Quantum chemical identification of blue and red forms of protonated pheophytin-a dianion

Abstract

Quantum mechanics/molecular mechanics (QM/MM) calculations were employed so as to determine the most stable structures of protonated forms of Pheophytin-a (Pheo) dianion in DMF. The protonated forms include PheoH and PheoH(2). Electrostatic potential-derived charges (ESP charges) employing the CHelpG scheme were obtained for Pheo(2-) and for possible blue and red forms of PheoH(-) by ROB3LYP methodology using 6-31 G(d) basis. The ESP charges and the most stable structures obtained by QM/MM calculations provide a clue to the probable site of protonation in Pheo(2-) and PheoH(-). A total of 18 different possible structures were investigated. From these calculations, we identify the structures of the blue and red forms of PheoH and PheoH(2). Furthermore, density functional treatment (DFT) along with dielectric ;polarizable continuum model (DPCM) calculations using 6-311+G(2d,2p) basis were done to determine the absolute free energy of reduction of Pheo to PheoH and PheoH2 in DMF. The calculated mid-point reduction potentials relative to the standard hydrogen electrode are 0.63 V (blue), -0.71 V (red) for reduction of Pheo to PheoH(-), and 0.15V (blue), 0.11 V(red) for reduction to PheoH(2). The calculated numbers are in good agreement with observed values, -0.68, -0.72, 0.15 and 0,05 V, respectively. (C) 2008