Investigation of the S1/ICT equilibrium in fucoxanthin by ultrafast pump-dump-probe and femtosecond stimulated Raman scattering spectroscopy

Raman Vibrational Signatures of Excited States of Echinenone in the Orange Carotenoid Protein (OCP) and Implications for its Photoactivation Mechanism

In this study, the vibrational characteristics of optically excited echinenone in various solvents and the Orange Carotenoid Protein (OCP) in red and orange states are systematically investigated through steady-state and time-resolved spectroscopy techniques. Time-resolved experiments, employing both Transient Absorption (TA) and Femtosecond Stimulated Raman Spectroscopy (FSRS), reveal different states in the OCP photoactivation process. The time-resolved studies indicate vibrational signatures of exited states positioned above the S1 state during the initial 140 fs of carotenoid evolution in OCP, an absence of a vibrational signature for the relaxed S1 state of echinenone in OCP, and more robust signatures of a highly excited ground state (GS) in OCP. Differences in S1 state vibration population signatures between OCP and solvents are attributed to distinct conformations of echinenone in OCP and hydrogen bonds at the keto group forming a short-lived intramolecular charge transfer (ICT) state. The vibrational dynamics of the hot GS in OCP show a more pronounced red shift of ground state CC vibration compared to echinenone in solvents, thus suggesting an unusually hot form of GS. The study proposes a hypothesis for the photoactivation mechanism of OCP, emphasizing the high level of vibrational excitation in longitudinal stretching modes as a driving force. In conclusion, the comparison of vibrational signatures reveals unique dynamics of energy dissipation in OCP, providing insights into the photoactivation mechanism and highlighting the impact of the protein environment on carotenoid behavior. The study underscores the importance of vibrational analysis in understanding the intricate processes involved in early phase OCP photoactivation.

Ultrafast spectroscopy of the hydrophilic carotenoid crocin at various pH

This study delves into the pH-dependent effects on the excited-state behavior of crocin, a hydrophilic carotenoid with diverse functions in biological systems. Steady-state spectroscopy demonstrates notable changes in absorption and fluorescence spectra, characterized by a pH-dependent blue shift and altered resolution of vibrational bands. Transient absorption spectra further elucidate these effects, highlighting a significant blue shift in the S1-Sn peak with increasing pH. Detailed kinetic analysis shows the pH-dependent dynamics of crocin's excited states. At pH 11, a shortening of effective conjugation is observed, resulting in a prolonged S1/ICT lifetime. Conversely, at pH 9, our data suggest a more complex scenario, suggesting the presence of two distinct crocin species with different relaxation patterns. This implies structural alterations within the crocin molecule, potentially linked to the deprotonation of hydroxyl groups in crocin and/or saponification at high pH.

Two-Dimensional Electronic Spectroscopy Characterization of Fucoxanthin-Chlorophyll Protein Reveals Excitonic Carotenoid-Chlorophyll Interactions

Fucoxanthin Chlorophyll Protein (FCP) is a Light Harvesting Complex found in diatoms and brown algae. It is particularly interesting for its efficiency in capturing the blue-green part of the light spectrum due to the presence of specific chromophores (fucoxanthin, chlorophyll a, and chlorophyll c). Recently, the crystallographic structure of FCP was solved, revealing the 3D arrangement of the pigments in the protein scaffold. While this information is helpful for interpreting the spectroscopic features of FCP, it has also raised new questions about the potential interactions between fucoxanthin and chlorophyll c. These interactions were suggested by their spatial closeness but have never been experimentally observed. To investigate this possible interaction mechanism, in this work, two-dimensional electronic spectroscopy (2DES) has been applied to study the ultrafast relaxation dynamics of FCP. The experiments captured an instantaneous delocalization of the excitation among fucoxanthin and chlorophyll c, suggesting the presence of a non-negligible coupling between the chromophores.

