Isomerization kinetics of bacteriochlorophyll b and bacteriopheophytin b under acidic conditions

Bacteriochlorophyll (BChl) b has a unique π-conjugation system, in which the bacteriochlorin macrocycle is conjugated with the C8-ethylidene group. This π-system is converted easily to the chlorin macrocycle. However, the effects of the central magnesium in BChl b on this conversion are unclear. In...

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Bibliographic Details
Published in:Photochemical & photobiological sciences 2022-07, Vol.21 (7), p.1193-1199
Main Authors: Takashima, Yusuke, Saga, Yoshitaka
Format: Article
Language:English
Subjects:
Biochemistry
Biomaterials
Chemistry
Chemistry and Materials Science
Original Papers
Physical Chemistry
Plant Sciences
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Summary:Bacteriochlorophyll (BChl) b has a unique π-conjugation system, in which the bacteriochlorin macrocycle is conjugated with the C8-ethylidene group. This π-system is converted easily to the chlorin macrocycle. However, the effects of the central magnesium in BChl b on this conversion are unclear. In this study, the isomerization kinetics of BChl b and its demetalated pigment, bacteriopheophytin (BPhe) b , was analyzed under weakly acidic conditions. BChl b exhibited faster acid-induced isomerization than BPhe b . These results were attributed to the stabilization of a cationic intermediate, whose C8-ethylidene group is protonated, during the isomerization of BChl b compared to BPhe b because of a difference in the electron densities of the π-conjugation systems between BChl b and BPhe b . High-performance liquid chromatography analyses indicated that BChl b was primarily isomerized to 3-acetyl Chl a , followed by demetalation. The reaction order was due to the slower demetalation kinetics of metallobacteriochlorins than metallochlorins. These results will be helpful for handling unstable BChl b and BPhe b . The reaction properties of BChl b and BPhe b demonstrated here will be helpful for understanding the in vivo formation of BPhe b , which acts as the primary electron acceptor in photosynthetic reaction center complexes in BChl b -containing purple photosynthetic bacteria. Graphical abstract
ISSN:1474-9092
1474-9092
DOI:10.1007/s43630-022-00207-1