Effects of peripheral substituents on epimerization kinetics of formylated chlorophylls

Abstract

C132-[Formula: see text]-epimers of chlorophyll (Chl) molecules are important cofactors in the photosystem I 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 C132-[Formula: see text]-epimers. We report herein the kinetic properties of the epimerization of formylated Chl molecules, Chl [Formula: see text] and Chl [Formula: see text], by use of triethylamine. Both Chl [Formula: see text] and Chl [Formula: see text] performed faster epimerization kinetics than Chl [Formula: see text], 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 [Formula: see text] was faster than that of Chl [Formula: see text]. This difference is rationalized by invoking a combination of the inductive effects of the C3- and C7-substituents in Chls; the sums of Hammett [Formula: see text] parameters of the C3- and C7-substituents exhibited high correlations with the epimerization rate constants of Chls [Formula: see text], [Formula: see text], and [Formula: see text].