4 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
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