INTERCONVERSION OF BACTERIOCHLOROPHYLL c AGGREGATES IN SOLID FILMS UPON ORGANIC VAPOR TREATMENT

Bacteriochlorophyll c (BChl c) solid films were prepared from a carbon tetrachloride solution on CaF₂ plates as artificial aggregates. Effects of organic vapor such as acetone and tetrahydrofuran (THF) on the BChl c films were studied by absorption and Fourier-transform infrared spectroscopy. Two major homologs (RE,E]BChl c_F and RP,E]BChl c_F) and one minor homolog (SI,E]BChl c) isolated from the green photosynthetic bacterium Chlorobium limicola strain 6230 were examined for the experiments. The BChl c polymeric aggregates absorbing at739–753 nm similar to those in the chlorosome were induced for all homologs upon the treatment of BChl c solid film with acetone vapor. The 13¹-keto C=O stretching band in the RE,E]BChl c_F solid film showed a downward shift from 1651 cm^?1to 1643 cm^?1 with a concomitant shift of the 3¹-OH stretching bands from 3337 and 3238 cm^?1 to 3163 cm^?1. It was suggested that the lower aggregates brought about by Mg…O=C(13¹) and (3¹)O…O=C(13¹) bonds were transformed into the higher aggregates strongly hydrogen-bonded in a Mg…(3¹)O-H…O=C(13^l) interaction. They were transformed to a monomer-like form absorbing at 667 nm upon exposure to THF vapor and were reversibly converted to the higher aggregates upon removal of THF molecules in vacuo.