14 research outputs found
Supplementary Material from Photoprotection through ultrafast charge recombination in photochemical reaction centres under oxidizing conditions
Materials and Methods; Figures S1-S10
Functional Compartmental Modeling of the Photosystems in the Thylakoid Membrane at 77 K
Time-resolved
fluorescence spectroscopy measurements at 77 K on
thylakoid membrane preparations and isolated photosynthetic complexes
thereof were investigated using target analysis with the aim of building
functional compartmental models for the photosystems in the thylakoid
membrane. Combining kinetic schemes with different spectral constraints
enabled us to resolve the energy transfer pathways and decay characteristics
of the different emissive species. We determined the spectral and
energetic properties of the red Chl pools in both photosystems and
quantified the formation of LHCII-LHCI-PSI supercomplexes in the transition
from native to unstacked thylakoid membranes
Excited States of the Inactive and Active Forms of the Orange Carotenoid Protein
The orange carotenoid protein (OCP)
is a crucial player in the process of nonphotochemical quenching in
a large number of cyanobacteria. This water-soluble protein binds
one pigment only, the keto carotenoid 3′-hydroxyechinenone,
and needs to be photoactivated by strong (blue-green) light in order
to induce energy dissipation within or from the phycobilisome, the
main light harvesting system of these organisms. We performed transient-absorption
spectroscopy on OCP samples frozen in the inactive and active forms
at 77 K. By making use of target analysis we determined the excited
state properties of the active form. Our results show that OCP photoactivation
modifies the carotenoid excited state energy landscape. More specifically
the photoactivated OCP is characterized by one state with predominantly
ICT character (ICT/S<sub>1</sub>) and a lifetime of 2.3 ps, and another
state with mainly S<sub>1</sub> character (S<sub>1</sub>/ICT) with
a lifetime of 7.6 ps. We also show that the kinetic model is fully
consistent with the RT data obtained earlier (Berera et al., <i>J. Phys. Chem.</i> <i>B</i> <b>2012</b>, <i>116</i>, 2568–2574). We propose that this ICT/S<sub>1</sub> state acts as the quencher in the OCP mediated nonphotochemical
quenching
Vibronic Coherence in the Charge Separation Process of the <i>Rhodobacter sphaeroides</i> Reaction Center
Two-dimensional
electronic spectroscopy was applied to a variant
of the reaction center (RC) of purple bacterium <i>Rhodobacter
sphaeroides</i> lacking the primary acceptor ubiquinone in order
to understand the ultrafast separation and transfer of charge between
the bacteriochlorin cofactors. For the first time, characteristic
2D spectra were obtained for the participating excited and charge-transfer
states, and the electron-transfer cascade (including two different
channels, the P* and B* channels) was fully mapped. By analyzing quantum
beats using 2D frequency maps, excited-state vibrational modes at
153 and 33 cm<sup>–1</sup> were identified. We speculate that
these modes couple to the charge separation (CS) process and collectively
optimize the CS and are responsible for the superhigh efficiency
Excitonic and Vibrational Coherence in the Excitation Relaxation Process of Two LH1 Complexes as Revealed by Two-Dimensional Electronic Spectroscopy
Ultrafast
excitation relaxation within a manifold exciton state
and long-lived vibrational coherence are two universal characteristics
of photosynthetic antenna complexes. In this work, we studied the
two-dimensional electronic spectra of two core light-harvesting (LH1)
complexes of <i>Thermochromatium</i> (<i>Tch.</i>) <i>tepidum</i>, native Ca<sup>2+</sup>-LH1 and modified
Ba<sup>2+</sup>-LH1. The role of the vibrational coherence in the
exciton relaxation was revealed by comparing the two LH1 with similar
structures but different electronic properties and by the evolution
of the exciton and vibrational coherence as a function of temperature
Achieving Exciton Delocalization in Quantum Dot Aggregates Using Organic Linker Molecules
The design of new complex structures
containing semiconductor quantum
dots offers a means to create a variety of new meso-solids and molecules.
The control of the coupling properties between the dots, accompanied
by the energetic tunability of the dots themselves, paves the way
toward the application and use of novel quantum properties. Here we
present our approach to alteration of interdot coupling using organic
linking molecules in a system of covalently bonded, aggregated quantum
dots. We used ultrafast transient absorption measurements to identify
marks of exciton delocalization over nearest neighbors to some extent.
