Changes in Molecular Structure upon Triplet Excitation of All-trans-Spheroidene in n-Hexane Solution and 15-cis-Spheroidene Bound to the Photo-Reaction Center from Rhodobacter sphaeroides As Revealed by Resonance-Raman Spectroscopy and Normal-Coordinate Analysis

Characterization of the Ultraviolet-B Absorption Band of Carotenoids Using Solvent-dependent Shifts in Steady-State and Transient Absorption Spectra

The versatile functions of carotenoids in biological systems are associated with the extended π-electron conjugation system. Strong visible absorption resulting from the optically allowed S2 (1Bu+) state and the low-lying optically forbidden S1 (2Ag-) state examined. Carotenoids also exhibit an absorption band in the ultraviolet-B region; however, the origin of this band (hereafter referred to as Suv state) is not well characterized. The Suv state is a candidate for the destination level of the well-known S1 → Sn transient absorption; however, an obvious energy mismatch has been observed. In this study, we examined the steady-state and picosecond transient absorption spectra of lycopene in various solvents. The Suv absorption of carotenoids with diverse conjugation lengths was also examined. The dependence of the energies on solvent polarizability and conjugation length revealed that both Suv and Sn are the "second" Bu+ state. The absorption spectrum for lycopene at 200 K revealed an additional vibrational band, which may be the vibrational origin of the S0 → Suv band. Considering the slow vibrational relaxation of the 2Ag- state, the S1 → Sn transition may represent the 2Ag- (v = 1) → 2Bu+ (v = 0) transition, and the energetic contradiction can be resolved.

Triplet-triplet energy transfer in artificial and natural photosynthetic antennas

In photosynthetic organisms, protection against photooxidative stress due to singlet oxygen is provided by carotenoid molecules, which quench chlorophyll triplet species before they can sensitize singlet oxygen formation. In anoxygenic photosynthetic organisms, in which exposure to oxygen is low, chlorophyll-to-carotenoid triplet-triplet energy transfer (T-TET) is slow, in the tens of nanoseconds range, whereas it is ultrafast in the oxygen-rich chloroplasts of oxygen-evolving photosynthetic organisms. To better understand the structural features and resulting electronic coupling that leads to T-TET dynamics adapted to ambient oxygen activity, we have carried out experimental and theoretical studies of two isomeric carotenoporphyrin molecular dyads having different conformations and therefore different interchromophore electronic interactions. This pair of dyads reproduces the characteristics of fast and slow T-TET, including a resonance Raman-based spectroscopic marker of strong electronic coupling and fast T-TET that has been observed in photosynthesis. As identified by density functional theory (DFT) calculations, the spectroscopic marker associated with fast T-TET is due primarily to a geometrical perturbation of the carotenoid backbone in the triplet state induced by the interchromophore interaction. This is also the case for the natural systems, as demonstrated by the hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of light-harvesting proteins from oxygenic (LHCII) and anoxygenic organisms (LH2). Both DFT and electron paramagnetic resonance (EPR) analyses further indicate that, upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.

Energy dissipative photoprotective mechanism of carotenoid spheroidene from the photoreaction center of purple bacteria Rhodobacter sphaeroides

Carotenoid spheroidene (SPO) functions for photoprotection in the photosynthetic reaction centers (RCs) and effectively dissipates its triplet excitation energy. Sensitized cis-to-trans isomerization was proposed as a possible mechanism for a singlet-triplet energy crossing for the 15,15'-cis-SPO; however, it has been questioned recently. To understand the dissipative photoprotective mechanism of this important SPO and to overcome the existing controversies on this issue, we carried out a theoretical investigation using density functional theory on the possible triplet energy relaxation mechanism through the cis-to-trans isomerization. Together with the earlier experimental observations, the possible mechanism was discussed for the triplet energy relaxation of the 15,15'-cis-SPO. The result shows that complete cis-to-trans isomerization is not necessary. Twisting the C15-C15' bond leads to singlet-triplet energy crossing at ϕ(14,15,15',14') = 77° with an energy 32.5 kJ mol(-1) (7.7 kcal mol(-1)) higher than that of the T1 15,15'-cis minimum. Further exploration of the minimum-energy intersystem crossing (MEISC) point shows that triplet relaxation could occur at a less distorted structure (ϕ = 58.4°) with the energy height of 26.5 KJ mol(-1) (6.3 kcal mol(-1)). Another important reaction coordinate to reach the MEISC point is the bond-length alternation. The model truncation effect, solvent effect, and spin-orbit coupling were also investigated. The singlet-triplet crossing was also investigated for the 13,14-cis stereoisomer and locked-13,14-cis-SPO. We also discussed the origin of the natural selection of the cis over trans isomer in the RC.

