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

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

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.

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.

Understanding Carotenoid Dynamics via the Vibronic Energy Relaxation Approach.

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.

Transient Absorption of Chlorophylls and Carotenoids after Two-Photon Excitation of LHCII.

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 ∼1.7 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.

Energy transfer pathways in the CAC light-harvesting complex of Rhodomonas salina.

Photosynthetic organisms had to evolve diverse mechanisms of light-harvesting to supply photosynthetic apparatus with enough energy. Cryptophytes represent one of the groups of photosynthetic organisms combining external and internal antenna systems. They contain one type of immobile phycobiliprotein located at the lumenal side of the thylakoid membrane, together with membrane-bound chlorophyll a/c antenna (CAC). Here we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the CAC proteins of cryptophyte Rhodomonas salina. The major CAC carotenoid, alloxanthin, is a cryptophyte-specific carotenoid, and it is the only naturally-occurring carotenoid with two triple bonds in its structure. In order to explore the energy transfer pathways within the CAC complex, three excitation wavelengths (505, 590, and 640 nm) were chosen to excite pigments in the CAC antenna. The excitation of Chl c at either 590 or 640 nm proves efficient energy transfer between Chl c and Chl a. The excitation of alloxanthin at 505 nm shows an active pathway from the S2 state with efficiency around 50%, feeding both Chl a and Chl c with approximately 1:1 branching ratio, yet, the S1-route is rather inefficient. The 57 ps energy transfer time to Chl a gives ~25% efficiency of the S1 channel. The low efficiency of the S1 route renders the overall carotenoid-Chl energy transfer efficiency low, pointing to the regulatory role of alloxanthin in the CAC antenna.

Equilibration Dependence of Fucoxanthin S1 and ICT Signatures on Polarity, Proticity, and Temperature by Multipulse Femtosecond Absorption Spectroscopy.

To demonstrate the value of the multipulse method in revealing the nature of coupling between excited states and explore the environmental dependencies of lowest excited singlet state (S1) and intramolecular charge transfer (ICT) state equilibration, we performed ultrafast transient absorption pump-dump-probe and pump-repump-probe spectroscopies on fucoxanthin in various solvent conditions. The effects of polarity, proticity, and temperature were tested in solvents methanol at 293 and 190 K, acetonitrile, and isopropanol. We show that manipulation of the kinetic traces can produce one trace reflecting the equilibration kinetics of the states, which reveals that lower polarity, proticity, and temperature delay S1/ICT equilibration. On the basis of a two-state model representing the S1 and ICT states on the same S1/ICT potential energy surface, we were able to show that the kinetics are strictly dependent on the initial relative populations of the states as well as the decay of the ICT state to the ground state. Informed by global analysis, a systematic method for target analysis based on this model allowed us to quantify the population transfer rates throughout the life of the S1/ICT state as well as separate the S1 and ICT spectral signatures. The results are consistent with the concept that the S1 and ICT states are part of one potential energy surface.