Structure-Dependent Demetalation Kinetics of Chlorophyll a Analogs under Acidic Conditions

Demetalation of chlorophyll (Chl) a and its analogs is an important reaction in oxygenic photosynthetic organisms, which produces the primary electron acceptors in photosystem II reaction centers and is crucial in the Chl degradation. From these viewpoints, demetalation reactions of four Chl a analogs, 3,8-divinyl-Chl a (DV-Chl a), 3-devinyl-3-ethyl-Chl a (mesoChl a), 132-demethoxycarbonyl-Chl a (pyroChl a) and protochlorophyll a (PChl a), were kinetically analyzed under weakly acidic conditions, and were compared with that of Chl a. DV-Chl a exhibited slower demetalation kinetics than did Chl a, whereas demetalation of mesoChl a was faster than that of Chl a. The difference in demetalation kinetics of the three chlorophyllous pigments originates from the electron-withdrawing ability of the vinyl group as the peripheral substituent compared with the ethyl group. Removal of the electron-withdrawing and homoconjugating 132-methoxycarbonyl group in Chl a (Chl a -> pyroChl a) accelerated demetalation kinetics by two-fold. PChl a possessing the porphyrin-type skeleton exhibited slower demetalation kinetics than Chl a. The structure-dependent demetalation properties of Chl a analogs will be useful for understanding in vivo Chl demetalation reactions in oxygenic photosynthetic organisms.