13 research outputs found
Excited-State Dynamics of Monomeric and Aggregated Carotenoid 8′-Apo-β-carotenal
Excited-state properties of monomeric and aggregated
carbonyl carotenoid
8′-apo-β-carotenal were studied by means of femtosecond
transient absorption spectroscopy. For monomers, the polarity-dependent
behavior characteristic of carotenoids with conjugated carbonyl group
was observed. In <i>n</i>-hexane the S<sub>1</sub> lifetime
is 25 ps, but it is shortened to 8 ps in methanol. This shortening
is accompanied by the appearance of new spectral bands in transient
absorption spectrum. On the basis of analysis of the transient absorption
spectra of monomeric 8′-apo-β-carotenal in <i>n</i>-hexane and methanol, we propose that the polarity-induced spectral
bands are due to the S<sub>1</sub>(A<sub>g</sub><sup>–</sup>)–S<sub>3</sub>(A<sub>g</sub><sup>+</sup>) transition, which
is enhanced upon breaking the symmetry of the molecule. This symmetry
breaking is caused by the conjugated carbonyl group; it is much stronger
in polar solvents where the S<sub>1</sub> state gains significant
charge-transfer character. Upon addition of water to methanol solution
of 8′-apo-β-carotenal we observed formation of aggregates
characterized by either blue-shifted (H-aggregate) or red-shifted
(J-aggregate) absorption spectrum. Both aggregate types exhibit excited-state
dynamics significantly different from those of monomeric 8′-apo-β-carotenal.
The lifetime of the relaxed S<sub>1</sub> state is 20 and 40 ps for
the H- and J-aggregates, respectively. In contrast to monomers, aggregation
promotes formation of triplet state, most likely by homofission occurring
between tightly packed molecules within the aggregate
A Unified Picture of S* in Carotenoids
In
π-conjugated chain molecules such as carotenoids, coupling
between electronic and vibrational degrees of freedom is of central
importance. It governs both dynamic and static properties, such as
the time scales of excited state relaxation as well as absorption
spectra. In this work, we treat vibronic dynamics in carotenoids on
four electronic states (|S<sub>0</sub>⟩, |S<sub>1</sub>⟩,
|S<sub>2</sub>⟩, and |S<sub>n</sub>⟩) in a physically
rigorous framework. This model explains all features previously associated
with the intensely debated S* state. Besides successfully fitting
transient absorption data of a zeaxanthin homologue, this model also
accounts for previous results from global target analysis and chain
length-dependent studies. Additionally, we are able to incorporate
findings from pump-deplete-probe experiments, which were incompatible
to any pre-existing model. Thus, we present the first comprehensive
and unified interpretation of S*-related features, explaining them
by vibronic transitions on either S<sub>1</sub>, S<sub>0</sub>, or
both, depending on the chain length of the investigated carotenoid
Tuning the Spectroscopic Properties of Aryl Carotenoids by Slight Changes in Structure
Two
carotenoids with aryl rings were studied by femtosecond transient
absorption spectroscopy and theoretical computational methods, and
the results were compared with those obtained from their nonaryl counterpart,
β-carotene. Although isorenieratene has more conjugated CC
bonds than β-carotene, its effective conjugation length, <i>N</i><sub>eff</sub>, is shorter than of β-carotene. This
is evidenced by a longer S<sub>1</sub> lifetime and higher S<sub>1</sub> energy of isorenieratene compared to the values for β-carotene.
On the other hand, although isorenieratene and renierapurpurin have
the same π-electron conjugated chain structure, <i>N</i><sub>eff</sub> is different for these two carotenoids. The S<sub>1</sub> lifetime of renierapurpurin is shorter than that of isorenieratene,
indicating a longer <i>N</i><sub>eff</sub> for renierapurpurin.
This conclusion is also consistent with a lower S<sub>1</sub> energy
of renierapurpurin compared to those of the other carotenoids. Density
functional theory (DFT) was used to calculate equilibrium geometries
of ground and excited states of all studied carotenoids. The terminal
ring torsion in the ground state of isorenieratene (41°) is very
close to that of β-carotene (45°), but equilibration of
the bond lengths within the aryl rings indicates that the each aryl
ring forms its own conjugated system. This results in partial detachment
of the aryl rings from the overall conjugation making <i>N</i><sub>eff</sub> of isorenieratene shorter than that of β-carotene.
