Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userProbing the Role of the Eighth Bacteriochlorophyll in holo-FMO Complex by Simulated Two-Dimensional Electronic Spectroscopy
143
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The Fenna-Matthews-Olson (FMO) protein-pigment complex acts as a molecular wire between the outer antenna system and the reaction center (RC); it is an important model system to study the excitonic energy transfer. Recent crystallographic studies report the existence of an additional (eighth) bacteriochlorophyll a (BChl a). To understand the functionality of this eighth BChl, we simulated the two-dimensional electronic spectra of both the 7-site (apo form) and the 8-site (holo form) variant of the FMO complex from green sulfur bacteria, Prosthecochloris aestuarii. By comparing the difference between the spectrum, it was found that the eighth BChl can affect two different excitonic energy transfer pathways, these being: (1) directly involve in the first pathway 6 $rightarrow$ 3 $rightarrow$ 1 of the apo form model by passing the excitonic energy to exciton 6; and (2) increase the excitonic wave function overlap between excitons 4 and 5 in the second pathway (7 $rightarrow$ 4,5 $rightarrow$ 2 $rightarrow$ 1) and thus increase the possible downward sampling routes across the BChls.
rate research
Read More
Using methods of condensed matter and statistical physics, we examine the transport of excitons through the Fenna-Matthews-Olson (FMO) complex from a receiving antenna to a reaction center. Writing the equations of motion for the exciton creation/annihilation operators, we are able to describe the exciton dynamics, even in the regime when the reorganization energy is of the order of the intra-system couplings. In particular, we obtain the well-known quantum oscillations of the site populations. We determine the exciton transfer efficiency in the presence of a quenching field and protein environment. While the majority of the protein vibronic modes are treated as a heat bath, we address the situation when specific modes are strongly coupled to excitons and examine the effects of these modes on the quantum oscillations and the energy transfer efficiency. We find that, for the vibronic frequencies below 16 meV, the exciton transfer is drastically suppressed. We attribute this effect to the formation of polaronic states where the exciton is transferred back and forth between the two pigments with the absorption/emission of the vibronic quanta, instead of proceeding to the reaction center. The same effect suppresses the quantum beating at the vibronic frequency of 25 meV. We also show that the efficiency of the energy transfer can be enhanced when the vibronic mode strongly couples to the third pigment only, instead of coupling to the entire system.
Recent interest in the role of quantum mechanics in the primary events of photosynthetic energy transfer has led to a convergence of nonlinear optical spectroscopy and quantum optics on the topic of energy-transfer dynamics in pigment-protein complexes. The convergence of these two communities has unveiled a mismatch between the background and terminology of the respective fields. To make connections, we provide a pedagogical guide to understanding the basics of two-dimensional electronic spectra aimed at researchers with a background in quantum optics.
We investigate how correlated fluctuations affect oscillatory features in rephasing and non-rephasing two-dimensional (2D) electronic spectra of a model dimer system. Based on a beating map analysis, we show that non-secular environmental couplings induced by uncorrelated fluctuations lead to oscillations centered at both cross- and diagonal-peaks in rephasing spectra as well as in non-rephasing spectra. Using an analytical approach, we provide a quantitative description of the non-secular effects in terms of the Feynman diagrams and show that the environment-induced mixing of different inter-excitonic coherences leads to oscillations in the rephasing diagonal-peaks and non-rephasing cross-peaks. We demonstrate that as correlations in the noise increase, the lifetime of oscillatory 2D signals is enhanced at rephasing cross-peaks and non-rephasing diagonal-peaks, while the other non-secular oscillatory signals are suppressed. We discuss that the asymmetry of 2D lineshapes in the beating map provides information on the degree of correlations in environmental fluctuations. Finally we investigate how the oscillatory features in 2D spectra are affected by inhomogeneous broadening.
The idea that excitonic state (electronic) coherences are of fundamental importance to natural photosynthesis gained popularity when, a decade ago, slowly dephasing quantum beats were observed in the two-dimensional electronic spectra of the Fenna-Matthews-Olson complex at 77 K. These were assigned to quantum superpositions of excitonic states; a controversial interpretation, as the spectral linewidths suggested fast dephasing arising from strong interactions with the environment. While it has been pointed out that vibrational motion produces similar spectral signatures, concrete assignment of these coherences to distinct physical processes is still lacking. Here we revisit the coherence dynamics of the Fenna-Matthews-Olson complex using polarization-controlled two-dimensional electronic spectroscopy, supported by theoretical modelling. We show that the long-lived quantum beats originate exclusively from vibrational coherences, whereas electronic coherences dephase entirely within 240 fs even at 77 K - a timescale too short to play a significant role in light harvesting. Additionally, we demonstrate that specific vibrational coherences are excited via vibronically coupled states. The detection of vibronic coupling indicates the relevance of this phenomenon for photosynthetic energy transfer.
In 2D electronic spectroscopy studies, long-lived quantum beats have recently been observed in photosynthetic systems, and it has been suggested that the beats are produced by quantum mechanically mixed electronic and vibrational states. Concerning the electronic-vibrational quantum mixtures, the impact of protein-induced fluctuations was examined by calculating the 2D electronic spectra of a weakly coupled dimer with vibrational modes in the resonant condition [J. Chem. Phys. 142, 212403 (2015)]. This analysis demonstrated that quantum mixtures of the vibronic resonance are rather robust under the influence of the fluctuations at cryogenic temperatures, whereas the mixtures are eradicated by the fluctuations at physiological temperatures. However, this conclusion cannot be generalized because the magnitude of the coupling inducing the quantum mixtures is proportional to the inter-pigment coupling. In this study, we explore the impact of the fluctuations on electronic-vibrational quantum mixtures in a strongly coupled dimer. with an off-resonant vibrational mode. Toward this end, we calculate electronic energy transfer (EET) dynamics and 2D electronic spectra of a dimer that corresponds to the most strongly coupled bacteriochlorophyll molecules in the Fenna-Matthews-Olson complex in a numerically accurate manner. The quantum mixtures are found to be robust under the exposure of protein-induced fluctuations at cryogenic temperatures, irrespective of the resonance. At 300 K, however, the quantum mixing is disturbed more strongly by the fluctuations, and therefore, the beats in the 2D spectra become obscure even in a strongly coupled dimer with a resonant vibrational mode. Further, the overall behaviors of the EET dynamics are demonstrated to be dominated by the environment and coupling between the 0-0 vibronic transitions as long as the Huang-Rhys factor of the vibrational mode is small.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
