Epimerization of chlorophyll a analogs: effects of the vinyl groups in the A-ring and B-ring of the chlorin macrocycle

Chlorophyll (Chl) a and 3,8-divinyl (DV)-Chl a are crucial pigments in efficient light-energy conversion in oxygenic photosynthesis. (C13²S)-Epimers of these pigments, Chl a' and DV-Chl a', form the primary electron donor in photosystem (PS)-I reaction center (RC) complexes. Despite the importance of Chl a' and DV-Chl a', no information is available on their in vivo formation. The substitution effects on epimerization reactions of Chl a and its analogs provide useful information to address this issue. In this study, the effects of vinyl groups in Chl pigments on epimerization properties were investigated using Chl a, DV-Chl a, and 3-devinyl-3-ethyl Chl a (mesoChl a). The vinyl group at the 8-position of DV-Chl a accelerated the epimerization reaction. This acceleration originates from the more electronegative C8-vinyl group in DV-Chl a compared to the C8-ethyl group in Chl a. By contrast, the transformation of the C3-ethyl to C3-vinyl group barely affects the epimerization kinetics of Chls. These results indicate that the vinyl group in the B-ring of the chlorin macrocycle has a larger effect on the epimerization of Chl a analogs than that in the A-ring. The thermodynamic properties of the epimerization of Chl a and its analogs were barely influenced by the vinyl groups at the 3- and 8-positions. The current results will provide a better understanding of the formation of the prime-type epimers of Chl a and DV-Chl a in PS-I RCs.