The vertical excitation energies and a lifetime of the two lowest singlet excited states of the conjugated polyenes from C2 to C22: Ab initio, DFT, and semiclassical MNDO-MD simulations

Electronic excited states in the series of polyene molecules were explored. Optimal ground-state geometry was used for the evaluation of vertical excitation energies. Results of a chosen set of functionals were compared to post-HF methods (EOM-CCSD, NEVPT2, CASPT2, and MRCI). In addition, the semiempirical OM2/MNDO method using MRCISD computational level was confronted with the above-mentioned techniques. Despite the fact that the first excited state has a significant double-excitation character some functionals were able to qualitatively determine the correct state order (where the lowest excited state has a A g - character). The most successful functionals in transition energies predictions were PBE, TPSS and BLYP in Tamm-Dancoff approach (TDA), which had the smallest root-mean-square deviation (RMSD) scoring towards the experimental values. Regarding RMSD scoring, the OM2/MNDO method performed fairly well, too. Besides absorption spectra, lifetimes of the first two excited states were estimated based on a stochastic approach exploring a swarm of OM2/MNDO hopping dynamics using the Tully fewest switch algorithm for each molecule. The longest lifetime of the first excited state (S₁ ) was found for decapentaene (about 5 ps). Further elongation of the conjugated chain caused a mild decrease of this value to ca 1.5 ps for docosaundecaene.

Dynamic Stark effect in β and γ carotenes induced by photoexcitation of bacteriochlorophyll c in chlorosomes from Chloroflexus aurantiacus

Chlorosomes of green bacteria can be considered as a prototype of future artificial light-harvesting devices due to their unique property of self-assembly of a large number of bacteriochlorophyll (BChl) c/d/e molecules into compact aggregates. The presence of carotenoids (Cars) in chlorosomes is very important for photoprotection, light harvesting and structure stabilization. In this work, we studied for the first time the electrochromic band shift (Stark effect) in Cars of the phototrophic filamentous green bacterium Chloroflexus (Cfx.) aurantiacus induced by fs light excitation of the main pigment, BChl c. The high accuracy of the spectral measurements permitted us to extract a small wavy spectral feature, which, obviously, can be associated with the dynamic shift of the Car absorption band. A global analysis of spectroscopy data and theoretical modeling of absorption spectra showed that near 60% of Cars exhibited a red Stark shift of ~ 25 cm^-1 and the remaining 40% exhibited a blue shift. We interpreted this finding as evidence of various orientations of Car in chlorosomes. We estimated the average value of the light-induced electric field strength in the place of Car molecules as ~ 10⁶ V/cm and the average distance between Car and the neighboring BChl c as ~ 10 Å. We concluded that the dynamics of the Car electrochromic band shift mainly reflected the dynamics of exciton migration through the chlorosome toward the baseplate within ~ 1 ps. Our work has unambiguously shown that Cars are sensitive indicators of light-induced internal electric fields in chlorosomes.

A photosynthetic antenna complex foregoes unity carotenoid-to-bacteriochlorophyll energy transfer efficiency to ensure photoprotection

Carotenoids play a number of important roles in photosynthesis, primarily providing light-harvesting and photoprotective energy dissipation functions within pigment-protein complexes. The carbon-carbon double bond (C=C) conjugation length of carotenoids (N), generally between 9 and 15, determines the carotenoid-to-(bacterio)chlorophyll [(B)Chl] energy transfer efficiency. Here we purified and spectroscopically characterized light-harvesting complex 2 (LH2) from Rhodobacter sphaeroides containing the N = 7 carotenoid zeta (ζ)-carotene, not previously incorporated within a natural antenna complex. Transient absorption and time-resolved fluorescence show that, relative to the lifetime of the S₁ state of ζ-carotene in solvent, the lifetime decreases ∼250-fold when ζ-carotene is incorporated within LH2, due to transfer of excitation energy to the B800 and B850 BChls a These measurements show that energy transfer proceeds with an efficiency of ∼100%, primarily via the S₁ → Q_x route because the S₁ → S₀ fluorescence emission of ζ-carotene overlaps almost perfectly with the Q_x absorption band of the BChls. However, transient absorption measurements performed on microsecond timescales reveal that, unlike the native N ≥ 9 carotenoids normally utilized in light-harvesting complexes, ζ-carotene does not quench excited triplet states of BChl a, likely due to elevation of the ζ-carotene triplet energy state above that of BChl a These findings provide insights into the coevolution of photosynthetic pigments and pigment-protein complexes. We propose that the N ≥ 9 carotenoids found in light-harvesting antenna complexes represent a vital compromise that retains an acceptable level of energy transfer from carotenoids to (B)Chls while allowing acquisition of a new, essential function, namely, photoprotective quenching of harmful (B)Chl triplets.

