5,537 research outputs found
Exciton coupling induces vibronic hyperchromism in light-harvesting complexes
The recently suggested possibility that weak vibronic transitions can be
excitonically enhanced in light-harvesting complexes is studied in detail. A
vibronic exciton dimer model which includes ground state vibrations is
investigated using multi-configuration time-dependent Hartree method with a
parameter set typical to photosynthetic light-harvesting complexes. Absorption
spectra are discussed in dependence on the Coulomb coupling, the detuning of
site energies, and the number of vibrational mode. Calculations of the
fluorescence spectra show that the spectral densities obtained from the low
temperature fluorescence line narrowing measurements of light-harvesting
systems need to be corrected for the exciton effects. For the J-aggregate
configuration, as in most of the light-harvesting complexes, the true spectral
density has larger amplitude than what is obtained from the measurement.Comment: revised version (minor
Quantum entanglement in photosynthetic light harvesting complexes
Light harvesting components of photosynthetic organisms are complex, coupled,
many-body quantum systems, in which electronic coherence has recently been
shown to survive for relatively long time scales despite the decohering effects
of their environments. Within this context, we analyze entanglement in
multi-chromophoric light harvesting complexes, and establish methods for
quantification of entanglement by presenting necessary and sufficient
conditions for entanglement and by deriving a measure of global entanglement.
These methods are then applied to the Fenna-Matthews-Olson (FMO) protein to
extract the initial state and temperature dependencies of entanglement. We show
that while FMO in natural conditions largely contains bipartite entanglement
between dimerized chromophores, a small amount of long-range and multipartite
entanglement exists even at physiological temperatures. This constitutes the
first rigorous quantification of entanglement in a biological system. Finally,
we discuss the practical utilization of entanglement in densely packed
molecular aggregates such as light harvesting complexes.Comment: 14 pages, 7 figures. Improved presentation, published versio
Superradiance Transition in Photosynthetic Light-Harvesting Complexes
We investigate the role of long-lasting quantum coherence in the efficiency
of energy transport at room temperature in Fenna-Matthews-Olson photosynthetic
complexes. The excitation energy transfer due to the coupling of the light
harvesting complex to the reaction center ("sink") is analyzed using an
effective non-Hermitian Hamiltonian. We show that, as the coupling to the
reaction center is varied, maximal efficiency in energy transport is achieved
in the vicinity of the superradiance transition, characterized by a segregation
of the imaginary parts of the eigenvalues of the effective non-Hermitian
Hamiltonian. Our results demonstrate that the presence of the sink (which
provides a quasi--continuum in the energy spectrum) is the dominant effect in
the energy transfer which takes place even in absence of a thermal bath. This
approach allows one to study the effects of finite temperature and the effects
of any coupling scheme to the reaction center. Moreover, taking into account a
realistic electric dipole interaction, we show that the optimal distance from
the reaction center to the Fenna-Matthews-Olson system occurs at the
superradiance transition, and we show that this is consistent with available
experimental data.Comment: 9 page
Modelling excitonic-energy transfer in light-harvesting complexes
The theoretical and experimental study of energy transfer in photosynthesis
has revealed an interesting transport regime, which lies at the borderline
between classical transport dynamics and quantum-mechanical interference
effects. Dissipation is caused by the coupling of electronic degrees of freedom
to vibrational modes and leads to a directional energy transfer from the
antenna complex to the target reaction-center. The dissipative driving is
robust and does not rely on fine-tuning of specific vibrational modes. For the
parameter regime encountered in the biological systems new theoretical tools
are required to directly compare theoretical results with experimental
spectroscopy data. The calculations require to utilize massively parallel
graphics processor units (GPUs) for efficient and exact computations.Comment: 20 pages, submitted to the AIP conference proceedings of the Latin
American School of Physics Marcos Moshinsky (ELAF 2013
Bloch-Redfield equations for modeling light-harvesting complexes
We challenge the misconception that Bloch-Redfield equations are a less
powerful tool than phenomenological Lindblad equations for modeling exciton
transport in photosynthetic complexes. This view predominantly originates from
an indiscriminate use of the secular approximation. We provide a detailed
description of how to model both coherent oscillations and several types of
noise, giving explicit examples. All issues with non-positivity are overcome by
a consistent straightforward physical noise model. Herein also lies the
strength of the Bloch-Redfield approach because it facilitates the analysis of
noise-effects by linking them back to physical parameters of the noise
environment. This includes temporal and spatial correlations and the strength
and type of interaction between the noise and the system of interest. Finally