The different position of the methyl group at the aryl ring of renierapurpurin
diminishes the aryl ring torsion to ∼20°. This planarization
results in a longer <i>N</i><sub>eff</sub> than that of
isorenieratene, rationalizing the observed differences in spectroscopic
properties
Role of Xanthophylls in Light Harvesting in Green Plants: A Spectroscopic Investigation of Mutant LHCII and Lhcb Pigment–Protein Complexes
The spectroscopic properties and energy transfer dynamics of the protein-bound chlorophylls and xanthophylls in monomeric, major LHCII complexes, and minor Lhcb complexes from genetically altered <i>Arabidopsis thaliana</i> plants have been investigated using both steady-state and time-resolved absorption and fluorescence spectroscopic methods. The pigment–protein complexes that were studied contain Chl <i>a</i>, Chl <i>b</i>, and variable amounts of the xanthophylls, zeaxanthin (Z), violaxanthin (V), neoxanthin (N), and lutein (L). The complexes were derived from mutants of plants denoted <i>npq1</i> (NVL), <i>npq2lut2</i> (Z), <i>aba4npq1lut2</i> (V), <i>aba4npq1</i> (VL), <i>npq1lut2</i> (NV), and <i>npq2</i> (LZ). The data reveal specific singlet energy transfer routes and excited state spectra and dynamics that depend on the xanthophyll present in the complex
Ultrafast Dynamics of Long Homologues of Carotenoid Zeaxanthin
Three zeaxanthin homologues with
conjugation lengths <i>N</i> of 15, 19, and 23 denoted as
Z15, Z19, and Z23 were studied by femtosecond
transient absorption spectroscopy, and the results were compared to
those obtained for zeaxanthin (Z11). The energies of S<sub>2</sub> decrease from 20 450 cm<sup>–1</sup> (Z11) to 18 280
cm<sup>–1</sup> (Z15), 17 095 cm<sup>–1</sup> (Z19), and 16 560 cm<sup>–1</sup> (Z23). Fitting the <i>N</i> dependence of the S<sub>2</sub> energies allowed the estimation
of E∞, the S<sub>2</sub> energy of a hypothetical
infinite zeaxanthin, to be ∼14 000 cm<sup>–1</sup>. Exciting the 0–0 band of the S<sub>2</sub> state produces
characteristic S<sub>1</sub>–S<sub><i>n</i></sub> spectral profiles in transient absorption spectra with maxima at
556 nm (Z11), 630 nm (Z15), 690 nm (Z19), and 740 nm (Z23). The red
shift of the S<sub>1</sub>–S<sub><i>n</i></sub> transition
with increasing conjugation length is caused by a decrease in the
S<sub>1</sub> state energy, resulting in S<sub>1</sub> lifetimes of
9 ps (Z11), 0.9 ps (Z15), 0.35 ps (Z19), and 0.19 ps (Z23). Essentially
the same lifetimes were obtained after excess energy excitation at
400 nm, but S<sub>1</sub>–S<sub><i>n</i></sub> becomes
broader, indicating a larger conformation disorder in the S<sub>1</sub> state after 400 nm excitation compared to excitation into the 0–0
band of the S<sub>2</sub> state. An S* signal was observed in all
samples, but only for Z15, Z19, and Z23 does the S* signal decay with
a lifetime different from that of the S<sub>1</sub> state. The S*
lifetimes are 2.9 and 1.6 ps for Z15 and Z19, respectively. In Z23
the S* signal needs two decay components yielding lifetimes of 0.24
and 2.3 ps. The S* signal is more pronounced after 400 nm excitation
Molecular Origin of Photoprotection in Cyanobacteria Probed by Watermarked Femtosecond Stimulated Raman Spectroscopy
Photoprotection
is fundamental in photosynthesis to avoid oxidative
photodamage upon excess light exposure. Excited chlorophylls (Chl)
are quenched by carotenoids, but the precise molecular origin remains
controversial. The cyanobacterial HliC protein belongs to the Hlip
family ancestral to plant light-harvesting complexes, and binds Chl <i>a</i> and β-carotene in 2:1 ratio. We analyzed HliC by
watermarked femtosecond stimulated Raman spectroscopy to follow the
time evolution of its vibrational modes. We observed a 2 ps rise of
the CC stretch band of the 2A<sub>g</sub><sup>–</sup> (S<sub>1</sub>) state of β-carotene upon Chl <i>a</i> excitation, demonstrating energy transfer quenching and fast excess-energy
dissipation. We detected two distinct β-carotene conformers
by the CC stretch frequency of the 2A<sub>g</sub><sup>–</sup> (S<sub>1</sub>) state, but only the β-carotene whose 2A<sub>g</sub><sup>–</sup> energy level is significantly lowered
and has a lower CC stretch frequency is involved in quenching.
