RESUMEN
Femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption data measured in a single experiment are used to determine the vibronic properties of the S1 state of linear carotenoids with different conjugation lengths. The Raman band corresponding to the CâC stretching mode in the S1 state peaks at 1799 cm-1 (neurosporene), 1802 cm-1 (spheroidene), and 1791 cm-1 (lycopene). Contrary to the ground state CâC mode, variation of the CâC stretching mode in the S1 state is small and does not follow a linear dependence on N. The lifetime of the Raman band matches the S1 decays obtained from transient absorption, confirming its S1 state origin. Direct comparison of transient absorption and FSRS signals allowed us to assign Raman signatures of nonrelaxed S1 and S0 states. For lycopene, FSRS data identified a component associated with a downshifted ground state CâC mode, which matches the dynamics of the S* signal observed in transient absorption data.
RESUMEN
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.
Asunto(s)
Carotenoides , Espectrometría Raman , Vibración , Carotenoides/química , Carotenoides/metabolismo , Proteínas Bacterianas/química , Proteínas Bacterianas/metabolismoRESUMEN
Cyanobacteria use large antenna complexes called phycobilisomes (PBSs) for light harvesting. However, intense light triggers non-photochemical quenching, where the orange carotenoid protein (OCP) binds to PBS, dissipating excess energy as heat. The mechanism of efficiently transferring energy from phycocyanobilins in PBS to canthaxanthin in OCP remains insufficiently understood. Using cryo-electron microscopy, we unveiled the OCP-PBS complex structure at 1.6- to 2.1-angstrom resolution, showcasing its inherent flexibility. Using multiscale quantum chemistry, we disclosed the quenching mechanism. Identifying key protein residues, we clarified how canthaxanthin's transition dipole moment in its lowest-energy dark state becomes large enough for efficient energy transfer from phycocyanobilins. Our energy transfer model offers a detailed understanding of the atomic determinants of light harvesting regulation and antenna architecture in cyanobacteria.
Asunto(s)
Cianobacterias , Ficobilisomas , Ficobilisomas/química , Ficobilisomas/metabolismo , Proteínas Bacterianas/metabolismo , Cantaxantina/metabolismo , Microscopía por Crioelectrón , Cianobacterias/metabolismoRESUMEN
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.
Asunto(s)
Carotenoides , Cinética , Análisis Espectral , Carotenoides/química , Concentración de Iones de HidrógenoRESUMEN
This work examines the influence of applied external voltage in bulk electrolysis on the excited-state properties of 8'-apo-ß-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 8â ps to 13â ps. Furthermore, switching off the applied voltage resulted in returning to no-voltage data within about 30â min. Our results show that the amplitude of the signal associated with the ICT state can be tuned by applying an external voltage.
RESUMEN
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, ß-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.
RESUMEN
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.
Asunto(s)
Carotenoides , Complejos de Proteína Captadores de Luz , Bacterioclorofilas/metabolismo , Beijerinckiaceae , Carotenoides/metabolismo , Glucósidos , Complejos de Proteína Captadores de Luz/metabolismoRESUMEN
Phycobilisome (PBS) structures are elaborate antennae in cyanobacteria and red algae1,2. These large protein complexes capture incident sunlight and transfer the energy through a network of embedded pigment molecules called bilins to the photosynthetic reaction centres. However, light harvesting must also be balanced against the risks of photodamage. A known mode of photoprotection is mediated by orange carotenoid protein (OCP), which binds to PBS when light intensities are high to mediate photoprotective, non-photochemical quenching3-6. Here we use cryogenic electron microscopy to solve four structures of the 6.2 MDa PBS, with and without OCP bound, from the model cyanobacterium Synechocystis sp. PCC 6803. The structures contain a previously undescribed linker protein that binds to the membrane-facing side of PBS. For the unquenched PBS, the structures also reveal three different conformational states of the antenna, two previously unknown. The conformational states result from positional switching of two of the rods and may constitute a new mode of regulation of light harvesting. Only one of the three PBS conformations can bind to OCP, which suggests that not every PBS is equally susceptible to non-photochemical quenching. In the OCP-PBS complex, quenching is achieved through the binding of four 34 kDa OCPs organized as two dimers. The complex reveals the structure of the active form of OCP, in which an approximately 60 Å displacement of its regulatory carboxy terminal domain occurs. Finally, by combining our structure with spectroscopic properties7, we elucidate energy transfer pathways within PBS in both the quenched and light-harvesting states. Collectively, our results provide detailed insights into the biophysical underpinnings of the control of cyanobacterial light harvesting. The data also have implications for bioengineering PBS regulation in natural and artificial light-harvesting systems.
