Dynamics of vibrational relaxation in the S1 state of carotenoids having 11 conjugated CC bonds

Energy dissipation efficiency in the CP43 assembly intermediate complex of photosystem II

Photosystem II in oxygenic organisms is a large membrane bound rapidly turning over pigment protein complex. During its biogenesis, multiple assembly intermediates are formed, including the CP43-preassembly complex (pCP43). To understand the energy transfer dynamics in pCP43, we first engineered a His-tagged version of the CP43 in a CP47-less strain of the cyanobacterium Synechocystis 6803. Isolated pCP43 from this engineered strain was subjected to advanced spectroscopic analysis to evaluate its excitation energy dissipation characteristics. These included measurements of steady-state absorption and fluorescence emission spectra for which correlation was tested with Stepanov relation. Comparison of fluorescence excitation and absorptance spectra determined that efficiency of energy transfer from β-carotene to chlorophyll a is 39 %. Time-resolved fluorescence images of pCP43-bound Chl a were recorded on streak camera, and fluorescence decay dynamics were evaluated with global fitting. These demonstrated that the decay kinetics strongly depends on temperature and buffer used to disperse the protein sample and fluorescence decay lifetime was estimated in 3.2-5.7 ns time range, depending on conditions. The pCP43 complex was also investigated with femtosecond and nanosecond time-resolved absorption spectroscopy upon excitation of Chl a and β-carotene to reveal pathways of singlet excitation relaxation/decay, Chl a triplet dynamics and Chl a → β-carotene triplet state sensitization process. The latter demonstrated that Chl a triplet in the pCP43 complex is not efficiently quenched by carotenoids. Finally, detailed kinetic analysis of the rise of the population of β-carotene triplets determined that the time constant of the carotenoid triplet sensitization is 40 ns.

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.

Achromatic frequency doubling of supercontinuum pulses for transient absorption spectroscopy

We present achromatic frequency doubling of supercontinuum pulses from a hollow core fiber as a technique for obtaining tunable ultrashort pulses in the near UV and blue spectral range. Pulse energies are stable on a 1.1% level, averaged over 100 000 shots. By the use of conventional optics only, we compress a 0.2 µJ pulse at a center wavelength of 475 nm to a pulse duration of 12 fs, as measured by X-FROG. We test the capabilities of the approach by employing the ASHG-pulses as a pump in a transient absorption experiment on β-carotene in solution.

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.

Photogeneration of Long-Lived Triplet States through Singlet Fission in Lycopene H-Aggregates

Molecular triplet excitons produced through singlet fission (SF) usually have shorter triplet lifetimes due to exciton-exciton recombination and relaxation pathways, thereby resulting in complex device architectures for SF-boosted solar cells. Using broadband transient absorption spectroscopy, we here show that the photoexcitation of nanostructured lycopene H-aggregates at room temperature produces free triplets with an unprecedented 35-fold enhancement in the lifetime compared to those localized on the monomer backbone. The observed rise of a spectrally blue-shifted correlated T-T pair state in ∼19 ps with distinct vibronic features provides the basis for SF-induced triplet generation.

Carotenoid Nuclear Reorganization and Interplay of Bright and Dark Excited States

We report quantum chemical calculations using multireference perturbation theory (MRPT) with the density matrix renormalization group (DMRG) plus photothermal deflection spectroscopy measurements to investigate the manifold of carotenoid excited states and establish their energies relative to the bright state (S₂) as a function of nuclear reorganization. We conclude that the primary photophysics and function of carotenoids are determined by interplay of only the bright (S₂) and lowest-energy dark (S₁) states. The lowest-lying dark state, far from being energetically distinguishable from the lowest-lying bright state along the entire excited-state nuclear reorganization pathway, is instead computed to be either the second or first excited state depending on what equilibrium geometry is considered. This result suggests that, rather than there being a dark intermediate excited state bridging a non-negligible energy gap from the lowest-lying dark state to the lowest-lying bright state, there is in fact no appreciable energy gap to bridge following photoexcitation. Instead, excited-state nuclear reorganization constitutes the bridge from S₂ to S₁, in the sense that these two states attain energetic degeneracy along this pathway.

Dynamics of aggregated states resolved by gated fluorescence in films of room temperature phosphorescent emitters

Phenazine derivative molecules were studied using steady state and time resolved fluorescence techniques and demonstrated to lead to strong formation of aggregated species, identified as dimers by time dependent density functional theory calculations. Blended films in a matrix of Zeonex®, produced at different concentrations, showed different contributions of dimer and monomer emissions in a prompt time frame, e.g. less than 50 ns. In contrast, the phosphorescence (e.g. emission from the triplet state) shows no significant effect on dimer formation, although strong dependence of the phosphorescence intensity on concentration is observed, leading to phosphorescence being quenched at higher concentration.