It implies that the low carotenoid S<sub>1</sub> energy that results
from specific pigment–protein or pigment–pigment interactions
is the key property for creating a dissipative energy channel. We
conclude that watermarked femtosecond stimulated Raman spectroscopy
constitutes a promising experimental method to assess energy transfer
and quenching mechanisms in oxygenic photosynthesis
Carotenoid Charge Transfer States and Their Role in Energy Transfer Processes in LH1–RC Complexes from Aerobic Anoxygenic Phototrophs
Light-harvesting complexes ensure
necessary flow of excitation
energy into photosynthetic reaction centers. In the present work,
transient absorption measurements were performed on LH1–RC
complexes isolated from two aerobic anoxygenic phototrophs (AAPs), <i>Roseobacter</i> sp. COL2P containing the carotenoid spheroidenone,
and <i>Erythrobacter</i> sp. NAP1 which contains the carotenoids
zeaxanthin and bacteriorubixanthinal. We show that the spectroscopic
data from the LH1–RC complex of <i>Roseobacter</i> sp. COL2P are very similar to those previously reported for <i>Rhodobacter sphaeroides</i>, including the transient absorption
spectrum originating from the intramolecular charge-transfer (ICT)
state of spheroidenone. Although the ICT state is also populated in
LH1–RC complexes of <i>Erythrobacter</i> sp. NAP1,
its appearance is probably related to the polarity of the bacteriorubixanthinal
environment rather than to the specific configuration of the carotenoid,
which we hypothesize is responsible for populating the ICT state of
spheroidenone in LH1–RC of <i>Roseobacter</i> sp.
COL2P. The population of the ICT state enables efficient S<sub>1</sub>/ICT-to-bacteriochlorophyll (BChl) energy transfer which would otherwise
be largely inhibited for spheroidenone and bacteriorubixanthinal due
to their low energy S<sub>1</sub> states. In addition, the triplet
states of these carotenoids appear well-tuned for efficient quenching
of singlet oxygen or BChl-a triplets, which is of vital importance
for oxygen-dependent organisms such as AAPs
Role of Carotenoids in Light-Harvesting Processes in an Antenna Protein from the Chromophyte <i>Xanthonema debile</i>
Chromophytes are an important group of microorganisms
that contribute
significantly to the carbon cycle on Earth. Their photosynthetic capacity
depends on efficiency of the light-harvesting system that differs
in pigment composition from that of green plants and other groups
of algae. Here we employ femtosecond transient absorption spectroscopy
to study energy transfer pathways in the main light-harvesting complex
of <i>Xanthonema debile</i>, denoted XLH, which contains
four carotenoidsdiadinoxanthin, heteroxanthin, diatoxanthin,
and vaucheriaxanthinand Chl-<i>a</i>. Overall carotenoid-to-chlorophyll
energy transfer efficiency is about 60%, but energy transfer pathways
are excitation wavelength dependent. Energy transfer from the carotenoid
S<sub>2</sub> state is active after excitation at both 490 nm (maximum
of carotenoid absorption) and 510 nm (red edge of carotenoid absorption),
but this channel is significantly more efficient after 510 nm excitation.
Concerning the energy transfer pathway from the S<sub>1</sub> state,
XLH contains two groups of carotenoids: those that have the S<sub>1</sub> route active (∼25%) and those having the S<sub>1</sub> pathway silent. For a fraction of carotenoids that transfer energy
via the S<sub>1</sub> channel, energy transfer is observed after both
excitation wavelengths, though energy transfer times are different,
yielding 3.4 ps (490 nm excitation) and 1.5 ps (510 nm excitation).