Spectroscopy and excited state dynamics of nearly infinite polyenes

Steady-state and transient absorption spectra with <50 fs time resolution were obtained for two conjugated polymers, both with ≈200 conjugated double bonds (N), constrained in planar, stable, polyene frameworks. Solutions of the polymers exhibit the same S2 → S1 → S* → S0 decay pathway observed for the N = 11-19 polyene oligomers and for zeaxanthin homologues with N = 11-23. Comparisons with the excited state dynamics of polydiactylene and a much longer, more disordered polyene polymer (poly(DEDPM)) show that the S2, S1, and S* lifetimes of the four polymers are almost identical. The S* signals in the polymers are assigned to absorption from vibrationally excited ground states. In spite of significant heterogeneities and variations in conjugation lengths in these long polyenes, their S0 → S2 absorptions are vibronically-resolved in room temperature solutions with electronic origins at ≈600 nm. The limiting wavelength for the S0 → S2 transitions is consistent with the persistence of bond length alternation in the electronic ground states and a HOMO-LUMO band gap in polyenes with N ≈ 200. The coincidence of the well-resolved S0 → S2 electronic origins and the convergence of the excited state lifetimes in the four polymers point to a common, "nearly infinite" polyene limit.

Potential TSPO Ligand and Photooxidation Quencher Isorenieratene from Arctic Ocean Rhodococcus sp. B7740

Due to its special aromatic structure, isorenieratene is thought to be an active natural antioxidant and photo/UV damage inhibitor. In this work, isorenieratene that was extracted from Rhodococcus sp. B7740 isolated from the Arctic Ocean, showed excellent scavenging ability of both singlet oxygen and hydroxyl radical in the UVB-induced auto-oxidation process using the EPR method. Within an ARPE-19 cell model damaged by UVB radiation, isorenieratene showed fine protective effects (1.13 ± 0.03 fold) compared with macular xanthophylls (MXs) through upregulating of tspo. The molecular docking was firstly performed to investigate the interaction of isorenieratene with TSPO as a special ligand. Results showed isorenieratene might form a better binding conformation (S-score −8.5438) than MXs and indicate that isorenieratene not only can function as a direct antioxidant but also activate tspo in ARPE-19 cells. Thus, isorenieratene might ease the UV-related damages including age-related macular degeneration (AMD).

Electronic and vibrational properties of carotenoids: from in vitro to in vivo

Carotenoids are among the most important organic compounds present in Nature and play several essential roles in biology. Their configuration is responsible for their specific photophysical properties, which can be tailored by changes in their molecular structure and in the surrounding environment. In this review, we give a general description of the main electronic and vibrational properties of carotenoids. In the first part, we describe how the electronic and vibrational properties are related to the molecular configuration of carotenoids. We show how modifications to their configuration, as well as the addition of functional groups, can affect the length of the conjugated chain. We describe the concept of effective conjugation length, and its relationship to the S₀ → S₂ electronic transition, the decay rate of the S₁ energetic level and the frequency of the ν₁ Raman band. We then consider the dependence of these properties on extrinsic parameters such as the polarizability of their environment, and how this information (S₀ → S₂ electronic transition, ν₁ band position, effective conjugation length and polarizability of the environment) can be represented on a single graph. In the second part of the review, we use a number of specific examples to show that the relationships can be used to disentangle the different mechanisms tuning the functional properties of protein-bound carotenoids.

