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

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

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

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

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

Nonconjugated Acyloxy Group Deactivates the Intramolecular Charge-Transfer State in the Carotenoid Fucoxanthin

We used ultrafast transient absorption spectroscopy to study excited-state dynamics of the keto-carotenoid fucoxanthin (Fx) and its two derivatives: 19′-butanoyloxyfucoxanthin (bFx) and 19′-hexanoyloxyfucoxanthin (hFx). These derivatives occur in some light-harvesting systems of photosynthetic microorganisms, and their presence is typically related to stress conditions. Even though the hexanoyl (butanoyl) moiety is not a part of the conjugated system of hFx (bFx), their absorption spectra in polar solvents exhibit more pronounced vibrational bands of the S₂ state than for Fx. The effect of the nonconjugated acyloxy moiety is further observed in transient absorption spectra, which for Fx exhibit characteristic features of an intramolecular charge transfer (ICT) state in all polar solvents. For bFx and hFx, however, much weaker ICT features are detected in methanol, and the spectral markers of the ICT state disappear completely in polar, but aprotic acetonitrile. The presence of the acyloxy moiety also alters the lifetimes of the S₁/ICT state. For Fx, the lifetimes are 60, 30, and 20 ps in <i>n</i>-hexane, acetonitrile, and methanol, whereas for bFx and hFx, these lifetimes yield 60, 60, and 40 ps, respectively. Testing the S₁/ICT state lifetimes of hFx in other solvents revealed that some ICT features can be induced only in polar, protic solvents (methanol, ethanol, and ethylene glycol). Thus, bFx and hFx represent a rather rare example of a system in which a nonconjugated functional group significantly alters excited-state dynamics. By comparison with other carotenoids, we show that a keto group at the acyloxy tail, even though it is not in conjugation, affects the electron distribution along the conjugated backbone, resulting in the observed decrease of the ICT character of the S₁/ICT state of bFx and hFx

Steady-state spectra of purified PS complexes from <i>G</i>. <i>phototrophica</i>.

<p>(A) Absorption spectra recorded at room temperature (red line) and at 77 K (blue line). (B) The thick line shows the LD (<i>LD</i> = <i>A</i>_H—<i>A</i>_V) spectra of the PS complex embedded in polyacrylamide gel. <i>A</i>_H and <i>A</i>_V correspond to absorbance of horizontally and vertically polarized light, respectively. For a flat, disk-like particle in a vertically compressed gel, the horizontal direction is parallel with the particle plane, vertical with particle normal. The thin line shows the reduced LD, <i>LD</i> / <i>Abs</i>., where <i>Abs</i>. is isotropic absorbance. (C) Circular dichroism spectrum of PS complexes in solution. All dichroic spectra were measured at room temperature. Abs, absorbance; CD, circular dichroism; LD, linear dichroism; mdeg, millidegree; PS, photosynthetic; RT, room temperature.</p