Asunto(s)
Ficobilisomas , Luz Solar , Proteínas Bacterianas/química , Proteínas Bacterianas/metabolismo , Transferencia de Energía/efectos de la radiación , Fotosíntesis/efectos de la radiación , Ficobilisomas/química , Ficobilisomas/metabolismo , Ficobilisomas/efectos de la radiación , Synechocystis/metabolismo , Synechocystis/efectos de la radiaciónRESUMEN
Carotenoids are an integral part of natural photosynthetic complexes, with tasks ranging from light harvesting to photoprotection. Their underlying energy deactivation network of optically dark and bright excited states is extremely efficient: after excitation of light with up to 2.5 eV of photon energy, the system relaxes back to ground state on a time scale of a few picoseconds. In this article, we summarize how a model based on the vibrational energy relaxation approach (VERA) explains the main characteristics of relaxation dynamics after one-photon excitation with special emphasis on the so-called S* state. Lineshapes after two-photon excitation are beyond the current model of VERA. We outline this future line of research in our article. In terms of experimental method development, we discuss which techniques are needed to better describe energy dissipation effects in carotenoids and within the first solvation shell.
Asunto(s)
Carotenoides , Proteínas del Complejo del Centro de Reacción Fotosintética , Complejos de Proteína Captadores de Luz , Fotones , VibraciónRESUMEN
Phototrophic Gemmatimonadetes evolved the ability to use solar energy following horizontal transfer of photosynthesis-related genes from an ancient phototrophic proteobacterium. The electron cryo-microscopy structure of the Gemmatimonas phototrophica photosystem at 2.4 Å reveals a unique, double-ring complex. Two unique membrane-extrinsic polypeptides, RC-S and RC-U, hold the central type 2 reaction center (RC) within an inner 16-subunit light-harvesting 1 (LH1) ring, which is encircled by an outer 24-subunit antenna ring (LHh) that adds light-gathering capacity. Femtosecond kinetics reveal the flow of energy within the RC-dLH complex, from the outer LHh ring to LH1 and then to the RC. This structural and functional study shows that G. phototrophica has independently evolved its own compact, robust, and highly effective architecture for harvesting and trapping solar energy.
RESUMEN
Life on Earth depends on photosynthesis, the conversion of light energy into chemical energy. Plants collect photons by light harvesting complexes (LHC)-abundant membrane proteins containing chlorophyll and xanthophyll molecules. LHC-like proteins are similar in their amino acid sequence to true LHC antennae, however, they rather serve a photoprotective function. Whether the LHC-like proteins bind pigments has remained unclear. Here, we characterize plant LHC-like proteins (LIL3 and ELIP2) produced in the cyanobacterium Synechocystis sp. PCC 6803 (hereafter Synechocystis). Both proteins were associated with chlorophyll a (Chl) and zeaxanthin and LIL3 was shown to be capable of quenching Chl fluorescence via direct energy transfer from the Chl Qy state to zeaxanthin S1 state. Interestingly, the ability of the ELIP2 protein to quench can be acquired by modifying its N-terminal sequence. By employing Synechocystis carotenoid mutants and site-directed mutagenesis we demonstrate that, although LIL3 does not need pigments for folding, pigments stabilize the LIL3 dimer.
Asunto(s)
Proteínas de Arabidopsis/química , Proteínas de Arabidopsis/metabolismo , Proteínas de Cloroplastos/química , Proteínas de Cloroplastos/metabolismo , Proteínas de Arabidopsis/genética , Carotenoides/metabolismo , Clorofila/metabolismo , Proteínas de Cloroplastos/genética , Transferencia de Energía , Mutación , Unión Proteica , Pliegue de Proteína , Multimerización de Proteína , Synechocystis/genética , Synechocystis/metabolismo , Xantófilas/metabolismo , Zeaxantinas/genética , Zeaxantinas/metabolismoRESUMEN
Here, we propose a possible photoactivation mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP), suggesting that the reaction involves the transient formation of a protonated ketocarotenoid (oxocarbenium cation) state. Taking advantage of engineering an OCP variant carrying the Y201W mutation, which shows superior spectroscopic and structural properties, it is shown that the presence of Trp201 augments the impact of one critical H-bond between the ketocarotenoid and the protein. This confers an unprecedented homogeneity of the dark-adapted OCP state and substantially increases the yield of the excited photoproduct S*, which is important for the productive photocycle to proceed. A 1.37 Å crystal structure of OCP Y201W combined with femtosecond time-resolved absorption spectroscopy, kinetic analysis, and deconvolution of the spectral intermediates, as well as extensive quantum chemical calculations incorporating the effect of the local electric field, highlighted the role of charge-transfer states during OCP photoconversion.
