8 research outputs found
Vibronic Wavepackets and Energy Transfer in Cryptophyte Light-Harvesting Complexes
Determining the key features of high-efficiency
photosynthetic energy transfer remains an ongoing task. Recently,
there has been evidence for the role of vibronic coherence in linking
donor and acceptor states to redistribute oscillator strength for
enhanced energy transfer. To gain further insights into the interplay
between vibronic wavepackets and energy-transfer dynamics, we systematically
compare four structurally related phycobiliproteins from cryptophyte
algae by broad-band pumpāprobe spectroscopy and extend a parametric
model based on global analysis to include vibrational wavepacket characterization.
The four phycobiliproteins isolated from cryptophyte algae are two
āopenā
structures and two āclosedā structures. The closed structures
exhibit strong exciton coupling in the central dimer. The dominant
energy-transfer pathway occurs on the subpicosecond timescale across
the largest energy gap in each of the proteins, from central to peripheral
chromophores. All proteins exhibit a strong 1585 cm<sup>ā1</sup> coherent oscillation whose relative amplitude, a measure of vibronic
intensity borrowing from resonance between donor and acceptor states,
scales with both energy-transfer rates and damping rates. Central
exciton splitting may aid in bringing the vibronically linked donor
and acceptor states into better resonance resulting in the observed
doubled rate in the closed structures. Several excited-state vibrational
wavepackets persist on timescales relevant to energy transfer, highlighting
the importance of further investigation of the interplay between electronic
coupling and nuclear degrees of freedom in studies on high-efficiency
photosynthesis
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
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 3ā²-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
Ultrafast Proton Shuttling in <i>Psammocora</i> Cyan Fluorescent Protein
Cyan, green, yellow, and red fluorescent
proteins (FPs) homologous
to green fluorescent protein (GFP) are used extensively as model systems
to study fundamental processes in photobiology, such as the capture
of light energy by protein-embedded chromophores, color tuning by
the protein matrix, energy conversion by FoĢrster resonance
energy transfer (FRET), and excited-state proton transfer (ESPT) reactions.
Recently, a novel cyan fluorescent protein (CFP) termed psamFP488
was isolated from the genus <i>Psammocora</i> of reef building
corals. Within the cyan color class, psamFP488 is unusual because
it exhibits a significantly extended Stokes shift. Here, we applied
ultrafast transient absorption and pumpādumpāprobe spectroscopy
to investigate the mechanistic basis of psamFP488 fluorescence, complemented
with fluorescence quantum yield and dynamic light scattering measurements.
Transient absorption spectroscopy indicated that, upon excitation
at 410 nm, the stimulated cyan emission rises in 170 fs. With pumpādumpāprobe
spectroscopy, we observe a very short-lived (110 fs) ground-state
intermediate that we assign to the deprotonated, anionic chromophore.
In addition, a minor fraction (14%) decays with 3.5 ps to the ground
state. Structural analysis of homologous proteins indicates that Glu-167
is likely positioned in sufficiently close vicinity to the chromophore
to act as a proton acceptor. Our findings support a model where unusually
fast ESPT from the neutral chromophore to Glu-167 with a time constant
of 170 fs and resulting emission from the anionic chromophore forms
the basis of the large psamFP488 Stokes shift. When dumped to the
ground state, the proton on neutral Glu is very rapidly shuttled back
to the anionic chromophore in 110 fs. Proton shuttling in excited
and ground states is a factor of 20ā4000 faster than in GFP,
which probably results from a favorable hydrogen-bonding geometry
between the chromophore phenolic oxygen and the glutamate acceptor,
possibly involving a short hydrogen bond. At any time in the reaction,
the proton is localized on either the chromophore or Glu-167, which
implies that most likely no low-barrier hydrogen bond exists between
these molecular groups. This work supports the notion that proton
transfer in biological systems, be it in an electronic excited or
ground state, can be an intrinsically fast process that occurs on
a 100 fs time scale. PsamFP488 represents an attractive model system
that poses an ultrafast proton transfer regime in discrete steps.