Lycopene crystalloids exhibit singlet exciton fission in tomatoes

Transient absorption studies conducted on in vitro lycopene aggregates, as well as on lycopene crystalloids inside tomato chromoplasts, reveal the appearance of a long-lived excited state, which we unambiguously identified as lycopene triplet. These triplet states must be generated by singlet exciton fission, which occurs from the lycopene 2Ag state. This is the first time the singlet fission process has ever been shown to occur in a biological material. We propose that the formation of carotenoid assemblies in chromoplasts may constitute a photoprotective process during chromoplast maturation, in addition to their function in signaling processes.

Understanding/unravelling carotenoid excited singlet states

Carotenoids are essential light-harvesting pigments in natural photosynthesis. They absorb in the blue-green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and thus expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet-singlet excitation energy transfer, and carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. The photochemistry and photophysics of carotenoids have often been interpreted by referring to those of simple polyene molecules that do not possess any functional groups. However, this may not always be wise because carotenoids usually have a number of functional groups that induce the variety of photochemical behaviours in them. These differences can also make the interpretation of the singlet excited states of carotenoids very complicated. In this article, we review the properties of the singlet excited states of carotenoids with the aim of producing as coherent a picture as possible of what is currently known and what needs to be learned.

Carotenoid to bacteriochlorophyll energy transfer in the RC-LH1-PufX complex from Rhodobacter sphaeroides containing the extended conjugation keto-carotenoid diketospirilloxanthin

RC-LH1-PufX complexes from a genetically modified strain of Rhodobacter sphaeroides that accumulates carotenoids with very long conjugation were studied by ultrafast transient absorption spectroscopy. The complexes predominantly bind the carotenoid diketospirilloxanthin, constituting about 75% of the total carotenoids, which has 13 conjugated C=C bonds, and the conjugation is further extended to two terminal keto groups. Excitation of diketospirilloxanthin in the RC-LH1-PufX complex demonstrates fully functional energy transfer from diketospirilloxanthin to BChl a in the LH1 antenna. As for other purple bacterial LH complexes having carotenoids with long conjugation, the main energy transfer route is via the S₂-Q_x pathway. However, in contrast to LH2 complexes binding diketospirilloxanthin, in RC-LH1-PufX we observe an additional, minor energy transfer pathway associated with the S₁ state of diketospirilloxanthin. By comparing the spectral properties of the S₁ state of diketospirilloxanthin in solution, in LH2, and in RC-LH1-PufX, we propose that the carotenoid-binding site in RC-LH1-PufX activates the ICT state of diketospirilloxanthin, resulting in the opening of a minor S₁/ICT-mediated energy transfer channel.

Carotenoid-to-bacteriochlorophyll energy transfer through vibronic coupling in LH2 from Phaeosprillum molischianum

The peripheral light-harvesting antenna complex (LH2) of purple photosynthetic bacteria is an ideal testing ground for models of structure-function relationships due to its well-determined molecular structure and ultrafast energy deactivation. It has been the target for numerous studies in both theory and ultrafast spectroscopy; nevertheless, certain aspects of the convoluted relaxation network of LH2 lack a satisfactory explanation by conventional theories. For example, the initial carotenoid-to-bacteriochlorophyll energy transfer step necessary on visible light excitation was long considered to follow the Förster mechanism, even though transfer times as short as 40 femtoseconds (fs) have been observed. Such transfer times are hard to accommodate by Förster theory, as the moderate coupling strengths found in LH2 suggest much slower transfer within this framework. In this study, we investigate LH2 from Phaeospirillum (Ph.) molischianum in two types of transient absorption experiments-with narrowband pump and white-light probe resulting in 100 fs time resolution, and with degenerate broadband 10 fs pump and probe pulses. With regard to the split Q_x band in this system, we show that vibronically mediated transfer explains both the ultrafast carotenoid-to-B850 transfer, and the almost complete lack of transfer to B800. These results are beyond Förster theory, which predicts an almost equal partition between the two channels.

