Exciton coupling in the photosystem I reaction center

Abstract

The point dipole approximation, transition charge method and transition density cube method are used to investigate the influences of the packing structures such as face-to-face distance and shift distance on the exciton coupling between theChlorophyll-a model dimers. The results show that the coupling strength obtained from three methods are very similar when the distance between the dimer molecules is larger than the molecule size; while the point dipole approximation will overestimate the exciton coupling and transition density cube method will give more reasonable coupling values when the distance is smaller than the molecule size. Based on these three method, we also calculated the couplings between Chlorophyll a pigments in both the crystal structure (1jb0.pdb) and the QM-MM optimized structure of the photosystem I reaction center. The exciton coupling of the special pair (P700) from the transition density cube method is 23.8 cm^-1 for optimized structure, which is very similar with the one 20 cm^-1 from full INDO Hamiltonian decomposition including Förster and Dexter term contributions. This explained that the strongest coupling dimer in the PSI reaction center is not the P700, but the dimer pair of the ecA1-ecB2 (-153 cm^-1) and ecB1-ecA2 (-152 cm^-1).