131 research outputs found
Two-Dimensional Electronic Spectroscopy of Chlorophyll a: Solvent Dependent Spectral Evolution
The interaction of the monomeric chlorophyll Q-band electronic transition with solvents of differing physical-chemical properties is investigated through two-dimensional electronic spectroscopy (2DES). Chlorophyll constitutes the key chromophore molecule in light harvesting complexes. It is well-known that the surrounding protein in the light harvesting complex fine-tunes chlorophyll electronic transitions to optimize energy transfer. Therefore, an understanding of the influence of the environment on the monomeric chlorophyll electronic transitions is important. The Q-band 2DES is inhomogeneous at early times, particularly in hydrogen bonding polar solvents, but also in nonpolar solvents like cyclohexane. Interestingly this inhomogeneity persists for long times, even up to the nanosecond time scale in some solvents. The reshaping of the 2DES occurs over multiple time scales and was assigned mainly to spectral diffusion. At early times the reshaping is Gaussian-like, hinting at a strong solvent reorganization effect. The temporal evolution of the 2DES response was analyzed in terms of a Brownian oscillator model. The spectral densities underpinning the Brownian oscillator fitting were recovered for the different solvents. The absorption spectra and Stokes shift were also properly described by this model. The extent and nature of inhomogeneous broadening was a strong function of solvent, being larger in H-bonding and viscous media and smaller in nonpolar solvents. The fastest spectral reshaping components were assigned to solvent dynamics, modified by interactions with the solute
Single-molecule spectroscopy of LHCSR1 protein dynamics identifies two distinct states responsible for multi-timescale photosynthetic photoprotection
In oxygenic photosynthesis, light harvesting is regulated to safely dissipate excess energy and prevent the formation of harmful photoproducts. Regulation is known to be necessary for fitness, but the molecular mechanisms are not understood. One challenge has been that ensemble experiments average over active and dissipative behaviours, preventing identification of distinct states. Here, we use single-molecule spectroscopy to uncover the photoprotective states and dynamics of the light-harvesting complex stress-related 1 (LHCSR1) protein, which is responsible for dissipation in green algae and moss. We discover the existence of two dissipative states. We find that one of these states is activated by pH and the other by carotenoid composition, and that distinct protein dynamics regulate these states. Together, these two states enable the organism to respond to two types of intermittency in solar intensity-step changes (clouds and shadows) and ramp changes (sunrise), respectively. Our findings reveal key control mechanisms underlying photoprotective dissipation, with implications for increasing biomass yields and developing robust solar energy devices
Full characterization of vibrational coherence in a porphyrin chromophore by two-dimensional electronic spectroscopy
In this work we present experimental and calculated two-dimensional electronic spectra for a 5,15-bisalkynyl porphyrin chromophore. The lowest energy electronic Qy transition couples mainly to a single 380 cm–1 vibrational mode. The two-dimensional electronic spectra reveal diagonal and cross peaks which oscillate as a function of population time. We analyze both the amplitude and phase distribution of this main vibronic transition as a function of excitation and detection frequencies. Even though Feynman diagrams provide a good indication of where the amplitude of the oscillating components are located in the excitation-detection plane, other factors also affect this distribution. Specifically, the oscillation corresponding to each Feynman diagram is expected to have a phase that is a function of excitation and detection frequencies. Therefore, the overall phase of the experimentally observed oscillation will reflect this phase dependence. Another consequence is that the overall oscillation amplitude can show interference patterns resulting from overlapping contributions from neighboring Feynman diagrams. These observations are consistently reproduced through simulations based on third order perturbation theory coupled to a spectral density described by a Brownian oscillator model
Multiscale photosynthetic exciton transfer
Photosynthetic light harvesting provides a natural blueprint for
bioengineered and biomimetic solar energy and light detection technologies.
Recent evidence suggests some individual light harvesting protein complexes
(LHCs) and LHC subunits efficiently transfer excitons towards chemical reaction
centers (RCs) via an interplay between excitonic quantum coherence, resonant
protein vibrations, and thermal decoherence. The role of coherence in vivo is
unclear however, where excitons are transferred through multi-LHC/RC aggregates
over distances typically large compared with intra-LHC scales. Here we assess
the possibility of long-range coherent transfer in a simple chromophore network
with disordered site and transfer coupling energies. Through renormalization we
find that, surprisingly, decoherence is diminished at larger scales, and
long-range coherence is facilitated by chromophoric clustering. Conversely,
static disorder in the site energies grows with length scale, forcing
localization. Our results suggest sustained coherent exciton transfer may be
possible over distances large compared with nearest-neighbour (n-n) chromophore
separations, at physiological temperatures, in a clustered network with small
static disorder. This may support findings suggesting long-range coherence in
algal chloroplasts, and provides a framework for engineering large chromophore
or quantum dot high-temperature exciton transfer networks.Comment: 9 pages, 6 figures. A significantly updated version is now published
online by Nature Physics (2012
Single-molecule identification of quenched and unquenched states of LHCII
In photosynthetic light harvesting, absorbed
sunlight is converted to electron flow with near-unity quantum
efficiency under low light conditions. Under high light
conditions, plants avoid damage to their molecular machinery
by activating a set of photoprotective mechanisms to
harmlessly dissipate excess energy as heat. To investigate
these mechanisms, we study the primary antenna complex in
green plants, light-harvesting complex II (LHCII), at the
single-complex level. We use a single-molecule technique, the Anti-Brownian Electrokinetic trap, which enables simultaneous
measurements of fluorescence intensity, lifetime, and spectra in solution. With this approach, including the first measurements of
fluorescence lifetime on single LHCII complexes, we access the intrinsic conformational dynamics. In addition to an unquenched
state, we identify two partially quenched states of LHCII. Our results suggest that there are at least two distinct quenching sites
with different molecular compositions, meaning multiple dissipative pathways in LHCII. Furthermore, one of the quenched
conformations significantly increases in relative population under environmental conditions mimicking high light.This material is based on work supported in part by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences under Award Number DE-FG02-07ER15892 (to W.E.M)
and by the Dutch organization for scientific research (NWO-ALW) via a Vici grant (to
R.C.). R.v.G. and T.P.J.K. were supported by the Netherlands Organization for Sciences,
Council of Chemical Sciences (NWO-CW) via a TOP-grant (700.58.305). R.v.G. was
further supported by an Advanced Investigator grant from the European Research
Council (no. 267333, PHOTPROT) and by the EU FP7 project PAPETS (GA 323901).