Effects of tunable excitation in carotenoids explained by the vibrational energy relaxation approach

Carotenoids are fundamental building blocks of natural light harvesters with convoluted and ultrafast energy deactivation networks. In order to disentangle such complex relaxation dynamics, several studies focused on transient absorption measurements and their dependence on the pump wavelength. However, such findings are inconclusive and sometimes contradictory. In this study, we compare internal conversion dynamics in [Formula: see text]-carotene, pumped at the first, second, and third vibronic progression peak. Instead of employing data fitting algorithms based on global analysis of the transient absorption spectra, we apply a fully quantum mechanical model to treat the high-frequency symmetric carbon-carbon (C=C and C-C) stretching modes explicitly. This model successfully describes observed population dynamics as well as spectral line shapes in their time-dependence and allows us to reach two conclusions: Firstly, the broadening of the induced absorption upon excess excitation is an effect of vibrational cooling in the first excited state ([Formula: see text]). Secondly, the internal conversion rate between the second excited state ([Formula: see text]) and [Formula: see text] crucially depends on the relative curve displacement. The latter point serves as a new perspective on solvent- and excitation wavelength-dependent experiments and lifts contradictions between several studies found in literature.

Femtosecond-Resolved Excited State Relaxation Dynamics of Copper (II) Tetraphenylporphyrin (CuTPP) After Soret Band Excitation

Excited state relaxation dynamics of Copper (II) tetraphenylporphyrin (CuTPP) after Soret band excitation have been investigated in various solvents by femtosecond broadband transient absorption spectroscopy. Significant role of charge transfer state has been confirmed from fast relaxation of triplet CuTPP in pyridine, giving τ ~ 26.5 ps. In piperidine, the transient measured at 480 nm shows biexponential behavior with distinct time constants of 300 fs and 27.4 ps. The fast component with τ ~ 300 fs is attributed to relaxation of the CuTPP-piperidine adduct populated in the ground state, giving the intrinsic relaxation rate of the CuTPP exciplex for the first time. For CuTPP in O-coordinating solvents of 1,4-dioxane and tetrahydrofuran (THF), a completely new relaxation channel via the ²[d_z2, d_x2−y2] state is opened. As the exciplex formation is diffusion controlled, triplet CuTPP lifetimes in pure solvents employed here are all measured to be more or less same to give ~30 ps, whereas the ²[d_z2, d_x2−y2] exciplex formed by the ligation with O-coordinating solvents is found to relax much slowly to the ground state, giving lifetimes of ~360 and ~270 ps in 1,4-dioxane and THF, respectively.

A DFT study of a set of natural dyes for organic electronics

We systematically investigate, at density functional theory level, the electronic properties of a set of ten carotenoid molecules with different conjugation length. Ground state geometries were fully optimized using both B3LYP and its long-range corrected version, i.e., the CAM-B3LYP functional. The time-dependent DFT approach (TD-DFT) was also performed for the calculation of the excited states of the optimized geometries and the results were compared to the experimental ones, when available. Our findings indicate a dependence of the transition vertical energies, oscillator strengths, and transition dipole moments on the extension of conjugation, as expected. We also investigate the impact of the intra-molecular vibrations on the absorption spectrum by means of the Franck-Condon (FC) and nuclear ensemble (NE) approach to spectra simulation. Our simulations suggest that the Franck-Condon approximation may not be suitable to appropriately characterize the vibronic progression of these molecules, whereas the NE approach provides a contribution that vary from negligible to meaningful depending on which molecule and energy region is under analysis.

Excited state properties of β-carotene analogs incorporating a lactone ring

Carotenoids possessing a carbonyl group along their polyene backbone exhibit unique excited state properties due to the occurrence of intramolecular charge transfer (ICT) in the excited state. In fact, the ICT characteristics of naturally occurring carbonyl carotenoids play an essential role in the highly efficient energy transfer that proceeds in aquatic photosynthetic antenna systems. In the present study, we synthesized two short-chain polyene carotenoids incorporating a lactone ring, denoted as BL-7 and BL-8, having seven and eight conjugated double bonds (n = 7 and 8), respectively. The excited state properties of these compounds were directly compared to those of their non-carbonyl counterparts to clarify the role of the carbonyl group in the generation of ICT. The energies of the optically allowed S₂ states for BL-7 and BL-8 were found to be more than 0.3 eV (2400 cm^-1) below those of non-carbonyl short β-carotene homologs. Ultrafast spectroscopic data demonstrated various solvent polarity-induced effects, including the appearance of stimulated emission in the near-IR region in the case of BL-7, and significant lifetime shortening of the lowest-lying singlet S₁ excited states of both BL-7 and BL-8. These results suggest that these compounds exhibit ICT characteristics.