In linking molecules incorporating a benzene ring, the delocalized
electron cloud displayed a profound influence over the interdot effects,
leading the way to easy coupling control in quantum-based devices,
under ambient conditions
Direct Observation of Energy Detrapping in LH1-RC Complex by Two-Dimensional Electronic Spectroscopy
The purple bacterial
core light harvesting antenna-reaction center
(LH1-RC) complex is the simplest system able to achieve the entire
primary function of photosynthesis. During the past decade, a variety
of photosynthetic proteins were studied by a powerful technique, two-dimensional
electronic spectroscopy (2DES). However, little attention has been
paid to LH1-RC, although its reversible uphill energy transfer, trapping,
and backward detrapping processes, represent a crucial step in the
early photosynthetic reaction dynamics. Thus, in this work, we employed
2DES to study two LH1-RC complexes of Thermochromatium (Tch.) tepidum. By direct observation of detrapping, the complex reversible process
was clearly identified and an overall scheme of the excitation evolution
in LH1-RC was obtained
Phycocyanin: One Complex, Two States, Two Functions
Solar energy captured
by pigments embedded in light-harvesting
complexes can be transferred to neighboring pigments, dissipated,
or emitted as fluorescence. Only when it reaches a reaction center
is the excitation energy stabilized in the form of a charge separation
and converted into chemical energy. Well-directed and regulated energy
transfer within the network of pigments is therefore of crucial importance
for the success of the photosynthetic processes. Using single-molecule
spectroscopy, we show that phycocyanin can dynamically switch between
two spectrally distinct states originating from two different conformations.
Unexpectedly, one of the two states has a red-shifted emission spectrum.
This state is not involved in energy dissipation; instead, we propose
that it is involved in direct energy transfer to photosystem I. Finally,
our findings suggest that the function of linker proteins in phycobilisomes
is to stabilize one state or the other, thus controlling the light-harvesting
functions of phycocyanin
Mechanistic Regimes of Vibronic Transport in a Heterodimer and the Design Principle of Incoherent Vibronic Transport in Phycobiliproteins
Following the observation
of coherent oscillations in nonlinear
spectra of photosynthetic pigment protein complexes, in particular,
phycobilliproteins such as PC645, coherent vibronic transport has
been suggested as a design principle for novel light-harvesting materials.
Vibronic transport between energetically remote pigments is coherent
when the presence of a vibration resonant with the electronic energy
gap supports transient delocalization between the electronic excited
states. We establish the mechanism of vibronic transport for a model
heterodimer across a wide range of molecular parameter values. The
resulting mechanistic map demonstrates that the molecular parameters
of phycobiliproteins in fact support incoherent vibronic transport.
This result points to an important design principle: Incoherent vibronic
transport is more efficient than a coherent mechanism when energetic
disorder exceeds the coupling between the donor and vibrationally
excited acceptor states. Finally, our results suggest that the role
of coherent vibronic transport in pigment protein complexes should
be reevaluated
The Photophysics of the Orange Carotenoid Protein, a Light-Powered Molecular Switch
To cope with the deleterious effects of excess illumination,
photosynthetic
organisms have developed photoprotective mechanisms that dissipate
the absorbed excess energy as heat from the antenna system. In cyanobacteria,
a crucial step in the process is the activation, by blue-green light,
of a soluble protein, known as orange carotenoid protein (OCP), which
binds the carotenoid 3′-hydroxyechinenone as its only pigment.
While the spectroscopic properties of the inactive form of OCP have
been described, the nature of the excited states in the active form
still awaits elucidation. We applied transient absorption spectroscopy
to the dark and the light activated forms of OCP to study and compare
the excited state dynamics of both forms. We show that excitation
of the photoactivated OCP leads to the population of new carotenoid
excited states. One of these states populated shortly after excitation
is characterized by a very pronounced charge transfer character and
a lifetime of about 0.6 ps. When the illuminated sample is exposed
to a dark relaxation period, it responds to excitation as the original
dark sample, showing that photoactivation and decay of the photoactivated
state are fully reversible. Thus OCP functions as a light-powered
molecular switch that modulates its spectroscopic properties as a
response to specific changes in light environment. We discuss the
importance of this switch in cyanobacteria photoprotection and propose
a mechanism wherein the red state of OCP echinenone acts as an energy
dissipator via its charge transfer state