R.v.G. gratefully acknowledges his Academy Professor grant from the Royal Netherlands
Academy of Arts and Sciences (KNAW). T.P.J.K. was further supported by University of
Pretoria’s Research Development Programme (grant no. A0W679). The authors would
like to acknowledge the following fellowships: a Postdoctoral Fellowship from the
Center for Molecular Analysis and Design at Stanford University (to G.S.S.-C.); a
Kenneth and Nina Tai Stanford Graduate Fellowship (to H.-Y.Y.); and a Long Term
Fellowship from EMBO (to M.G.).http://pubs.acs.org/journal/jpclcdhb2017Physic
Single-molecule identification of quenched and unquenched states of LHCII
In photosynthetic light harvesting, absorbed
sunlight is converted to electron flow with near-unity quantum
efficiency under low light conditions. Under high light
conditions, plants avoid damage to their molecular machinery
by activating a set of photoprotective mechanisms to
harmlessly dissipate excess energy as heat. To investigate
these mechanisms, we study the primary antenna complex in
green plants, light-harvesting complex II (LHCII), at the
single-complex level. We use a single-molecule technique, the Anti-Brownian Electrokinetic trap, which enables simultaneous
measurements of fluorescence intensity, lifetime, and spectra in solution. With this approach, including the first measurements of
fluorescence lifetime on single LHCII complexes, we access the intrinsic conformational dynamics. In addition to an unquenched
state, we identify two partially quenched states of LHCII. Our results suggest that there are at least two distinct quenching sites
with different molecular compositions, meaning multiple dissipative pathways in LHCII. Furthermore, one of the quenched
conformations significantly increases in relative population under environmental conditions mimicking high light.This material is based on work supported in part by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences under Award Number DE-FG02-07ER15892 (to W.E.M)
and by the Dutch organization for scientific research (NWO-ALW) via a Vici grant (to
R.C.). R.v.G. and T.P.J.K. were supported by the Netherlands Organization for Sciences,
Council of Chemical Sciences (NWO-CW) via a TOP-grant (700.58.305). R.v.G. was
further supported by an Advanced Investigator grant from the European Research
Council (no. 267333, PHOTPROT) and by the EU FP7 project PAPETS (GA 323901).
R.v.G. gratefully acknowledges his Academy Professor grant from the Royal Netherlands
Academy of Arts and Sciences (KNAW). T.P.J.K. was further supported by University of
Pretoria’s Research Development Programme (grant no. A0W679). The authors would
like to acknowledge the following fellowships: a Postdoctoral Fellowship from the
Center for Molecular Analysis and Design at Stanford University (to G.S.S.-C.); a
Kenneth and Nina Tai Stanford Graduate Fellowship (to H.-Y.Y.); and a Long Term
Fellowship from EMBO (to M.G.).http://pubs.acs.org/journal/jpclcdhb2017Physic
Genotyping a second growth coast redwood forest : a high throughput methodology
The idea that excitonic (electronic) coherences are of fundamental importance to natural photosynthesis gained popularity when slowly dephasing quantum beats (QBs) were observed in the two-dimensional electronic spectra of the Fenna–Matthews–Olson (FMO) complex at 77 K. These were assigned to superpositions of excitonic states, a controversial interpretation, as the strong chromophore–environment interactions in the complex suggest fast dephasing. Although it has been pointed out that vibrational motion produces similar spectral signatures, a concrete assignment of these oscillatory signals to distinct physical processes is still lacking. Here we revisit the coherence dynamics of the FMO complex using polarization-controlled two-dimensional electronic spectroscopy, supported by theoretical modelling. We show that the long-lived QBs are exclusively vibrational in origin, whereas the dephasing of the electronic coherences is completed within 240 fs even at 77 K. We further find that specific vibrational coherences are produced via vibronically coupled excited states. The presence of such states suggests that vibronic coupling is relevant for photosynthetic energy transfer
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Pseudo-Rephasing and Pseudo-Free-Induction-Decay Mechanism in Two-Color Three-Pulse Photon Echo of a Binary System
Fine control of chlorophyll-carotenoid interactions defines the functionality of light-harvesting proteins in plants
V.B. and C.D.P.D. acknowledge the support from the Leverhulme Trust RPG-2015-337. This research utilized Queen Mary’s MidPlus computational facilities, supported by QMUL Research-IT and funded by EPSRC grant EP/K000128/1. W.P.B acknowledges support from the Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0001035 for initial development of the TDC calculation code, as well as support from Army Research Office (ARO-MURI) Award W911NF1210420 for further development
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