Torsional Dynamics and Intramolecular Charge Transfer in the S2 (1(1)Bu(+)) Excited State of Peridinin: A Mechanism for Enhanced Mid-Visible Light Harvesting

Of the carotenoids known in photosynthetic organisms, peridinin exhibits one of the highest quantum efficiencies for excitation energy transfer to chlorophyll (Chl) a acceptors. The mechanism for this enhanced performance involves an order-of-magnitude slowing of the S2 (1(1)Bu(+)) → S1 (2(1)Ag(-)) nonradiative decay pathway compared to carotenoids lacking carbonyl substitution. Using femtosecond transient grating spectroscopy with optical heterodyne detection, we have obtained the first evidence that the nonradiative decay of the S2 state of peridinin is promoted by large-amplitude torsional motions. The decay of an intermediate state termed Sx, which we assign to a twisted form of the S2 state, is substantially slowed by solvent friction in peridinin due to its intramolecular charge transfer (ICT) character.

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

Time-resolved multi-pulse spectroscopic methods-pump-dump-probe (PDP) and femtosecond stimulated Raman spectroscopy-were used to investigate the excited state photodynamics of the carbonyl group containing carotenoid fucoxanthin (FX). PDP experiments show that S1 and ICT states in FX are strongly coupled and that the interstate equilibrium is rapidly (<5 ps) reestablished after one of the interacting states is deliberately depopulated. Femtosecond stimulated Raman scattering experiments indicate that S1 and ICT are vibrationally distinct species. Identification of the FSRS modes on the S1 and ICT potential energy surfaces allows us to predict a possible coupling channel for the state interaction.

Carotenoids and Photosynthesis

Carotenoids are ubiquitous and essential pigments in photosynthesis. They absorb in the blue-green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and so expand the wavelength range of light that is able to drive photosynthesis. This is an example of singlet-singlet energy transfer, and so carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. Carotenoids also act to protect photosynthetic organisms from the harmful effects of excess exposure to light. Triplet-triplet energy transfer from chlorophylls to carotenoids plays a key role in this photoprotective reaction. In the light-harvesting pigment-protein complexes from purple photosynthetic bacteria and chlorophytes, carotenoids have an additional role of structural stabilization of those complexes. In this article we review what is currently known about how carotenoids discharge these functions. The molecular architecture of photosynthetic systems will be outlined first to provide a basis from which to describe carotenoid photochemistry, which underlies most of their important functions in photosynthesis.

Ultrafast Dynamics of Long Homologues of Carotenoid Zeaxanthin

Three zeaxanthin homologues with conjugation lengths N 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 N dependence of the S2 energies allowed the estimation of [Formula: see text], 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-Sn 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-Sn 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-Sn 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.

Different Response of Carbonyl Carotenoids to Solvent Proticity Helps To Estimate Structure of the Unknown Carotenoid from Chromera velia

In order to estimate the possible structure of the unknown carbonyl carotenoid related to isofucoxanthin from Chromera velia 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.

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, Neff, 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, Neff is different for these two carotenoids. The S1 lifetime of renierapurpurin is shorter than that of isorenieratene, indicating a longer Neff 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 Neff 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 Neff than that of isorenieratene, rationalizing the observed differences in spectroscopic properties.

Vibronic energy relaxation approach highlighting deactivation pathways in carotenoids

Energy relaxation between two electronic states of a molecule is mediated by a set of relevant vibrational states.

Collisional relaxation of apocarotenals: identifying the S* state with vibrationally excited molecules in the ground electronic state S0*

The S* signal of carotenoids corresponds to vibrationally hot molecules in the ground electronic state S₀*.

Ultrafast time-resolved vibrational spectroscopies of carotenoids in photosynthesis

This review discusses the application of time-resolved vibrational spectroscopies to the studies of carotenoids in photosynthesis. The focus is on the ultrafast time regime and the study of photophysics and photochemistry of carotenoids by femtosecond time-resolved stimulated Raman and four-wave mixing spectroscopies. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Spectroscopic properties of the Chlorophyll a-Chlorophyll c 2-Peridinin-Protein-Complex (acpPC) from the coral symbiotic dinoflagellate Symbiodinium

Femtosecond time-resolved transient absorption spectroscopy was performed on the chlorophyll a-chlorophyll c 2-peridinin-protein-complex (acpPC), a major light-harvesting complex of the coral symbiotic dinoflagellate Symbiodinium. The measurements were carried out on the protein as well on the isolated pigments in the visible and the near-infrared region at 77 K. The data were globally fit to establish inter-pigment energy transfer paths within the scaffold of the complex. In addition, microsecond flash photolysis analysis was applied to reveal photoprotective capabilities of carotenoids (peridinin and diadinoxanthin) in the complex, especially the ability to quench chlorophyll a triplet states. The results demonstrate that the majority of carotenoids and other accessory light absorbers such as chlorophyll c 2 are very well suited to support chlorophyll a in light harvesting. However, their performance in photoprotection in the acpPC is questionable. This is unusual among carotenoid-containing light-harvesting proteins and may explain the low resistance of the acpPC complex against photoinduced damage under even moderate light conditions.