Low-Light Anoxygenic Photosynthesis and Fe-S-Biogeochemistry in a Microbial Mat

We report extremely low-light-adapted anoxygenic photosynthesis in a thick microbial mat in Magical Blue Hole, Abaco Island, The Bahamas. Sulfur cycling was reduced by iron oxides and organic carbon limitation. The mat grows below the halocline/oxycline at 30 m depth on the walls of the flooded sinkhole. In situ irradiance at the mat surface on a sunny December day was between 0.021 and 0.084 μmol photons m^-2 s^-1, and UV light (<400 nm) was the most abundant part of the spectrum followed by green wavelengths (475–530 nm). We measured a light-dependent carbon uptake rate of 14.5 nmol C cm^-2 d^-1. A 16S rRNA clone library of the green surface mat layer was dominated (74%) by a cluster (>97% sequence identity) of clones affiliated with Prosthecochloris, a genus within the green sulfur bacteria (GSB), which are obligate anoxygenic phototrophs. Typical photopigments of brown-colored GSB, bacteriochlorophyll e and (β-)isorenieratene, were abundant in mat samples and their absorption properties are well-adapted to harvest light in the available green and possibly even UV-A spectra. Sulfide from the water column (3–6 μmol L^-1) was the main source of sulfide to the mat as sulfate reduction rates in the mats were very low (undetectable-99.2 nmol cm^-3 d^-1). The anoxic water column was oligotrophic and low in dissolved organic carbon (175–228 μmol L^-1). High concentrations of pyrite (FeS₂; 1–47 μmol cm^-3) together with low microbial process rates (sulfate reduction, CO₂ fixation) indicate that the mats function as net sulfide sinks mainly by abiotic processes. We suggest that abundant Fe(III) (4.3–22.2 μmol cm^-3) is the major source of oxidizing power in the mat, and that abiotic Fe-S-reactions play the main role in pyrite formation. Limitation of sulfate reduction by low organic carbon availability along with the presence of abundant sulfide-scavenging iron oxides considerably slowed down sulfur cycling in these mats.

Role of Ring Ortho Substituents on the Configuration of Carotenoid Polyene Chains

The 9-(Z)-configuration was exclusively obtained in the carotenoid polyene chain irrespective of olefination and disconnection methods for terminal ortho-unsubstituted benzene rings. The 2,6-dimethyl substituents in the terminal rings secure an all-(E)-polyene structure. The single molecular conductance of the pure 9-(Z)-carotene was measured for the first time to be 1.53 × 10^-4 ± 6.37 × 10^-5G₀, whose value was 47% that of the all-(E)-carotene ((3.23 × 10^-4) ± (1.23 × 10^-4) G₀).

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.

Energy transfer in aggregates of bacteriochlorophyll c self-assembled with azulene derivatives

Bacteriochlorophyll (BChl) c is the main light-harvesting pigment of certain photosynthetic bacteria. It is found in the form of self-assembled aggregates in the so-called chlorosomes. Here we report the results of co-aggregation experiments of BChl c with azulene and its tailored derivatives. We have performed spectroscopic and quantum chemical characterization of the azulenes, followed by self-assembly experiments. The results show that only azulenes with sufficient hydrophobicity are able to induce aggregation of BChl c. Interestingly, only azulene derivatives possessing a conjugated phenyl ring were capable of efficient (∼50%) excitation energy transfer to BChl molecules. These aggregates represent an artificial light-harvesting complex with enhanced absorption between 220 and 350 nm compared to aggregates of pure BChl c. The results provide insight into the principles of self-assembly of BChl aggregates and suggest an important role of the π-π interactions in efficient energy transfer.

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.

Excited state lifetimes and energies of okenone and chlorobactene, exemplary keto and non-keto aryl carotenoids

Photophysical properties of two typical aryl carotenoids, okenone and chlorobactene, were studied with application of femtosecond and microsecond time-resolved absorption spectroscopies.

Genome rearrangements of Streptomyces albus J1074 lead to the carotenoid gene cluster activation

Streptomyces albus J1074 is a derivative of the S. albus G1 strain defective in SalG1 restriction-modification system. Genome sequencing of S. albus J1074 revealed that the size of its chromosome is 6.8 Mb with unusually short terminal arms of only 0.3 and 0.4 Mb. Here we present our attempts to evaluate the dispensability of subtelomeric regions of the S. albus J1074 chromosome. A number of large site-directed genomic deletions led to circularization of the S. albus J1074 chromosome and to the overall genome reduction by 307 kb. Two spontaneous mutants with an activated carotenoid cluster were obtained. Genome sequencing and transcriptome analysis indicated that phenotypes of these mutants resulted from the right terminal 0.42 Mb chromosomal region deletion, followed by the carotenoid cluster amplification. Our results indicate that the right terminal 0.42 Mb fragment is dispensable under laboratory conditions. In contrast, the left terminal arm of the S. albus J1074 chromosome contains essential genes and only 42 kb terminal region is proved to be dispensable. We identified overexpressed carotenoid compounds and determined fitness costs of the large genomic rearrangements.

