Redox potential of chlorophyll d in vitro

Chlorophyll (Chl) d is a major chlorophyll in a novel oxygenic prokaryote Acaryochloris marina. Here we first report the redox potential of Chl d in vitro. The oxidation potential of Chl d was + 0.88 V vs. SHE in acetonitrile; the value was higher than that of Chl a (+ 0.81 V) and lower than that of Chl b (+ 0.94 V). The oxidation potential order, Chl b > Chl d > Chl a, can be explained by inductive effect of substituent groups on the conjugated π-electron system on the macrocycle. Corresponding pheophytins showed the same order; Phe b (+ 1.25 V) > Phe d (+ 1.21 V) > Phe a (+ 1.14 V), but the values were significantly higher than those of Chls, which are rationalized in terms of an electron density decrease in the π-system by the replacement of magnesium with more electronegative hydrogen. Consequently, oxidation potential of Chl a was found to be the lowest among Chls and Phes. The results will help us to broaden our views on photosystems in A. marina.