Effect of protein aggregation on the spectroscopic properties and excited state kinetics of the LHCII pigment–protein complex from green plants

Steady-state and time-resolved absorption and fluorescence spectroscopic experiments have been carried out at room and cryogenic temperatures on aggregated and unaggregated monomeric and trimeric LHCII complexes isolated from spinach chloroplasts. Protein aggregation has been hypothesized to be one of the mechanistic factors controlling the dissipation of excess photo-excited state energy of chlorophyll during the process known as nonphotochemical quenching. The data obtained from the present experiments reveal the role of protein aggregation on the spectroscopic properties and dynamics of energy transfer and excited state deactivation of the protein-bound chlorophyll and carotenoid pigments.

Low-temperature time-resolved spectroscopic study of the major light-harvesting complex of Amphidinium carterae

The major light-harvesting complex of Amphidinium (A.) carterae, chlorophyll-a-chlorophyll-c 2-peridinin-protein complex (acpPC), was studied using ultrafast pump-probe spectroscopy at low temperature (60 K). An efficient peridinin-chlorophyll-a energy transfer was observed. The stimulated emission signal monitored in the near-infrared spectral region was stronger when redder part of peridinin pool was excited, indicating that these peridinins have the S1/ICT (intramolecular charge-transfer) state with significant charge-transfer character. This may lead to enhanced energy transfer efficiency from "red" peridinins to chlorophyll-a. Contrary to the water-soluble antenna of A. carterae, peridinin-chlorophyll-a protein, the energy transfer rates in acpPC were slower under low-temperature conditions. This fact underscores the influence of the protein environment on the excited-state dynamics of pigments and/or the specificity of organization of the two pigment-protein complexes.

Energetics and dynamics of the low-lying electronic states of constrained polyenes: implications for infinite polyenes

Steady-state and ultrafast transient absorption spectra were obtained for a series of conformationally constrained, isomerically pure polyenes with 5-23 conjugated double bonds (N). These data and fluorescence spectra of the shorter polyenes reveal the N dependence of the energies of six (1)B(u)(+) and two (1)A(g)(-) excited states. The (1)B(u)(+) states converge to a common infinite polyene limit of 15,900 ± 100 cm(-1). The two excited (1)A(g)(-) states, however, exhibit a large (~9000 cm(-1)) energy difference in the infinite polyene limit, in contrast to the common value previously predicted by theory. EOM-CCSD ab initio and MNDO-PSDCI semiempirical MO theories account for the experimental transition energies and intensities. The complex, multistep dynamics of the 1(1)B(u)(+) → 2(1)A(g)(-) → 1(1)A(g)(-) excited state decay pathways as a function of N are compared with kinetic data from several natural and synthetic carotenoids. Distinctive transient absorption signals in the visible region, previously identified with S* states in carotenoids, also are observed for the longer polyenes. Analysis of the lifetimes of the 2(1)A(g)(-) states, using the energy gap law for nonradiative decay, reveals remarkable similarities in the N dependence of the 2(1)A(g)(-) decay kinetics of the carotenoid and polyene systems. These findings are important for understanding the mechanisms by which carotenoids carry out their roles as light-harvesting molecules and photoprotective agents in biological systems.

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), Roseobacter sp. COL2P containing the carotenoid spheroidenone, and Erythrobacter sp. NAP1 which contains the carotenoids zeaxanthin and bacteriorubixanthinal. We show that the spectroscopic data from the LH1-RC complex of Roseobacter sp. COL2P are very similar to those previously reported for Rhodobacter sphaeroides, 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 Erythrobacter 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 Roseobacter 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 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 Arabidopsis thaliana 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 a, Chl b, and variable amounts of the xanthophylls, zeaxanthin (Z), violaxanthin (V), neoxanthin (N), and lutein (L). The complexes were derived from mutants of plants denoted npq1 (NVL), npq2lut2 (Z), aba4npq1lut2 (V), aba4npq1 (VL), npq1lut2 (NV), and npq2 (LZ). The data reveal specific singlet energy transfer routes and excited state spectra and dynamics that depend on the xanthophyll present in the complex.