Comparison of P700 and P740 based on the redox potentials

Abstract

In PS I of A. marina, the primary electron donor P740 is a Chl d/d' heterodimer, and the midpoint potential E_m was initially reported as +335 mV, being as much as 100 mV lower than P700 (ca. +430 mV), and hence the Chl d was supposed to have an oxidation potential E_ox lower than that of Chl a. We previously reported that E_ox value of Chl d (+0.88 V ) in acetonitrile was higher than that of Chl a (+0.81 V ). Recently, E_m of P740 was re-examined and found to be +430 mV by Benjamin et al., which is almost equal P700 value, suggesting that the interaction between the special pair chlorophylls (Chl d and d') of P740 might be stronger than that between Chl a and a' of P700, due to the presence of the coordination of formyl group to Mg in P740.