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 S₁ 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 S₁(A_g^–)–S₃(A_g⁺) 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 S₁ 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 S₁ 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 (|S₀⟩, |S₁⟩, |S₂⟩, and |S_n⟩) 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 S₁, S₀, 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 CC bonds than β-carotene, its effective conjugation length, <i>N</i>_eff, is shorter than of β-carotene. This is evidenced by a longer S₁ lifetime and higher S₁ 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>_eff is different for these two carotenoids. The S₁ lifetime of renierapurpurin is shorter than that of isorenieratene, indicating a longer <i>N</i>_eff for renierapurpurin. This conclusion is also consistent with a lower S₁ 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>_eff 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>_eff 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 S₂ decrease from 20 450 cm^–1 (Z11) to 18 280 cm^–1 (Z15), 17 095 cm^–1 (Z19), and 16 560 cm^–1 (Z23). Fitting the <i>N</i> dependence of the S₂ energies allowed the estimation of E∞, the S₂ energy of a hypothetical infinite zeaxanthin, to be ∼14 000 cm^–1. Exciting the 0–0 band of the S₂ state produces characteristic S₁–S_<i>n</i> 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 S₁–S_<i>n</i> transition with increasing conjugation length is caused by a decrease in the S₁ state energy, resulting in S₁ 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 S₁–S_<i>n</i> becomes broader, indicating a larger conformation disorder in the S₁ state after 400 nm excitation compared to excitation into the 0–0 band of the S₂ 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 S₁ 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 CC stretch band of the 2A_g^– (S₁) 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 CC stretch frequency of the 2A_g^– (S₁) state, but only the β-carotene whose 2A_g^– energy level is significantly lowered and has a lower CC stretch frequency is involved in quenching. It implies that the low carotenoid S₁ 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 S₁/ICT-to-bacteriochlorophyll (BChl) energy transfer which would otherwise be largely inhibited for spheroidenone and bacteriorubixanthinal due to their low energy S₁ 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 carotenoidsdiadinoxanthin, heteroxanthin, diatoxanthin, and vaucheriaxanthinand 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 S₂ 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 S₁ state, XLH contains two groups of carotenoids: those that have the S₁ route active (∼25%) and those having the S₁ pathway silent. For a fraction of carotenoids that transfer energy via the S₁ 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 S₁ 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 S₁ energies in solution lower than the energy of the acceptor state, Q_<i>y</i> state of Chl-<i>a</i>. Thus, binding of these carotenoids to XLH must tune their S₁ 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 S₁ 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 (S₁) 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 S₁/ICT equilibration. On the basis of a two-state model representing the S₁ and ICT states on the same S₁/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 S₁/ICT state as well as separate the S₁ and ICT spectral signatures. The results are consistent with the concept that the S₁ 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 S₁/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 S₁/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