Asunto(s)
Proteínas Bacterianas/química , Proteínas Bacterianas/metabolismo , Carotenoides/metabolismo , Fotoquímica , Cristalografía , Enlace de Hidrógeno , Cinética , Modelos Moleculares , Conformación ProteicaRESUMEN
Femtosecond transient absorption spectroscopy following two-photon excitation (2PE) is used to determine the contributions of carotenoids and chlorophylls to the 2PE signals in the main plant light-harvesting complex (LHCII). For 2PE, excitation at 1210 and 1300 nm was used, being within the known 2PE profile of LHCII. At both excitation wavelengths, the transient absorption spectra exhibit a shape characteristic of excited chlorophylls with only a minor contribution from carotenoids. We compare the 2PE data measured for LHCII with those obtained from 2PE of a lutein/chlorophyll a mixture in acetone. We estimate that although the 2PE cross section of a single carotenoid in acetone is â¼1.7 times larger than that of a Chl a, due to the 1:3.5 carotenoid/Chl ratio in LHCII, only one-third of the absorbed 2PE photons excite carotenoids in LHCII in the 1200-1300 nm range.
RESUMEN
Higher plants defend themselves from bursts of intense light via the mechanism of Non-Photochemical Quenching (NPQ). It involves the Photosystem II (PSII) antenna protein (LHCII) adopting a conformation that favors excitation quenching. In recent years several structural models have suggested that quenching proceeds via energy transfer to the optically forbidden and short-lived S 1 states of a carotenoid. It was proposed that this pathway was controlled by subtle changes in the relative orientation of a small number of pigments. However, quantum chemical calculations of S 1 properties are not trivial and therefore its energy, oscillator strength and lifetime are treated as rather loose parameters. Moreover, the models were based either on a single LHCII crystal structure or Molecular Dynamics (MD) trajectories about a single minimum. Here we try and address these limitations by parameterizing the vibronic structure and relaxation dynamics of lutein in terms of observable quantities, namely its linear absorption (LA), transient absorption (TA) and two-photon excitation (TPE) spectra. We also analyze a number of minima taken from an exhaustive meta-dynamical search of the LHCII free energy surface. We show that trivial, Coulomb-mediated energy transfer to S 1 is an unlikely quenching mechanism, with pigment movements insufficiently pronounced to switch the system between quenched and unquenched states. Modulation of S 1 energy level as a quenching switch is similarly unlikely. Moreover, the quenching predicted by previous models is possibly an artifact of quantum chemical over-estimation of S 1 oscillator strength and the real mechanism likely involves short-range interaction and/or non-trivial inter-molecular states.
RESUMEN
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.
RESUMEN
We present the first examples of alkylated derivatives of the macropolyhedral boron hydride, anti-B18H22, which is the gain medium in the first borane laser. This new series of ten highly stable and colorless organic-inorganic hybrid clusters are capable of the conversion of UVA irradiation to blue light with fluorescence quantum yields of unity. This study gives a comprehensive description of their synthesis, isolation, and structural characterization together with a delineation of their photophysical properties using a combined theoretical and experimental approach. Treatment of anti-B18H22 1 with RI (where R = Me or Et) in the presence of AlCl3 gives a series of alkylated derivatives, Rx-anti-B18H22-x (where x = 2 to 6), compounds 2-6, in which the 18-vertex octadecaborane cluster architectures are preserved and yet undergo a linear "polyhedral swelling", depending on the number of cluster alkyl substituents. The use of dichloromethane solvent in the synthetic procedure leads to dichlorination of the borane cluster and increased alkylation to give Me11-anti-B18H9Cl2 11, Me12-anti-B18H8Cl2 12, and Me13-anti-B18H7Cl2 13. All new alkyl derivatives are highly stable, extremely efficient (ΦF = 0.76-1.0) blue fluorophores (λems = 423-427 nm) and are soluble in a wide range of organic solvents and also a polystyrene matrix. The Et4-anti-B18H18 derivative 4b crystallizes from pentane solution in two phases with consequent multiabsorption and multiemission photophysical properties. An ultrafast transient UV-vis absorption spectroscopic study of compounds 4a and 4b reveals that an efficient excited-state absorption at the emission wavelength inhibits the laser performance of these otherwise remarkable luminescent molecules. All these new compounds add to the growing portfolio of octadecaborane-based luminescent species, and in an effort to broaden the perspective on their highly emissive photophysical properties, we highlight emerging patterns that successive substitutions have on the molecular size of the 18-vertex borane cluster structure and the distribution of the electron density within.
