Electron transfer pathways in photosystem I reaction centers

Quantum Interferences and Electron Transfer in Photosystem I

“…29 Contrary to early models, 60,64 the split orbital method leads to weak couplings between the Chl's, and even weaker interactions between the Chl's and PhQ's and between the PhQ's and the iron sulfur cluster. 29 We have computed these charge transfer integrals using the fragment approach implemented in the Amsterdam Density Functional (ADF) code. 65 The charge transfer integrals are directly computed via the off diagonal matrix elements of the Fock operator and consequently account for the nonzero spatial overlap between the molecular orbitals of the fragments.…”

“…On the contrary, the fluctuations on the on-site energies are independent of their original values and follow a Gaussian distribution of width: η × E 0 with E 0 = 0.1 eV. The introduction of a detuning in the on-site energy of the PhQ is motivated by recent experimental studies that tend to show that the on-site energy of one the PhQ could lie as much as few hundred millielectronvolts below the other, probably due to slight asymmetric modifications on the two PhQ’s local environments. ,, This detuning, noted Δ E , is introduced in our simulation by decreasing the energy of one of the PhQ from −460 to −860 meV. The equivalent cases, where the energy of either |Q 7 ⟩ or |Q 8 ⟩ is modified, are considered and correspond respectively to a negative or a positive Δ E .…”

“…25,26 Asymmetries between the propagation over the two branches are most often observed, leading to a biphasic rate, 21,27 probably due to slight differences in the environment of the phylloquinones. 28,29 However, this biphasic rate is significantly reduced for given species or by fully reducing the clusters F a and F b . 20 Other important open questions remain, such as the capability of the PSI to shut down whenever subjected to intense illumination.…”

“…Many crucial points have been intensely debated: the propagation of the electron over one single branch or both of them, − the utilization of the PhQ’s and the F x cluster as intermediary electron acceptors, − the location of the principal electron donor and the importance of exciton propagation over the Chl’s. − The current consensus is that starting from the special pair, the electron propagates over both branches and reaches the Chl-A 0 ’s after 0.5–20 ps (with a complex unexplored subpicosecond dynamics). The electron is then transferred to the PhQ’s after 20–200 ps and finally to the F x cluster after 20–200 ns. , Asymmetries between the propagation over the two branches are most often observed, leading to a biphasic rate, , probably due to slight differences in the environment of the phylloquinones. , However, this biphasic rate is significantly reduced for given species or by fully reducing the clusters F a and F b …”

Bidirectional Electron Transfer in the Reaction Centre of Photosystem I

“…The most evident asymmetry in the primary sequence of the PsaA and the PsaB RC subunits, in the surrounding PhQ binding sites relates to the presence of a tryptophan in the PsaB subunit (PsaB‐Trp673) where a glycine is found in the homologous position of PsaA. This has led to the suggestion, based on quantum chemical calculations, that the PsaB‐Trp673 might play an active role in ET events (Ivashin and Larsson 2003).…”