Effects of peripheral substituents on epimerization kinetics of formylated chlorophylls

C13(2)-S-epimers of chlorophyll (Chl) molecules are important cofactors in the photosystem 1 reaction centers in oxygenic photosynthetic organisms; however, their production mechanism is still unclear. The reaction properties of Chl epimerization are helpful for a better understanding of the molecular mechanism of the in vivo formation of Chl C13(2)-S-epimers. We report herein the kinetic properties of the epimerization of formylated Chl molecules, Chl b and Chl d, by use of triethylamine. Both Chl b and Chl d performed faster epimerization kinetics than Chl a, indicating that the electron-withdrawing ability of the formyl groups directly linked to the chlorin macrocycle is responsible for acceleration of the epimerization. Comparing the rate constants of the two mono-formylated Chl molecules indicated that the epimerization of Chl b was faster than that of Chl d. This difference is rationalized by invoking a combination of the inductive effects of the C3- and C7-substituents in Chls; the sums of Hammett sigma parameters of the C3- and C7-substituents exhibited high correlations with the epimerization rate constants of Chls b, d, and a.