Superradiance Transition and Nonphotochemical Quenching in Photosynthetic Complexes

Abstract

We demonstrate numerically that superradiance could play a significant role in nonphotochemical quenching (NPQ) in light-harvesting complexes. Our model consists of a network of five interconnected sites (discrete excitonic states) that are responsible for the NPQ mechanism. Damaging and charge transfer states are linked to their sinks (independent continuum electron spectra), in which the chemical reactions occur. The superradiance transition in the charge transfer (or in the damaging) channel, occurs at particular electron transfer rates from the discrete to the continuum electron spectra, and can be characterized by a segregation of the imaginary parts of the eigenvalues of the effective non-Hermitian Hamiltonian. All five excitonic sites interact with their protein environment that is modeled by a random stochastic process. We find the region of parameters in which the superradiance transition into the charge transfer channel takes place. We demonstrate that this superradiance transition has the capability of producing optimal NPQ performance. W hen sunlight intensity is too high, damaging processes (such as singlet oxygen production) occur, that can destroy the photosynthetic organisms. To survive intense sunlight fluctuations, photosynthetic organisms have evolved many protective strategies, including nonphotochemical quenching (NPQ) [1]. (See also references therein.) Using this strategy, light-harvesting complexes (LHCs) experience geometrical reorganizations due to the conformational changes of their protein environments. As a result, the damaging channels are partly suppressed, and the excessive sunlight energy is transfered to the non-damaging, quenching channel(s), that are opened in this regime.In modeling the NPQ mechanisms, it is possible to characterize the sites of the LHCs by the discrete excitonic energy states, |n , n being the number of site, associated with the light-sensitive chlorophyll or carotenoid molecule. Then, both the damaging and undamaging channels can be characterized by their corresponding sinks, |Sn , that provide independent continuum electron energy spectra [2]. These sinks can have very complex structures, and they can be responsible for primary charge separation processes, as in the photosynthetic reaction centers (RCs) [3,4], for creation of charge transfer states, for singlet oxygen production [2], and for many other quasi-reversible chemical reactions. Generally, a particular sink, |Sn , is connected to a particular site, |n . The energy transfer from this state to its sink is characterized by the electron transfer (ET) rate, Γn. For those sites which are not connected to sinks, the corresponding ET rates vanish. Under reasonable assumptions, this type of model can be described by an effective non-Hermitian Hamiltonian [2,3,4,5,6].Then, this approach becomes similar to those used in describing the so-called "superradiance transition" (ST) in systems in which the discrete (intrinsic) energy states interact with the continuum spectra [7,8,9,10,11,12]. In these systems, th...

On improving the performance of nonphotochemical quenching in CP29 light-harvesting antenna complex

“…Energy transfer processes of the NPQ mechanism can be modeled by describing the sites associated with light-sensitive chlorophyll or carotenoid molecules by discrete excitonic energy states, |n (where n enumerates the sites of the LHC). Both the damaging and the quenching channels can be characterized by their corresponding energy sinks, |S n , that provide independent continuum electron energy spectra [7,8]. These sinks can have very complex structures.…”

“…These sinks can have very complex structures. They can be responsible for many quasi-reversible chemical reactions, such as primary charge separation processes in the photosynthetic RCs [9,10], creation of CTS and singlet oxygen production [7,8], and coherent quantum effects [11,12,13,14] even at ambient temperature. Generally, each sink, |S n , is connected to a particular site, |n .…”

“…In the presence of the protein-solvent noisy environment, the evolution of the system can be described by the following effective non-Hermitian Hamiltonian [7,8]…”

“…The evolution of the average components of the density matrix is described by the following system of ordinary differential equations [7,8]:…”

“…Specifically, we analyze stage (3) of the NPQ mechanism in CP29 LHCs of plants and green algae by a charge-transfer state (CTS), discussed in [3] (see also [4,5,6]) 1 . We build on the model developed in [7,8], and demonstrate numerically that the performance of the NPQ mechanism can be significantly improved by varying (i) characteristics of the chlorophyll dimer, (ii) resonant properties of protein-solvent interaction, and (iii) the ET rates to sinks. Our analysis suggests strategies for restoring NPQ efficiency in response to environmental changes.…”