Artificial Photosynthetic Reaction Centers: A Semiempirical Conformation Analysis of Various Donor-Acceptor Systems

Abstract

The electronic ground states of five covalently linked electron donor acceptor systems have been investigated by semiempirical methods using the AM1 Hamiltonian. A tetraphenylporphyrine (TPP) unit serves as an electron donor with various electron acceptors. In the porphyrine-anthracene (P-A) dyad, both subunits are separated by a biphenyl-like spacer, in the free base porphyrine-flavine (P-F) and the zinc porphyrine-flavine (P_Zn-F) dyad by a terphenyl-like aromatic spacer. The components of the porphyrine-quinone (P-Q) dyad are separated by a peptide-methylene (-NHCO-CH₂-) unit. A benzoquinone is chosen as acceptor, because it is a derivative of the acceptor in the natural photosynthetic reaction centre (PSRC). Addition of a carotenoid unit generates the carotenoid-porphyrine-quinone (C-P-Q) triad, which is an excellent model for artificial photosynthesis. Geometries, conformational energies and activation barriers for rotation about the single bonds connecting the various subunits are characterised for all five systems. A direct comparison of the geometry with experimental results corroborates the theoretical computations where available. The strong dependence of the electron transfer rate upon distance and orientation of both the donor and acceptor subunit is the main reason for the detailed study of the flexibility of conformational degrees of freedom. Solvent effects are included in the computations by using the self consistent reaction field (SCRF) approach.