Abstract
Chlorophylls undergo epimerization, and an epimer pair is denoted with a non-primed and primed symbol, e.g., as Chl a/a'. Chl d' functions as P740 in Acaryochloris marina as a heterodimer of d/d', like a/a' for P700. The ratios of a'/a, b'/b and d'/d in vivo are 0.2-1%, 0% and 1%. A comparison of such a biased epimer constitution in vivo with features of a'/a, b'/b and d'/d epimerization in vitro would provide an insight into the existence of a' and d' in PS I. We compared the base-catalyzed epimerization of a'/a, b'/b and d'/d in diethyl ether. The equilibrium mole fraction of a', b' and d' were 0.241, 0.254 and 0.225, at 20oC. The order of the k + k' value is b'/b > d'/d > a'/a with approximate relative ratios of 4:2:1. The order is inverse to that of the ring π-electron density affected by an inductive effect of the substituents, but is well in line with the redox potentials. The oxidation potential and epimerization rate of b'/b are too high for plants to use them as the electron transfer parts in the RC. In contrast, the oxidation potential and the epimerization rate of a'/a are lowest and smallest, and hence a'/a may be widely used by plants.