RESUMEN
The Orange Carotenoid Protein (OCP) is a water-soluble protein that governs photoprotection in many cyanobacteria. The 35 kDa OCP is structurally and functionally modular, consisting of an N-terminal effector domain (NTD) and a C-terminal regulatory domain (CTD); a carotenoid spans the two domains. The CTD is a member of the ubiquitous Nuclear Transport Factor-2 (NTF2) superfamily (pfam02136). With the increasing availability of cyanobacterial genomes, bioinformatic analysis has revealed the existence of a new family of proteins, homologs to the CTD, the C-terminal domain-like carotenoid proteins (CCPs). Here we purify holo-CCP2 directly from cyanobacteria and establish that it natively binds canthaxanthin (CAN). We use small-angle X-ray scattering (SAXS) to characterize the structure of this carotenoprotein in two distinct oligomeric states. A single carotenoid molecule spans the two CCPs in the dimer. Our analysis with X-ray footprinting-mass spectrometry (XFMS) identifies critical residues for carotenoid binding that likely contribute to the extreme red shift (ca. 80 nm) of the absorption maximum of the carotenoid bound by the CCP2 dimer and a further 10 nm shift in the tetramer form. These data provide the first structural description of carotenoid binding by a protein consisting of only an NTF2 domain.
Asunto(s)
Proteínas Bacterianas/ultraestructura , Cantaxantina/química , Cianobacterias/ultraestructura , Proteínas de Transporte Nucleocitoplasmático/ultraestructura , Proteínas Bacterianas/química , Cristalografía por Rayos X , Cianobacterias/química , Proteínas de Transporte Nucleocitoplasmático/química , Proteínas de Transporte Nucleocitoplasmático/genética , Unión Proteica/efectos de los fármacos , Dominios Proteicos/genética , Dispersión del Ángulo PequeñoRESUMEN
The major light-harvesting complex of photosystem II (LHCII) is the main contributor to sunlight energy harvesting in plants. The flexible design of LHCII underlies a photoprotective mechanism whereby this complex switches to a dissipative state in response to high light stress, allowing the rapid dissipation of excess excitation energy (non-photochemical quenching, NPQ). In this work, we locked single LHCII trimers in a quenched conformation after immobilization of the complexes in polyacrylamide gels to impede protein interactions. A comparison of their pigment excited-state dynamics with quenched LHCII aggregates in buffer revealed the presence of a new spectral band at 515 nm arising after chlorophyll excitation. This is suggested to be the signature of a carotenoid excited state, linked to the quenching of chlorophyll singlet excited states. Our data highlight the marked sensitivity of pigment excited-state dynamics in LHCII to structural changes induced by the environment.
RESUMEN
Xanthophylls in light harvesting complexes perform a number of functions ranging from structural support to light-harvesting and photoprotection. In the major light harvesting complex of photosystem II in plants (LHCII), the innermost xanthophyll binding pockets are occupied by lutein molecules. The conservation of these sites within the LHC protein family suggests their importance in LHCII functionality. In the present work, we induced the photoprotective switch in LHCII isolated from the Arabidopsis mutant npq1lut2, where the lutein molecules are exchanged with violaxanthin. Despite the differences in the energetics of the pigments and the impairment of chlorophyll fluorescence quenching in vivo, we show that isolated complexes containing violaxanthin are still able to induce the quenching switch to a similar extent to wild type LHCII monomers. Moreover, the same spectroscopic changes take place, which suggest the involvement of the terminal emitter site (L1) in energy dissipation in both complexes. These results indicate the robust nature of the L1 xanthophyll binding domain in LHCII, where protein structural cues are the major determinant of the function of the bound carotenoid.