256 research outputs found
Disentangling the low-energy states of the major light-harvesting complex of plants and their role in photoprotection
The ability to dissipate large fractions of their absorbed light energy as heat is a vital photoprotective function of
the peripheral light-harvesting pigment–protein complexes in photosystemII of plants. The major component of
this process, known as qE, is characterised by the appearance of low-energy (red-shifted) absorption and fluorescence
bands. Although the appearance of these red states has been established, the molecular mechanism, their
site and particularly their involvement in qE are strongly debated. Here, room-temperature single-molecule fluorescence
spectroscopy was used to study the red emission states of the major plant light-harvesting complex
(LHCII) in different environments, in particular conditions mimicking qE. It was found that most states correspond
to peak emission at around 700 nm and are unrelated to energy dissipative states, though their frequency
of occurrence increased under conditions that mimicked qE. Longer-wavelength emission appeared to be directly
related to energy dissipative states, in particular emission beyond 770nm. The ensemble average of the red emission
bands shares many properties with those obtained from previous bulk in vitro and in vivo studies. We propose
the existence of at least three excitation energy dissipating mechanisms in LHCII, each of which is associated
with a different spectral signature and whose contribution to qE is determined by environmental control of protein
conformational disorder. Emission at 700 nmis attributed to a conformational change in the Lut 2 domain,which is
facilitated by the conformational change associated with the primary quenching mechanism involving Lut 1.This work was supported by the EU FP7Marie Curie Reintegration Grant (ERG 224796) (C.I.); the CEA-Eurotalents Program(PCOFUNDGA- 2008-228664) (C.I.); research and equipment grants from UK BBSRC and EPSRC (M.P.J. and A.V.R.); Grants from the Netherlands Organization for Scientific Research (700.58.305 and 700.56.014 from the Foundation of Chemical Sciences) (T.P.J.K., C.I., and R.v.G.),and the Advanced Investigator Grant (267333, PHOTPROT) from the European Research Council (ERC) (C.I., T.P.J.K., and R.v.G.).http://www.elsevier.com/locate/bbabiohb2014ai201
Quantum physics meets biology
Quantum physics and biology have long been regarded as unrelated disciplines,
describing nature at the inanimate microlevel on the one hand and living
species on the other hand. Over the last decades the life sciences have
succeeded in providing ever more and refined explanations of macroscopic
phenomena that were based on an improved understanding of molecular structures
and mechanisms. Simultaneously, quantum physics, originally rooted in a world
view of quantum coherences, entanglement and other non-classical effects, has
been heading towards systems of increasing complexity. The present perspective
article shall serve as a pedestrian guide to the growing interconnections
between the two fields. We recapitulate the generic and sometimes unintuitive
characteristics of quantum physics and point to a number of applications in the
life sciences. We discuss our criteria for a future quantum biology, its
current status, recent experimental progress and also the restrictions that
nature imposes on bold extrapolations of quantum theory to macroscopic
phenomena.Comment: 26 pages, 4 figures, Perspective article for the HFSP Journa
Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy
In this work, the transfer of excitation energy was studied in native and cation-depletion induced, unstacked thylakoid membranes of spinach by steady-state and time-resolved fluorescence spectroscopy. Fluorescence emission spectra at 5 K show an increase in photosystem I (PSI) emission upon unstacking, which suggests an increase of its antenna size. Fluorescence excitation measurements at 77 K indicate that the increase of PSI emission upon unstacking is caused both by a direct spillover from the photosystem II (PSII) core antenna and by a functional association of light-harvesting complex II (LHCII) to PSI, which is most likely caused by the formation of LHCII-LHCI-PSI supercomplexes. Time-resolved fluorescence measurements, both at room temperature and at 77 K, reveal differences in the fluorescence decay kinetics of stacked and unstacked membranes. Energy transfer between LHCII and PSI is observed to take place within 25 ps at room temperature and within 38 ps at 77 K, consistent with the formation of LHCII-LHCI-PSI supercomplexes. At the 150-160 ps timescale, both energy transfer from LHCII to PSI as well as spillover from the core antenna of PSII to PSI is shown to occur at 77 K. At room temperature the spillover and energy transfer to PSI is less clear at the 150 ps timescale, because these processes compete with charge separation in the PSII reaction center, which also takes place at a timescale of about 150 ps. © 2007 Springer Science+Business Media B.V
Excitation Dynamics and Relaxation in a Molecular Heterodimer
The exciton dynamics in a molecular heterodimer is studied as a function of
differences in excitation and reorganization energies, asymmetry in transition
dipole moments and excited state lifetimes. The heterodimer is composed of two
molecules modeled as two-level systems coupled by the resonance interaction.