How do surrounding environments influence the electronic and vibrational properties of spheroidene?

Absorption and Raman spectra of spheroidene dissolved in various organic solvents and bound to peripheral light-harvesting LH2 complexes from photosynthetic purple bacteria Rhodobacter (Rba.) sphaeroides 2.4.1 were measured. The results showed that the peak energies of absorption and C-C and C=C stretching Raman lines are linearly proportional to the polarizability of solvents, as has already been reported. When comparing these results with those measured on LH2 complexes, it was confirmed that spheroidene is surrounded by a media with high polarizability. However, the change in the spectral width of the Raman lines, which reflect vibrational decay time, cannot be explained simply by a similar dependence of solvent polarizability. The experimental results were analyzed using a potential theoretical model. Consequently, a systematic change in the Raman line widths in the ground state can be satisfactorily explained as a function of the viscosity of the surrounding media. Even when the absorption peaks appear at the same energy, the vibrational decay time of spheroidene in the LH2 complexes is approximately 15-20 % slower than that in organic solvents.

Molecular adaptation of photoprotection: triplet states in light-harvesting proteins

The photosynthetic light-harvesting systems of purple bacteria and plants both utilize specific carotenoids as quenchers of the harmful (bacterio)chlorophyll triplet states via triplet-triplet energy transfer. Here, we explore how the binding of carotenoids to the different types of light-harvesting proteins found in plants and purple bacteria provides adaptation in this vital photoprotective function. We show that the creation of the carotenoid triplet states in the light-harvesting complexes may occur without detectable conformational changes, in contrast to that found for carotenoids in solution. However, in plant light-harvesting complexes, the triplet wavefunction is shared between the carotenoids and their adjacent chlorophylls. This is not observed for the antenna proteins of purple bacteria, where the triplet is virtually fully located on the carotenoid molecule. These results explain the faster triplet-triplet transfer times in plant light-harvesting complexes. We show that this molecular mechanism, which spreads the location of the triplet wavefunction through the pigments of plant light-harvesting complexes, results in the absence of any detectable chlorophyll triplet in these complexes upon excitation, and we propose that it emerged as a photoprotective adaptation during the evolution of oxygenic photosynthesis.

Selective Binding of Carotenoids with a Shorter Conjugated Chain to the LH2 Antenna Complex and Those with a Longer Conjugated Chain to the Reaction Center from Rubrivivax gelatinosus

Rubrivivax gelatinosus having both the spheroidene and spirilloxanthin biosynthetic pathways produces carotenoids (Cars) with a variety of conjugated chains, which consist of different numbers of conjugated double bonds (n), including the C=C (m) and C=O (o) bonds. When grown under anaerobic conditions, the wild type produces Cars for which n = m = 9-13, whereas under semiaerobic conditions, it additionally produces Cars for which n = m + o = 10 + 1, 13 + 1, and 13 + 2. On the other hand, a mutant, in which the latter pathway is genetically blocked, produces only Cars for which n = 9 and 10 under anaerobic conditions and n = 9, 10, and 10 + 1 under semianaerobic conditions. Those Cars that were extracted from the LH2 complex (LH2) and the reaction center (RC), isolated from the wild-type and the mutant Rvi. gelatinosus, were analyzed by HPLC, and their structures were determined by mass spectrometry and 1H NMR spectroscopy. The selective binding of Cars to those pigment-protein complexes has been characterized as follows. (1) Cars with a shorter conjugated chain are selectively bound to LH2 whereas Cars with a longer conjugated chain to the RC. (2) Shorter chain Cars with a hydroxyl group are bound to LH2 almost exclusively. This rule holds either in the absence or in the presence of the keto group. The natural selection of shorter chain Cars by LH2 and longer chain Cars by the RC is discussed, on the basis of the results now available, in relation to the light-harvesting and photoprotective functions of Cars.

