Identity of Green Plant Reaction Centers from Quantum Chemical Determination of Redox Potentials of Special Pairs

Abstract

We have carried out quantum chemical computations on the special pairs of chlorophyll-a molecules so as to resolve the ambiguity of the large oxidation redox potential of P680 and establish the identities of P680 and P700 by comparing the calculated potentials with the observed ones. The methodology adopted here has been INDO. At first the oxidation potential of chlorophyll-a has been determined from the calculations on model structures prepared from the crystallographic structure of ethyl chlorophyllide-a dihydrate with variously truncated side chains. A good value of the oxidation potential is found only for structures that have most of the side chains and hydrogen-bonded water molecules intact. The calculated oxidation potentials are about 0.68 V in dichloromethane and 0.77 V in acetonitrile at pH 4.5 at 298.15 K. The calculated numbers are in good agreement with the observed values 0.74−0.86 and 0.76 V, respectively, for chlorophyll-a in these solvents. The intact structures have been used to form the...