Time-Resolved Spectroelectrochemical Dynamics of Carotenoid 8'-apo-β-Carotenal

This work examines the influence of applied external voltage in bulk electrolysis on the excited-state properties of 8'-apo-β-carotenal in acetonitrile by steady-state and ultrafast time-resolved absorption spectroscopy. The data collected under bulk electrolysis were compared with those taken without applied voltage. The steady-state measurements showed that although intensity of the S0 -S2 absorption band varies with the applied voltage, the spectral position remain nearly constant. Comparison of transient absorption spectra shows that the magnitude of the ICT-like band decreases during the experiment under applied voltage condition, and is associated with a prolongation of the S1 /ICT-like lifetime from 8 ps to 13 ps. Furthermore, switching off the applied voltage resulted in returning to no-voltage data within about 30 min. Our results show that the amplitude of the signal associated with the ICT state can be tuned by applying an external voltage.

Origin of Energy Dissipation in the Oligomeric Fucoxanthin-Chlorophyll a/c Binding Proteins

Fucoxanthin-chlorophyll proteins (FCPs) are a family of photosynthetic light-harvesting complex (LHC) proteins found in diatoms. They efficiently capture photons and regulate their functions, ensuring diatom survival in highly fluctuating light. FCPs are present in different oligomeric states in vivo, but functional differences among these FCP oligomers are not yet fully understood. Here we characterized two types of antenna complexes (FCP-B/C dimers and FCP-A tetramers) that coexist in the marine centric diatom Chaetoceros gracilis using both time-resolved fluorescence and transient absorption spectroscopy. We found that the FCP-B/C complex did not show fluorescence quenching, whereas FCP-A was severely quenched, via an ultrafast excitation energy transfer (EET) pathway from Chl a Qy to the fucoxanthin S1/ICT state. These results highlight the functional differences between FCP dimers and tetramers and indicate that the EET pathway from Chl a to carotenoids is an energy dissipation mechanism conserved in a variety of photosynthetic organisms.

Relaxation dynamics of high-energy excited states of carotenoids studied by UV excitation and pump-repump-probe transient absorption spectroscopy

The excited states of carotenoids have been a subject of numerous studies. While a majority of these reports target the excited state dynamics initiated by the excitation of the S2 state, the upper excited state(s) absorbing in the UV spectral region (denoted as SUV) has been only scarcely studied. Moreover, the relation between the SUV and Sn, the final state of the well-known S1-Sn transition of carotenoids, remains unknown. To address this yet-unresolved issue, we compared the excited state dynamics of two carotenoids, namely, β-carotene and astaxanthin, after excitation of either the SUV or Sn state. The SUV state was excited directly by UV light, and the excitation of the Sn state was achieved via re-pumping the S1-Sn transition. The results indicated that direct SUV excitation produces an S1-Sn band that is significantly broader than that obtained after S2 excitation, most probably due to the generation of multiple S1 conformations produced by excess energy. No such broadening is observed if the Sn state is excited by the re-pump pulse. This shows that the Sn and SUV states are different, each initializing a specific relaxation pathway. We propose that the Sn state retains the coupled triplet pair character of the S1 state, while the SUV state is the higher state of Bu+ symmetry accessible by one-photon transition.

The Energy Transfer Yield between Carotenoids and Chlorophylls in Peridinin Chlorophyll a Protein Is Robust against Mutations

The energy transfer (ET) from carotenoids (Cars) to chlorophylls (Chls) in photosynthetic complexes occurs with almost unitary efficiency thanks to the synergistic action of multiple finely tuned channels whose photophysics and dynamics are not fully elucidated yet. We investigated the energy flow from the Car peridinin (Per) to Chl a in the peridinin chlorophyll a protein (PCP) from marine algae Amphidinium carterae by using two-dimensional electronic spectroscopy (2DES) with a 10 fs temporal resolution. Recently debated hypotheses regarding the S₂-to-S₁ relaxation of the Car via a conical intersection and the involvement of possible intermediate states in the ET were examined. The comparison with an N89L mutant carrying the Per donor in a lower-polarity environment helped us unveil relevant details on the mechanisms through which excitation was transferred: the ET yield was conserved even when a mutation perturbed the optimization of the system thanks to the coexistence of multiple channels exploited during the process.