It constitutes a valuable model system in addition to wild type GFP,
where proton transfer is relatively slow, and the S65T/H148D GFP mutant,
where the effects of low-barrier hydrogen bonds dominate
Hydrogen Bond Switching among Flavin and Amino Acids Determines the Nature of Proton-Coupled Electron Transfer in BLUF Photoreceptors
BLUF domains are flavin-binding photoreceptors that can
be reversibly
switched from a dark-adapted state to a light-adapted state. Proton-coupled
electron transfer (PCET) from a conserved tyrosine to the flavin that
results in a neutral flavin semiquinone/tyrosyl radical pair constitutes
the photoactivation mechanism of BLUF domains. Whereas in the dark-adapted
state PCET occurs in a sequential fashion where electron transfer
precedes proton transfer, in the light-adapted state the same radical
pair is formed by a concerted mechanism. We propose that the altered
nature of the PCET process results from a hydrogen bond switch between
the flavin and its surrounding amino acids that preconfigures the
system for proton transfer. Hence, BLUF domains represent an attractive
biological model system to investigate and understand PCET in great
detail
Subpicosecond Excited-State Proton Transfer Preceding Isomerization During the Photorecovery of Photoactive Yellow Protein
The ultrafast excited-state dynamics underlying the receptor state photorecovery is resolved in the M100A mutant of the photoactive yellow protein (PYP) from Halorhodospira halophila. The M100A PYP mutant, with its distinctly slower photocycle than wt PYP, allows isolation of the pB signaling state for study of the photodynamics of the protonated chromophore <i>cis-p</i>-coumaric acid. Transient absorption signals indicate a subpicosecond excited-state proton-transfer reaction in the pB state that results in chromophore deprotonation prior to the cisātrans isomerization required in the photorecovery dynamics of the pG state. Two terminal photoproducts are observed, a blue-absorbing species presumed to be deprotonated <i>trans-p</i>-coumaric acid and an ultraviolet-absorbing protonated photoproduct. These two photoproducts are hypothesized to originate from an equilibrium of open and closed folded forms of the signaling state, I<sub>2</sub> and I<sub>2</sub>ā²
Photoionization and Electron Radical Recombination Dynamics in Photoactive Yellow Protein Investigated by Ultrafast Spectroscopy in the Visible and Near-Infrared Spectral Region
Photoinduced ionization of the chromophore
inside photoactive yellow
protein (PYP) was investigated by ultrafast spectroscopy in the visible
and near-infrared spectral regions. An absorption band that extended
from around 550 to 850 nm was observed and ascribed to solvated electrons,
ejected from the <i>p</i>-hydroxycinnamic acid anion chromophore
upon the absorption of two 400 nm photons. Global kinetic analysis
showed that the solvated electron absorption decayed in two stages:
a shorter phase of around 10 ps and a longer phase of more than 3
ns. From a simulation based on a diffusion model we conclude that
the diffusion rate of the electron is about 0.8 Ć
<sup>2</sup>/ps in wild type PYP, and that the electron is ejected to a short
distance of only several angstroms away from the chromophore. The
chromophoreāprotein pocket appears to provide a water-similar
local environment for the electron. Because mutations at different
places around the chromophore have different effect on the electron
recombination dynamics, we suggest that solvated electrons could provide
a new method to investigate the local dielectric environment inside
PYP and thus help to understand the role of the protein in the photoisomerization
process
Excitation Energy Trapping and Dissipation by Ni-Substituted Bacteriochlorophyll <i>a</i> in Reconstituted LH1 Complexes from Rhodospirillum rubrum
Bacteriochlorophyll <i>a</i> with Ni<sup>2+</sup> replacing
the central Mg<sup>2+</sup> ion was used as an ultrafast excitation
energy dissipation center in reconstituted bacterial LH1 complexes.
B870, a carotenoid-less LH1 complex, and B880, an LH1 complex containing
spheroidene, were obtained via reconstitution from the subunits isolated
from chromatophores of Rhodospirillum rubrum. Ni-substituted bacteriochlorophyll <i>a</i> added to
the reconstitution mixture partially substituted the native pigment
in both forms of LH1. The excited-state dynamics of the reconstituted
LH1 complexes were probed by femtosecond pumpāprobe transient
absorption spectroscopy in the visible and near-infrared spectral
region. Spheroidene-binding B880 containing no excitation dissipation
centers displayed complex dynamics in the time range of 0.1ā10
ps, reflecting internal conversion and intersystem crossing in the
carotenoid, exciton relaxation in BChl complement, and energy transfer
from carotenoid to the latter. In B870, some aggregation-induced excitation
energy quenching was present. The binding of Ni-BChl <i>a</i> to both B870 and B880 resulted in strong quenching of the excited
states with main deexcitation lifetime of ca. 2 ps. The LH1 excited-state
lifetime could be modeled with an intrinsic decay time constant in
Ni-substituted bacteriochlorophyll <i>a</i> of 160 fs. The
presence of carotenoid in LH1 did not influence the kinetics of energy
trapping by Ni-BChl unless the carotenoid was directly excited, in
which case the kinetics was limited by a slower carotenoid S<sub>1</sub> to bacteriochlorophyll energy transfer