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

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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 (R[E,E]BChl cF and R[P,E]BChl cF) and one minor homolog (S[I,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 R[E,E]BChl cF 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(13l) 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.