Electronic and Vibrational Properties of Allene Carotenoids

Carotenoids are conjugated linear molecules built from the repetition of terpene units, which display a large structural diversity in nature. They may, in particular, contain several types of side or end groups, which tune their functional properties, such as absorption position and photochemistry. We report here a detailed experimental study of the absorption and vibrational properties of allene-containing carotenoids, together with an extensive modeling of these experimental data. Our calculations can satisfactorily explain the electronic properties of vaucheriaxanthin, where the allene group introduces the equivalent of one C=C double bond into the conjugated C=C chain. The position of the electronic absorption of fucoxanthin and butanoyloxyfucoxanthin requires long-range corrections to be found correctly on the red side of that of vaucheriaxanthin; however, these corrections tend to overestimate the effect of the conjugated and nonconjugated C=O groups in these molecules. We show that the resonance Raman spectra of these carotenoids are largely perturbed by the presence of the allene group, with the two major Raman contributions split into two components. These perturbations are satisfactorily explained by modeling, through a gain in the Raman intensity of the C=C antisymmetric stretching mode, induced by the presence of the allene group in the carotenoid C=C chain.

Solvent-Dependent Characterization of Fucoxanthin through 2D Electronic Spectroscopy Reveals New Details on the Intramolecular Charge-Transfer State Dynamics

The electronic state manifolds of carotenoids and their relaxation dynamics are the object of intense investigation because most of the subtle details regulating their photophysics are still unknown. In order to contribute to this quest, here, we present a solvent-dependent 2D Electronic Spectroscopy (2DES) characterization of fucoxanthin, a carbonyl carotenoid involved in the light-harvesting process of brown algae. The 2DES technique allows probing its ultrafast relaxation dynamics in the first 1000 fs after photoexcitation with a 10 fs time resolution. The obtained results help shed light on the dynamics of the first electronic state manifold and, in particular, on an intramolecular charge-transfer state (ICT), whose photophysical properties are particularly elusive given its (almost) dark nature.

Excited-State Evolution of Keto-Carotenoids after Excess Energy Excitation in the UV Region

Carotenoids are molecules with rich photophysics that are in many biological systems involved in photoprotection. Yet, their response to excess energy excitation is only scarcely studied. Here we have explored excited state properties of three keto-carotenoids, echinenone, canthaxanthin and rhodoxanthin after excess energy excitation to a singlet state absorbing in UV. Though the basic spectral features and kinetics of S₂ , hot S₁ , relaxed S₁ states remain unchanged upon UV excitation, the clear increase of the S* signal is observed after excess energy excitation, associated with increased S* lifetime. A multiple origin of the S* signal, originating either from specific conformations in the S₁ state or from a non-equilibrated ground state, is confirmed in this work. We propose that the increased amount of energy stored in molecular vibrations, induced by the UV excitation, is the reason for the enhanced S* signal observed after UV excitation. Our data also suggest that a fraction of the UV excited state population may proceed through a non-sequential pathway, bypassing the S₂ state.

Intramolecular charge-transfer state of carotenoids siphonaxanthin and siphonein: function of non-conjugated acyl-oxy group

We used ultrafast transient absorption spectroscopy to study excited-state dynamics of two keto-carotenoids, siphonaxanthin and siphonein. These two carotenoids differ in the presence of dodecanoyl-oxy group in siphonein, which is attached to the C19 carbon on the same side of the molecule as the conjugated keto group. We show that this dodecanoyl-oxy group, though not in conjugation, is still capable of modifying excited state properties. While spectroscopic properties of siphonein and siphonaxanthin are nearly identical in a non-polar solvent, they become markedly different in polar solvents. In a polar solvent, siphonein, having the dodecanoyl-oxy moiety, exhibits less pronounced vibrational bands in the absorption spectrum and has significantly enhanced characteristic features of an intramolecular charge-transfer (ICT) state in transient absorption spectra compared to siphonaxanthin. The presence of the dodecanoyl-oxy moiety also alters the lifetimes of the S₁/ICT state. For siphonaxanthin, the lifetimes are 60, 20, and 14 ps in n-hexane, acetonitrile, and methanol, whereas for siphonein these lifetimes yield 60, 11, and 10 ps. Thus, we show that even a non-conjugated functional group can affect the charge-transfer character of the S₁/ICT state. By comparison with fucoxanthin acyl-oxy derivatives, we show that position of the acyl-oxy group in respect to the conjugated keto group is the key feature determining whether the polarity-dependent behavior is enhanced or suppressed.

