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Register a new userThe B850/B875 Photosynthetic Complex Ground and Excited State are Both Coherent
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R. H. Squire
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A bacterial photosynthetic light harvesting complex PLHC absorbs a photon and transfers this energy almost perfectly at room temperature RT to a Reaction Center RC, where charge separation occurs. While there are a number of possible light absorbers involved in this process, our focus is the B850 and B875 complexes. We propose that the dominant feature of the ground states in the B850 ring and the B875 open chain are pseudo one dimensional metals due to each bacteriochlorophyll a BChl containing a coordinated magnesium ion Mg2+. The Mg ion structure undergoes a static Peierls distortion that results in symmetry breaking that changes the even spacing of the Mg/BChl molecules comprising the chains to the experimentally observed Mg/BChl dimers. The results are charge density waves CDW, one for each type of the two complexes that result in an energy gap in the single particle electronic spectrum and coherent phonon s spanning the entire rings. The ground state CDWs seem to have two functions the first is to form a stable optical platform and the second is to suppress radical formation and energy dissipation of the coherent excited state by creating single particle energy gaps. After excitation by a photon, the B850 exciton delocalizes on the ring a second photon can form a two-level exciton polariton that could be an alternative explanation for the splitting of the B850 exciton band. The coherent polariton formed could actively participate in uphill electronic energy transfer EET. Additionally we suggest other possible energy storage mechanisms and entanglement possibilities. We suggest experimental studies to clarify these proposals.
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The prospect of coherent dynamics and excitonic delocalization across several light-harvesting structures in photosynthetic membranes is of considerable interest, but challenging to explore experimentally. Here we demonstrate theoretically that the excitonic delocalization across extended domains involving several light-harvesting complexes can lead to unambiguous signatures in the optical response, specifically, linear absorption spectra. We characterize, under experimentally established conditions of molecular assembly and protein-induced inhomogeneities, the optical absorption in these arrays from polarized and unpolarized excitation, and demonstrate that it can be used as a diagnostic tool to determine the coherent coupling among iso-energetic light-harvesting structures. The knowledge of these couplings would then provide further insight into the dynamical properties of transfer, such as facilitating the accurate determination of Forster rates.
There is a remarkable characteristic of photosynthesis in nature, that is, the energy transfer efficiency is close to 100%. Recently, due to the rapid progress made in the experimental techniques, quantum coherent effects have been experimentally demonstrated. Traditionally, the incoherent theories are capable of calculating the energy transfer efficiency, e.g., (generalized) Forster theory and modified Redfield theory. However, in order to describe the quantum coherent effects in photosynthesis, the coherent theories have been developed, such as hierarchical equation of motion, quantum path integral, coherent modified Redfield theory, small-polaron quantum master equation, and general Bloch-Redfield theory in addition to the Redfield theory. Here, we summarize the main points of the above approaches, which might be beneficial to the quantum simulation of quantum dynamics of exciton energy transfer in natural photosynthesis, and shed light on the design of artificial light-harvesting devices.
The electronic excitation population and coherence dynamics in the chromophores of the photosynthetic light harvesting complex 2 (LH2) B850 ring from purple bacteria (Rhodopseudomonas acidophila) have been studied theoretically at both physiological and cryogenic temperatures. Similar to the well-studied Fenna-Matthews-Olson (FMO) protein, oscillations of the excitation population and coherence in the site basis are observed in LH2 by using a scaled hierarchical equation of motion (HEOM) approach. However, this oscillation time (300 fs) is much shorter compared to the FMO protein (650 fs) at cryogenic temperature. Both environment and high temperature are found to enhance the propagation speed of the exciton wave packet yet they shorten the coherence time and suppress the oscillation amplitude of coherence and the population. Our calculations show that a long-lived coherence between chromophore electronic excited states can exist in such a noisy biological environment.
Since in coupled-cluster (CC) theory ground-state and excitation energies are eigenvalues of a non-Hermitian matrix, these energies can in principle take on complex values. In this paper we discuss the appearance of complex energy values in CC calculations from a mathematical perspective. We analyze the behaviour of the eigenvalues of Hermitian matrices that are perturbed (in a non-Hermitian manner) by a real parameter. Based on these results we show that for CC calculations with real-valued Hamiltonian matrices the ground-state energy generally takes a real value. Furthermore, we show that in the case of real-valued Hamiltonian matrices complex excitation energies only occur in the context of conical intersections. In such a case, unphysical consequences are encountered such as a wrong dimension of the intersection seam, large numerical deviations from full configuration-interaction (FCI) results, and the square-root-like behaviour of the potential surfaces near the conical intersection. In the case of CC calculations with complex-valued Hamiltonian matrix elements, it turns out that complex energy values are to be expected for ground and excited states when no symmetry is present. We confirm the occurrence of complex energies by sample calculations using a six-state model and by CC calculations for the H2O molecule in a strong magnetic field. We furthermore show that symmetry can prevent the occurrence of complex energy values. Lastly, we demonstrate that in most cases the real part of the complex energy values provides a very good approximation to the FCI energy.
We have investigated the structural dynamics in photoexcited 1,2-diiodotetrafluoroethane molecules (C2F4I2) in the gas phase experimentally using ultrafast electron diffraction and theoretically using FOMO-CASCI excited state dynamics simulations. The molecules are excited by an ultra-violet femtosecond laser pulse to a state characterized by a transition from the iodine 5p orbital to a mixed 5p|| hole and CF2 antibonding orbital, which results in the cleavage of one of the carbon-iodine bonds. We have observed, with sub-Angstrom resolution, the motion of the nuclear wavepacket of the dissociating iodine atom followed by coherent vibrations in the electronic ground state of the C2F4I radical. The radical reaches a stable classical (non-bridged) structure in less than 200 fs.
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