Redox potential of chlorophyll d in vitro

Chlorophyll (Chl) d is a major chlorophyll in a novel oxygenic prokaryoteAcaryochloris 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 washigher than that of Chl a (+0.81 V) and lower than that of Chl b (+0.93 V). Theoxidation potential order, Chl b > Chl d > Chl a, can be explained by inductive effectof substituent groups on the conjugated ;-electron system on the macrocycle.Corresponding pheophytins showed the same order; Phe b (+ 1.25 V) > Phe d (+1.21V) > 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 ;-system by thereplacement of magnesium with more electronegative hydrogen. Consequently,oxidation potential of Chl a was found to be the lowest among Chls and Phes. Theresults will help us to broaden our views on photosystems in A. marina