This corresponds to efficiencies of the S<sub>1</sub> channel of ∼85%
that is rather unusual for a donor–acceptor pair consisting
of a noncarbonyl carotenoid and Chl-<i>a</i>. Moreover,
major carotenoids in XLH, diadinoxanthin and diatoxanthin, have their
S<sub>1</sub> energies in solution lower than the energy of the acceptor
state, Q<sub><i>y</i></sub> state of Chl-<i>a</i>. Thus, binding of these carotenoids to XLH must tune their S<sub>1</sub> energy to allow for efficient energy transfer. Besides the
light-harvesting function, carotenoids in XLH also have photoprotective
role; they quench Chl-<i>a</i> triplets via triplet–triplet
energy transfer from Chl-<i>a</i> to carotenoid
Equilibration Dependence of Fucoxanthin S<sub>1</sub> and ICT Signatures on Polarity, Proticity, and Temperature by Multipulse Femtosecond Absorption Spectroscopy
To
demonstrate the value of the multipulse method in
revealing the nature of coupling between excited states and explore
the environmental dependencies of lowest excited singlet state (S<sub>1</sub>) and intramolecular charge transfer (ICT) state equilibration,
we performed ultrafast transient absorption pump–dump–probe
and pump–repump–probe spectroscopies on fucoxanthin
in various solvent conditions. The effects of polarity, proticity,
and temperature were tested in solvents methanol at 293 and 190 K,
acetonitrile, and isopropanol. We show that manipulation of the kinetic
traces can produce one trace reflecting the equilibration kinetics
of the states, which reveals that lower polarity, proticity, and temperature
delay S<sub>1</sub>/ICT equilibration. On the basis of a two-state
model representing the S<sub>1</sub> and ICT states on the same S<sub>1</sub>/ICT potential energy surface, we were able to show that the
kinetics are strictly dependent on the initial relative populations
of the states as well as the decay of the ICT state to the ground
state. Informed by global analysis, a systematic method for target
analysis based on this model allowed us to quantify the population
transfer rates throughout the life of the S<sub>1</sub>/ICT state
as well as separate the S<sub>1</sub> and ICT spectral signatures.
The results are consistent with the concept that the S<sub>1</sub> and ICT states are part of one potential energy surface
Different Response of Carbonyl Carotenoids to Solvent Proticity Helps To Estimate Structure of the Unknown Carotenoid from <i>Chromera velia</i>
In
order to estimate the possible structure of the unknown carbonyl
carotenoid related to isofucoxanthin from <i>Chromera velia</i> denoted as isofucoxanthin-like carotenoid (Ifx-l), we employed steady-state
and ultrafast time-resolved spectroscopic techniques to investigate
spectroscopic properties of Ifx-l in various solvents. The results
were compared with those measured for related carotenoids with known
structure: fucoxanthin (Fx) and isofucoxanthin (Ifx). The experimental
data were complemented by quantum chemistry calculations and molecular
modeling. The data show that Ifx-l must have longer effective conjugation
length than Ifx. Yet, the magnitude of polarity-dependent changes
in Ifx-l is larger than for Ifx, suggesting significant differences
in structure of these two carotenoids. The most interesting spectroscopic
feature of Ifx-l is its response to solvent proticity. The transient
absorption data show that (1) the magnitude of the ICT-like band of
Ifx-l in acetonitrile is larger than in methanol and (2) the S<sub>1</sub>/ICT lifetime of Ifx-l in acetonitrile, 4 ps, is markedly
shorter than in methanol (10 ps). This is opposite behavior than for
Fx and Ifx whose S<sub>1</sub>/ICT lifetimes are always shorter in
protic solvent methanol (20 and 13 ps) than in aprotic acetonitrile
(30 and 17 ps). Comparison with other carbonyl carotenoids reported
earlier showed that proticity response of Ifx-l is consistent with
presence of a conjugated lactone ring. Combining the experimental
data and quantum chemistry calculations, we estimated a possible structure
of Ifx-l