we analyze a prototypical dimer system as well as a 7-site Fenna-Matthews-Olson
(FMO) complex in regards to spatial correlation length of the noise, noise
strength, temperature and their connection to the transfer time and transfer
Identifying the quantum correlations in light-harvesting complexes
One of the major efforts in the quantum biological program is to subject
biological systems to standard tests or measures of quantumness. These tests
and measures should elucidate if non-trivial quantum effects may be present in
biological systems. Two such measures of quantum correlations are the quantum
discord and the relative entropy of entanglement. Here, we show that the
relative entropy of entanglement admits a simple analytic form when dynamics
and accessible degrees of freedom are restricted to a zero- and
single-excitation subspace. We also simulate and calculate the amount of
quantum discord that is present in the Fenna-Matthews-Olson protein complex
during the transfer of an excitation from a chlorosome antenna to a reaction
center. We find that the single-excitation quantum discord and relative entropy
of entanglement are equal for all of our numerical simulations, but a proof of
their general equality for this setting evades us for now. Also, some of our
simulations demonstrate that the relative entropy of entanglement without the
single-excitation restriction is much lower than the quantum discord. The first
picosecond of dynamics is the relevant timescale for the transfer of the
excitation, according to some sources in the literature. Our simulation results
indicate that quantum correlations contribute a significant fraction of the
total correlation during this first picosecond in many cases, at both cryogenic
and physiological temperature.Comment: 15 pages, 7 figures, significant improvements including (1) an
analytical formula for the single-excitation relative entropy of entanglement
(REE), (2) simulations indicating that the single-excitation REE is equal to
the single-excitation discord, and (3) simulations indicating that the full
REE can be much lower than the single-excitation RE
Dimerization-assisted energy transport in light-harvesting complexes
We study the role of the dimer structure of light-harvesting complex II (LH2)
in excitation transfer from the LH2 (without a reaction center (RC)) to the LH1
(surrounding the RC), or from the LH2 to another LH2. The excited and
un-excited states of a bacteriochlorophyll (BChl) are modeled by a quasi-spin.
In the framework of quantum open system theory, we represent the excitation
transfer as the total leakage of the LH2 system and then calculate the transfer
efficiency and average transfer time. For different initial states with various
quantum superposition properties, we study how the dimerization of the B850
BChl ring can enhance the transfer efficiency and shorten the average transfer
time.Comment: 11 pages, 6 figure
Strong antenna-enhanced fluorescence of a single light-harvesting complex shows photon antibunching
The nature of the highly efficient energy transfer in photosynthetic light-harvesting complexes is a subject of intense research. Unfortunately, the low fluorescence efficiency and limited photostability hampers the study of individual light-harvesting complexes at ambient conditions. Here we demonstrate an over 500-fold fluorescence enhancement of light-harvesting complex 2 (LH2) at the single-molecule level by coupling to a gold nanoantenna. The resonant antenna produces an excitation enhancement of circa 100 times and a fluorescence lifetime shortening to ~\n20 ps. The radiative rate enhancement results in a 5.5-fold-improved fluorescence quantum efficiency. Exploiting the unique brightness, we have recorded the first photon antibunching of a single light-harvesting complex under ambient conditions, showing that the 27 bacteriochlorophylls coordinated by LH2 act as a non-classical single-photon emitter. The presented bright antenna-enhanced LH2 emission is a highly promising system to study energy transfer and the role of quantum coherence at the level of single complexes
Strategies to enhance the excitation energy-transfer efficiency in a light-harvesting system using the intra-molecular charge transfer character of carotenoids
Fucoxanthin is a carotenoid that is mainly found in light-harvesting complexes from brown algae and diatoms. Due to the presence of a carbonyl group attached to polyene chains in polar environments, excitation produces an excited intra-molecular charge transfer. This intra-molecular charge transfer state plays a key role in the highly efficient (∼95%) energy-transfer from fucoxanthin to chlorophyll a in the light-harvesting complexes from brown algae. In purple bacterial light-harvesting systems the efficiency of excitation energy-transfer from carotenoids to bacteriochlorophylls depends on the extent of conjugation of the carotenoids. In this study we were successful, for the first time, in incorporating fucoxanthin into a light-harvesting complex 1 from the purple photosynthetic bacterium, Rhodospirillum rubrum G9+ (a carotenoidless strain). Femtosecond pump-probe spectroscopy was applied to this reconstituted light-harvesting complex in order to determine the efficiency of excitation energy-transfer from fucoxanthin to bacteriochlorophyll a when they are bound to the light-harvesting 1 apo-proteins
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