The system-bath coupling is taken into account as a modulating factor of the
energy gap of the molecular excitation, while the relaxation to the ground
state is treated phenomenologically. Comparison of the description of the
excitation dynamics modeled using either the Redfield equations (secular and
full forms) or the Hierarchical quantum master equation (HQME) is demonstrated
and discussed. Possible role of the dimer as an excitation quenching center in
photosynthesis self-regulation is discussed. It is concluded that the
system-bath interaction rather than the excitonic effect determines the
excitation quenching ability of such a dimer
Ultrafast transient absorption spectroscopy: principles and application to photosynthetic systems
The photophysical and photochemical reactions, after light absorption by a photosynthetic pigment–protein complex, are among the fastest events in biology, taking place on timescales ranging from tens of femtoseconds to a few nanoseconds. The advent of ultrafast laser systems that produce pulses with femtosecond duration opened up a new area of research and enabled investigation of these photophysical and photochemical reactions in real time. Here, we provide a basic description of the ultrafast transient absorption technique, the laser and wavelength-conversion equipment, the transient absorption setup, and the collection of transient absorption data. Recent applications of ultrafast transient absorption spectroscopy on systems with increasing degree of complexity, from biomimetic light-harvesting systems to natural light-harvesting antennas, are presented. In particular, we will discuss, in this educational review, how a molecular understanding of the light-harvesting and photoprotective functions of carotenoids in photosynthesis is accomplished through the application of ultrafast transient absorption spectroscopy
Excitons in a Photosynthetic Light-Harvesting System: A Combined Molecular Dynamics/Quantum Chemistry and Polaron Model Study
The dynamics of pigment-pigment and pigment-protein interactions in
light-harvesting complexes is studied with a novel approach which combines
molecular dynamics (MD) simulations with quantum chemistry (QC) calculations.
The MD simulations of an LH-II complex, solvated and embedded in a lipid
bilayer at physiological conditions (with total system size of 87,055 atoms)
revealed a pathway of a water molecule into the B800 binding site, as well as
increased dimerization within the B850 BChl ring, as compared to the
dimerization found for the crystal structure. The fluctuations of pigment (B850
BChl) excitation energies, as a function of time, were determined via ab initio
QC calculations based on the geometries that emerged from the MD simulations.
From the results of these calculations we constructed a time-dependent
Hamiltonian of the B850 exciton system from which we determined the linear
absorption spectrum. Finally, a polaron model is introduced to describe quantum
mechanically both the excitonic and vibrational (phonon) degrees of freedom.
The exciton-phonon coupling that enters into the polaron model, and the
corresponding phonon spectral function are derived from the MD/QC simulations.
It is demonstrated that, in the framework of the polaron model, the absorption
spectrum of the B850 excitons can be calculated from the autocorrelation
function of the excitation energies of individual BChls, which is readily
available from the combined MD/QC simulations. The obtained result is in good
agreement with the experimentally measured absorption spectrum.Comment: REVTeX3.1, 23 pages, 13 (EPS) figures included. A high quality PDF
file of the paper is available at
http://www.ks.uiuc.edu/Publications/Papers/PDF/DAMJ2001/DAMJ2001.pd
Revisiting the optical properties of the FMO protein
We review the optical properties of the FMO complex as found by spectroscopic studies of the Qy band over the last two decades. This article emphasizes the different methods used, both experimental and theoretical, to elucidate the excitonic structure and dynamics of this pigment–protein complex
Linear dichroism and circular dichroism in photosynthesis research
The efficiency of photosynthetic light energy conversion depends largely on the molecular architecture of the photosynthetic membranes. Linear- and circular-dichroism (LD and CD) studies have contributed significantly to our knowledge of the molecular organization of pigment systems at different levels of complexity, in pigment–protein complexes, supercomplexes, and their macroassemblies, as well as in entire membranes and membrane systems. Many examples show that LD and CD data are in good agreement with structural data; hence, these spectroscopic tools serve as the basis for linking the structure of photosynthetic pigment–protein complexes to steady-state and time-resolved spectroscopy. They are also indispensable for identifying conformations and interactions in native environments, and for monitoring reorganizations during photosynthetic functions, and are important in characterizing reconstituted and artificially constructed systems. This educational review explains, in simple terms, the basic physical principles, and theory and practice of LD and CD spectroscopies and of some related quantities in the areas of differential polarization spectroscopy and microscopy
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