted in a strong decrease of the quantum yield of bleaching. The absorption spectrum was determined for photo-bleached bacteriochlorophyll in different solvents. Furthermore, the effect of reversibility of photo-bleaching was studied as a function of temperature. A reversible “chemical” bleaching was found to occut after addition of ferric salts, iodine, or potassium permanganate. The absorption spectrum was found to be analogous to that of chlorophyll a under similar conditions. This reversible “bleaching” proved to occur not only in methanol, but also in acetone or ether. Fluorescence spectra were determined before and after bleaching of “pure” and “crude” bacteriochlorophyll solutions. The quenching of fluorescence after addition of quinone and the effect of this addition on the degree of polarisation for bacteriochlorophyll and chlorophyll a was also investigated