Conjugation-Length Dependence of the T1 Lifetimes of Carotenoids Free in Solution and Incorporated into the LH2, LH1, RC, and RC-LH1 Complexes: Possible Mechanisms of Triplet-Energy Dissipation

In addition to the roles of antioxidant and spacer, carotenoids (Cars) in purple photosynthetic bacteria pursue two physiological functions, i.e., light harvesting and photoprotection. To reveal the mechanisms of the photoprotective function, i.e., quenching triplet bacteriochlorophyll to prevent the sensitized generation of singlet oxygen, the triplet absorption spectra were recorded for Cars, where the number of conjugated double bonds (n) is in the region of 9-13, to determine the dependence on n of the triplet lifetime. The Cars examined include those in (a) solution; (b) the reconstituted LH1 complexes; (c) the native LH2 complexes from Rba. sphaeroides G1C, Rba. sphaeroides 2.4.1, Rsp. molischianum, and Rps. acidophila 10050; (d) the RCs from Rba. sphaeroides G1C, Rba. sphaeroides 2.4.1, and Rsp. rubrum S1; and (e) the RC-LH1 complexes from Rba. sphaeroides G1C, Rba. sphaeroides 2.4.1, Rsp. molischianum, Rps. acidophila 10050, and Rsp. rubrum S1. The results lead us to propose the following mechanisms: (i) A substantial shift of the linear dependence to shorter lifetimes on going from solution to the LH2 complex was ascribed to the twisting of the Car conjugated chain. (ii) A substantial decrease in the slope of the linear dependence on going from the reconstituted LH1 to the LH1 component of the RC-LH1 complex was ascribed to the minor-component Car forming a leak channel of triplet energy. (iii) The loss of conjugation-length dependence on going from the isolated RC to the RC component of the RC-LH1 complex was ascribed to the presence of a triplet-energy reservoir consisting of bacteriochlorophylls in the RC component.

Triplet-State Conformational Changes in 15-cis-Spheroidene Bound to the Reaction Center from Rhodobacter sphaeroides 2.4.1 as Revealed by Time-Resolved EPR Spectroscopy: Strengthened Hypothetical Mechanism of Triplet-Energy Dissipation

Time-resolved EPR spectra of 15-cis-spheroidene bound to the reaction center from Rhodobacter sphaeroides 2.4.1 were recorded at low temperatures. (1) A three-component analysis of the spectral-data matrices by singular-value decomposition followed by global fitting identified the transformation of the triplet carotenoid, (3)Car(I) --> (3)Car(II); during this process, the leak of the triplet population was suggested. A four-component analysis suggested the presence of a representative intermediate, (3)Car(R), that forms a leak channel of the triplet population. (2) A theoretical calculation of the zero-field splitting parameters, |D| and |E|, by the use of a polyene model, showed that the transformation, (3)Car(I) --> (3)Car(R) --> (3)Car(II), accompanies the conformational changes of (0 degrees , 0 degrees , 0 degrees ) --> (+20 degrees , -20 degrees , +20 degrees ) --> (+45 degrees , -40 degrees , +40 degrees ) around the central cis C15=C15', trans C13=C14, and trans C11=C12 bonds, respectively. (3) The initial, rapid decrease followed by the inversion of spin polarization along the z axis of (3)Car was observed, which was correlated with a change in the spin angular momentum. (4) In reference to the binding pocket of the Car, determined by X-ray crystallography, the conformational changes were ascribed to the intrinsic isomerization property of 15-cis (3)Car as well as the Car-peptide intermolecular interaction; a detailed picture was proposed. All of the above results support the mechanism of triplet-energy dissipation proposed previously: the rotational motions around the central double bonds cause a change in the orbital angular momentum and, through the spin-orbit coupling, a change in the spin angular momentum, which enhances the T(1) --> S(0) intersystem crossing dissipating the triplet energy.

Synthesis of 18O-Labelled chlorophyll derivatives at carbonyl oxygen atoms by acidic hydrolysis of the ethylene ketal and acetal

The ethylene ketal of pyropheophorbides, chlorophylls possessing the 13-keto carbonyl group and lacking the 13(2)-methoxycarbonyl group, reacted with H(2)(18)O (ca. 95% 18O atom) by acidic hydrolysis to give efficiently and regioselectively 13(1)-18O-oxo-labelled compounds (ca. 92% 18O). The resulting 18O-labelled chlorin was modified by several chemical reactions to afford some derivatives with little loss of the 18O atom. Following the same procedures, 3(1),13(1)-doubly-18O-labelled pyrochlorophyll derivatives were also prepared. All the synthetic 18O-labelled compounds were identified by FAB-mass and vibrational spectra. Especially, in the vibrational spectroscopic results including IR and resonance Raman spectra, an about 30 cm(-1) wavenumber down-shift of the 3- and/or 13-C[double bond]O stretching vibrational bands was observed by exchanging 3(1)- or 13(1)-oxo-oxygen atom from 16O to 18O.