Modeling Dynamic Conformations of Organic Molecules: Alkyne Carotenoids in Solution

Calculating the spectroscopic properties of complex conjugated organic molecules in their relaxed state is far from simple. An additional complexity arises for flexible molecules in solution, where the rotational energy barriers are low enough so that nonminimum conformations may become dynamically populated. These metastable conformations quickly relax during the minimization procedures preliminary to density functional theory calculations, and so accounting for their contribution to the experimentally observed properties is problematic. We describe a strategy for stabilizing these nonminimum conformations in silico, allowing their properties to be calculated. Diadinoxanthin and alloxanthin present atypical vibrational properties in solution, indicating the presence of several conformations. Performing energy calculations in vacuo and polarizable continuum model calculations in different solvents, we found three different conformations with values for the δ dihedral angle of the end ring ca. 0, 180, and 90° with respect to the plane of the conjugated chain. The latter conformation, a nonglobal minimum, is not stable during the minimization necessary for modeling its spectroscopic properties. To circumvent this classical problem, we used a Car–Parinello MD supermolecular approach, in which diadinoxanthin was solvated by water molecules so that metastable conformations were stabilized by hydrogen-bonding interactions. We progressively removed the number of solvating waters to find the minimum required for this stabilization. This strategy represents the first modeling of a carotenoid in a distorted conformation and provides an accurate interpretation of the experimental data.

Probing Structure and Reaction Dynamics of Proteins Using Time-Resolved Resonance Raman Spectroscopy

The mechanistic understanding of protein functions requires insight into the structural and reaction dynamics. To elucidate these processes, a variety of experimental approaches are employed. Among them, time-resolved (TR) resonance Raman (RR) is a particularly versatile tool to probe processes of proteins harboring cofactors with electronic transitions in the visible range, such as retinal or heme proteins. TR RR spectroscopy offers the advantage of simultaneously providing molecular structure and kinetic information. The various TR RR spectroscopic methods can cover a wide dynamic range down to the femtosecond time regime and have been employed in monitoring photoinduced reaction cascades, ligand binding and dissociation, electron transfer, enzymatic reactions, and protein un- and refolding. In this account, we review the achievements of TR RR spectroscopy of nearly 50 years of research in this field, which also illustrates how the role of TR RR spectroscopy in molecular life science has changed from the beginning until now. We outline the various methodological approaches and developments and point out current limitations and potential perspectives.

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 n-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.

Ultrafast spectroscopy tracks carotenoid configurations in the orange and red carotenoid proteins from cyanobacteria

A quenching mechanism mediated by the orange carotenoid protein (OCP) is one of the ways cyanobacteria protect themselves against photooxidative stress. Here, we present a femtosecond spectroscopic study comparing OCP and RCP (red carotenoid protein) samples binding different carotenoids. We confirmed significant changes in carotenoid configuration upon OCP activation reported by Leverenz et al. (Science 348:1463-1466. doi: 10.1126/science.aaa7234 , 2015) by comparing the transient spectra of OCP and RCP. The most important marker of these changes was the magnitude of the transient signal associated with the carotenoid intramolecular charge-transfer (ICT) state. While OCP with canthaxanthin exhibited a weak ICT signal, it increased significantly for canthaxanthin bound to RCP. On the contrary, a strong ICT signal was recorded in OCP binding echinenone excited at the red edge of the absorption spectrum. Because the carbonyl oxygen responsible for the appearance of the ICT signal is located at the end rings of both carotenoids, the magnitude of the ICT signal can be used to estimate the torsion angles of the end rings. Application of two different excitation wavelengths to study OCP demonstrated that the OCP sample contains two spectroscopically distinct populations, none of which is corresponding to the photoactivated product of OCP.