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

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.

The Orange Carotenoid Protein Triggers Cyanobacterial Photoprotection by Quenching Bilins via a Structural Switch of Its Carotenoid

Cyanobacteria were the first microorganisms that released oxygen into the atmosphere billions of years ago. To do it safely under intense sunlight, they developed strategies that prevent photooxidation in the photosynthetic membrane, by regulating the light-harvesting activity of their antenna complexes-the phycobilisomes-via the orange-carotenoid protein (OCP). This water-soluble protein interacts with the phycobilisomes and triggers nonphotochemical quenching (NPQ), a mechanism that safely dissipates overexcitation in the membrane. To date, the mechanism of action of OCP in performing NPQ is unknown. In this work, we performed ultrafast spectroscopy on a minimal NPQ system composed of the active domain of OCP bound to the phycobilisome core. The use of this system allowed us to disentangle the signal of the carotenoid from that of the bilins. Our results demonstrate that the binding to the phycobilisomes modifies the structure of the ketocarotenoid associated with OCP. We show that this molecular switch activates NPQ, by enabling excitation-energy transfer from the antenna pigments to the ketocarotenoid.

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.

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.

Excitation-Dependence of Excited-State Dynamics and Vibrational Relaxation of Lutein Explored by Multiplex Transient Grating

Multiplex transient grating (MTG) spectroscopy was applied to lutein in ethanol to investigate the excitation-energy dependence of the excited-state dynamics and vibrational relaxation. The transient spectra obtained upon low (480 nm) and high-energy (380 nm) excitation both recorded a strong excited-state absorption (ESA) of S₁ → S _n as well as a broad band in the blue wavelength that was previously proposed as the S* state. By means of Gaussian decomposition and global fitting of the ESA band, a long-time component assigned to the triplet state was derived from the kinetic trace of 480 nm excitation. Moreover, the MTG signal with a resolution of 110 fs displayed the short-time quantum beat signal. In order to unveil the vibrational coherence in the excited-state decay, the linear and non-linear simulations of the steady spectrum and dynamic signals were presented in which at least three fundamental modes standing for C-C stretching (ν₁), C=C stretching (ν₂), and O-H valence vibrations (ν₃) were considered to analyze the experimental signals. It was identified that the vibrational coherence between ν₁ and ν₃ or ν₂ and ν₃ was responsible for quantum beat that may be associated with the triplet state. We concluded that upon low- or high-energy excitation into the S₂ state, the photo-isomerization of the molecule and structural recovery on the time-scale of vibrational cooling are the key factors to form a mixed conformation in the hot-S₁ state that is the precursor of a long life-time triplet.

Carotenoid responds to excess energy dissipation in the LH2 complex from Rhodoblastus acidophilus

The functions of both (bacterio) chlorophylls and carotenoids in light-harvesting complexes have been extensively studied during the past decade, yet, the involvement of BChl a high-energy Soret band in the cascade of light-harvesting processes still remains a relatively unexplored topic. Here, we present transient absorption data recorded after excitation of the Soret band in the LH2 complex from Rhodoblastus acidophilus. Comparison of obtained data to those recorded after excitation of rhodopin glucoside and B800 BChl a suggests that no Soret-to-Car energy transfer pathway is active in LH2 complex. Furthermore, a spectrally rich pattern observed in the spectral region of rhodopin glucoside ground state bleaching (420-550 nm) has been assigned to an electrochromic shift. The results of global fitting analysis demonstrate two more features. A 6 ps component obtained exclusively after excitation of the Soret band has been assigned to the response of rhodopin glucoside to excess energy dissipation in LH2. Another time component, ~ 450 ps, appearing independently of the excitation wavelength was assigned to BChl a-to-Car triplet-triplet transfer. Presented data demonstrate several new features of LH2 complex and its behavior following the excitation of the Soret band.

Combined contributions of carotenoids and chlorophylls in two-photon spectra of photosynthetic pigment-protein complexes-A new way to quantify carotenoid dark state to chlorophyll energy transfer?

Transitions into the first excited state of carotenoids, Car S₁, are optically forbidden in conventional one-photon excitation (OPE) but are possible via two-photon excitation (TPE). This can be used to quantify the amount of Car S₁ to Chlorophyll (Chl) energy transfer in pigment-protein complexes and plants by observing the chlorophyll fluorescence intensity after TPE in comparison to the intensity observed after direct chlorophyll OPE. A parameter, Φ_Coupling ^{Car S1-Chl}, can be derived that directly reflects relative differences or changes in the Car S₁ → Chl energy transfer of different pigment-protein complexes and even living plants. However, very careful calibrations are necessary to ensure similar OPE and TPE excitation probabilities and transition energies. In plants, the exact same sample spot must be observed at the same time. All this is experimentally quite demanding. Φ_Coupling ^{Car S1-Chl} also corrects intrinsically for direct chlorophyll TPE caused by larger chlorophyll excesses in the complexes, but recently it turned out that in certain TPE wavelengths ranges, its contribution can be quite large. Fortunately, this finding opens also the possibility of determining Φ_Coupling ^{Car S1-Chl} in a much easier way by directly comparing values in TPE spectra observed at wavelengths that are either more dominated by Cars or Chls. This avoids tedious comparisons of OPE and TPE experiments and potentially allows measurement at even only two TPE wavelengths. Here, we explored this new approach to determine Φ_Coupling ^{Car S1-Chl} directly from single TPE spectra and present first examples using known experimental spectra from Cars, Chl a, Chl b, LHC II, and PS 1.