Chlorophyll Excitations in Photosystem I of Synechococcus elongatus

Abstract

The Q y excitation energies of the 96 chlorophyll molecules in photosystem I of Synechococcus elongatus, both in and out of their protein environments, were obtained by using the semiempirical INDO/S method and the crystal structure geometries. The dipole-dipole approximation was used to calculate the coupling between the Q y states of chlorophylls; in the case of closely separated chlorophylls INDO/S dimer calculations were used to determine the couplings. The effective Hamiltonian for chlorophyll Q y excitations was constructed, enabling tentative assignment of red chlorophylls and calculation of the absorption spectrum of PSI.

α-Helices direct excitation energy flow in the Fenna–Matthews–Olson protein

“…Single amino acid side chains, either charged or neutral, cause shift magnitudes of at most Ϸ180 cm Ϫ1 and tend to compensate each other (SI Tables 3-5). Accordingly, the charged amino acids do not play the dominant role that was attributed to them in earlier studies (15)(16)(17)28), where large parts of the protein charge distribution, in particular, the backbone, were not modeled, and standard protonation states of titratable residues were assumed (15)(16)(17). In qualitative agreement with experimental results on other antenna systems (29), hydrogen bond donors to BChla cause red shifts of the site energies, but in the present system this shift never exceeds Ϸ130 cm Ϫ1 (SI Table 4).…”

“…Directed energy transport results from energetic relaxation transferring population between exciton states of different spatial extents. The latter depend crucially on excitonic couplings and site energies, so that the elucidation of energy-transfer mechanisms on the basis of spectroscopic data (2-4) and crystal structures (5-7) requires knowledge of both these quantities (8, 9), which are not directly available from experiments.Whereas there are various methods at different levels of approximation to accurately determine excitonic couplings from structural data (10-15), the calculation of site energies has been a challenge (15)(16)(17). In earlier attempts to calculate site energies directly by using the information from crystal structures, quantum chemical methods were applied to the pigment cofactors and their immediate protein environment (16,17).…”

“…Whereas there are various methods at different levels of approximation to accurately determine excitonic couplings from structural data (10-15), the calculation of site energies has been a challenge (15)(16)(17). In earlier attempts to calculate site energies directly by using the information from crystal structures, quantum chemical methods were applied to the pigment cofactors and their immediate protein environment (16,17).…”

“…In earlier attempts to calculate site energies directly by using the information from crystal structures, quantum chemical methods were applied to the pigment cofactors and their immediate protein environment (16,17). A shortcoming of a solely quantum chemical method is that important long-range electrostatic pigment-protein couplings are not taken into account.…”

Characterization of Low-Energy Chlorophylls in the PSI-LHCI Supercomplex from Chlamydomonas reinhardtii. A Site-Selective Fluorescence Study

“…The processes of energy equilibration between these low-energy Chls and bulk Chls were extensively studied (see reviews 1,2 ), but the nature, location, and function of these red Chls are still not fully understood. [3][4][5][6][7][8][9][10][11][12] The red forms of Chls were characterized in detail for PSI core complexes from several species of cyanobacteria. At 6 K, two spectral pools of these Chls were identified with steadystate absorption/fluorescence line narrowing techniques: one with absorption maximum at 703 nm (C703, Synechococcus PCC 7942) or 708 nm (C708; Synechocystis PCC 6803, Thermosynechococcus elongatus (T. elongatus), Spirulina platensis (S. platensis)) and the other one with absorption maximum at 719 nm (T. elongatus) or at 740 nm (S. platensis).…”