Successfully controlled isomerization and pheophytinization of bacteriochlorophyll b by weak acid in the dark in vitro

The infrared absorbance peak at 795 nm rapidly disappears while the absorbance at 675 nm increases simultaneously with the addition of acid to a solution of bacteriochlorophyll b (BChl b) in acetone in the dark. These optical changes result from light-independent isomerization, which converts BChl b to 3-desvinyl-3-acetyl-chlorophyll a. However, in aqueous acetone in the presence of acid, BChl b undergoes pheophytinization (demetalation) to yield bacteriopheophytin b. Both isomerization and pheophytinization of BChl b were successfully controlled by acid in the dark.