Structural and functional roles of carotenoids in chlorosomes

Chlorosomes are large light-harvesting complexes found in three phyla of anoxygenic photosynthetic bacteria. Chlorosomes are primarily composed of self-assembling pigment aggregates. In addition to the main pigment, bacteriochlorophyll c, d, or e, chlorosomes also contain variable amounts of carotenoids. Here, we use X-ray scattering and electron cryomicroscopy, complemented with absorption spectroscopy and pigment analysis, to compare the morphologies, structures, and pigment compositions of chlorosomes from Chloroflexus aurantiacus grown under two different light conditions and Chlorobaculum tepidum. High-purity chlorosomes from C. aurantiacus contain about 20% more carotenoid per bacteriochlorophyll c molecule when grown under low light than when grown under high light. This accentuates the light-harvesting function of carotenoids, in addition to their photoprotective role. The low-light chlorosomes are thicker due to the overall greater content of pigments and contain domains of lamellar aggregates. Experiments where carotenoids were selectively extracted from intact chlorosomes using hexane proved that they are located in the interlamellar space, as observed previously for species belonging to the phylum Chlorobi. A fraction of the carotenoids are localized in the baseplate, where they are bound differently and cannot be removed by hexane. In C. tepidum, carotenoids cannot be extracted by hexane even from the chlorosome interior. The chemical structure of the pigments in C. tepidum may lead to π-π interactions between carotenoids and bacteriochlorophylls, preventing carotenoid extraction. The results provide information about the nature of interactions between bacteriochlorophylls and carotenoids in the protein-free environment of the chlorosome interior.

Molecular factors controlling photosynthetic light harvesting by carotenoids

Carotenoids are naturally occurring pigments that absorb light in the spectral region in which the sun irradiates maximally. These molecules transfer this energy to chlorophylls, initiating the primary photochemical events of photosynthesis. Carotenoids also regulate the flow of energy within the photosynthetic apparatus and protect it from photoinduced damage caused by excess light absorption. To carry out these functions in nature, carotenoids are bound in discrete pigment-protein complexes in the proximity of chlorophylls. A few three-dimensional structures of these carotenoid complexes have been determined by X-ray crystallography. Thus, the stage is set for attempting to correlate the structural information with the spectroscopic properties of carotenoids to understand the molecular mechanism(s) of their function in photosynthetic systems. In this Account, we summarize current spectroscopic data describing the excited state energies and ultrafast dynamics of purified carotenoids in solution and bound in light-harvesting complexes from purple bacteria, marine algae, and green plants. Many of these complexes can be modified using mutagenesis or pigment exchange which facilitates the elucidation of correlations between structure and function. We describe the structural and electronic factors controlling the function of carotenoids as energy donors. We also discuss unresolved issues related to the nature of spectroscopically dark excited states, which could play a role in light harvesting. To illustrate the interplay between structural determinations and spectroscopic investigations that exemplifies work in the field, we describe the spectroscopic properties of four light-harvesting complexes whose structures have been determined to atomic resolution. The first, the LH2 complex from the purple bacterium Rhodopseudomonas acidophila, contains the carotenoid rhodopin glucoside. The second is the LHCII trimeric complex from higher plants which uses the carotenoids lutein, neoxanthin, and violaxanthin to transfer energy to chlorophyll. The third, the peridinin-chlorophyll-protein (PCP) from the dinoflagellate Amphidinium carterae, is the only known complex in which the bound carotenoid (peridinin) pigments outnumber the chlorophylls. The last is xanthorhodopsin from the eubacterium Salinibacter ruber. This complex contains the carotenoid salinixanthin, which transfers energy to a retinal chromophore. The carotenoids in these pigment-protein complexes transfer energy with high efficiency by optimizing both the distance and orientation of the carotenoid donor and chlorophyll acceptor molecules. Importantly, the versatility and robustness of carotenoids in these light-harvesting pigment-protein complexes have led to their incorporation in the design and synthesis of nanoscale antenna systems. In these bioinspired systems, researchers are seeking to improve the light capture and use of energy from the solar emission spectrum.