Vibrational Relaxation and Redistribution in the 2Ag- State of all-trans-Lycopene As Revealed by Picosecond Time-Resolved Absorption Spectroscopy

The full dynamics of energy relaxation in large organic molecules: from photo-excitation to solvent heating

In some molecular systems, such as nucleobases, polyenes or sunscreens, substantial amounts of photo-excitation energy are dissipated on a sub-picosecond time scale. Where does this energy go or among which degrees of freedom it is being distributed at such early times?

In some molecular systems, such as nucleobases, polyenes or the active ingredients of sunscreens, substantial amounts of photo-excitation energy are dissipated on a sub-picosecond time scale, raising questions such as: where does this energy go or among which degrees of freedom it is being distributed at such early times? Here we use transient absorption spectroscopy to track excitation energy dispersing from the optically accessible vibronic subsystem into the remaining vibrational subsystem of the solute and solvent. Monitoring the flow of energy during vibrational redistribution enables quantification of local molecular heating. Subsequent heat dissipation away from the solute molecule is characterized by classical thermodynamics and molecular dynamics simulations. Hence, we present a holistic approach that tracks the internal temperature and vibronic distribution from the act of photo-excitation to the restoration of the global equilibrium. Within this framework internal vibrational redistribution and vibrational cooling are emergent phenomena. We demonstrate the validity of the framework by examining a highly controversial example, carotenoids. We show that correctly accounting for the local temperature unambiguously explains their energetically and temporally congested spectral dynamics without the ad hoc postulation of additional ‘dark’ states. An immediate further application of this approach would be to monitor the excitation and thermal dynamics of pigment–protein systems.

A Unified Picture of S* in Carotenoids

In π-conjugated chain molecules such as carotenoids, coupling between electronic and vibrational degrees of freedom is of central importance. It governs both dynamic and static properties, such as the time scales of excited state relaxation as well as absorption spectra. In this work, we treat vibronic dynamics in carotenoids on four electronic states (|S0⟩, |S1⟩, |S2⟩, and |Sn⟩) in a physically rigorous framework. This model explains all features previously associated with the intensely debated S* state. Besides successfully fitting transient absorption data of a zeaxanthin homologue, this model also accounts for previous results from global target analysis and chain length-dependent studies. Additionally, we are able to incorporate findings from pump-deplete-probe experiments, which were incompatible to any pre-existing model. Thus, we present the first comprehensive and unified interpretation of S*-related features, explaining them by vibronic transitions on either S1, S0, or both, depending on the chain length of the investigated carotenoid.

Astaxanthin Protecting Membrane Integrity against Photosensitized Oxidation through Synergism with Other Carotenoids

Incorporation of astaxanthin or zeaxanthin in giant unilamellar vesicles (GUVs) of phosphatidylcholine resulted in a longer lag phase than incorporation of β-carotene or lycopene for the onset of budding induced by chlorophyll a photosensitization and quantified by a dimensionless entropy parameter using optical microscopy and digital image heterogeneity analysis. The lowest initial rate of GUV budding after the lag phase was seen for GUVs with astaxanthin as the least reducing carotenoid, while the lowest final level of entropy appeared for those with lycopene or β-carotene as a more reducing carotenoid. The combination of astaxanthin and lycopene gave optimal protection against budding with respect to both a longer lag phase and lower final level of entropy by combining good electron acceptance and good electron donation. Quenching of singlet oxygen by carotenoids close to chlorophyll a in the membrane interior in parallel with scavenging of superoxide radicals by astaxanthin anchored in the surface may explain the synergism between carotenoids involving both type I and type II photosensitization by chlorophyll a.

Dynamics of the time-resolved stimulated Raman scattering spectrum in presence of transient vibronic inversion of population on the example of optically excited trans-β-apo-8'-carotenal

We have studied the effect of transient vibrational inversion of population in trans-β-apo-8(')-carotenal on the time-resolved femtosecond stimulated Raman scattering (TR-FSRS) signal. The experimental data are interpreted by applying a quantum mechanical approach, using the formalism of projection operators for constructing the theoretical model of TR-FSRS. Within this theoretical frame we explain the presence of transient Raman losses on the Stokes side of the TR-FSRS spectrum as the effect of vibrational inversion of population. In view of the obtained experimental and theoretical results, we conclude that the excited S2 electronic level of trans-β-apo-8(')-carotenal relaxes towards the S0 ground state through a set of four vibrational sublevels of S1 state.

Electron transfer from plant phenolates to carotenoid radical cations. Antioxidant interaction entering the Marcus theory inverted region

β-Carotene, lycopene, and zeaxanthin are maximally regenerated by plant phenolates from their radical cations formed during laser flash photolysis in 9:1 (v/v) chloroform/methanol for a driving force corresponding to the reorganization energy according to the Marcus theory. For β-carotene, the reorganization energy has values of 0.41 ± 0.04 and 0.40 ± 0.04 eV for the plant phenols in the presence of 1 and 2 equiv of base, respectively, at 23 °C. For a driving force lower than the reorganization energy, regeneration of the carotenoids is less efficient as is seen for m-hydroxybenzoic acid, vanillic acid, and p-coumaric acid. For a driving force above the maximum rate as determined to have kET = 6.3 × 10(9) L·mol(-1)·s(-1) for syringic acid and β-carotene, the reaction becomes gradually slower and regeneration less efficient as is seen for the more reducing caffeic acid, rutin, and quercetin corresponding to an inverted region for the rate of electron transfer. Lycopene and zeaxanthin show a similar behavior for the same series of plant phenols with slightly lower reorganization energy, in agreement with the lower reduction potential of their radical cations, while, for the ketocarotenoids astaxanthin and canthaxanthin, fast reactions with a solvent of radical cations inhibit regeneration from being detected. Intermediate reducing plant phenols accordingly yield maximal protection of carotenoids against photobleaching in foods and beverages.

Characterization and thermal isomerization of (all-E)-lycopene

A large amount of (all-E)-lycopene was successfully purified from tomato paste using an improved method that included a procedure to wash crystalline powder with acetone. The total yield of the pure (all-E) form was at least 30%. The melting point of (all-E)-lycopene was determined to be 176.35 °C by differential scanning calorimetry (DSC) measurements. Bathochromic shifts were observed in the absorption maxima of all solvents tested (at most a 36 nm shift for λ2 in carbon disulfide, as was observed in hexane) and were accompanied by absorbance decreases, namely, a hypochromic effect, showing a higher correlation between the position and the intensity of the main absorption bands. This bathochromic shift was dependent upon the polarizability of the solvent rather than its polarity. The structure of (all-E)-lycopene in CDCl3 and C6D6 was identified on the basis of one- and two-dimensional nuclear magnetic resonance (NMR) spectra, including (1)H and (13)C NMR, homonuclear correlation spectroscopy ((1)H-(1)H COSY), heteronuclear multiple-quantum coherence (HMQC), and heteronuclear multiple-bond connectivity (HMBC). The rate constants of the decrease in (all-E)-lycopene with hexane and benzene were calculated to be 3.19 × 10(-5) and 3.55 × 10(-5) s(-1), respectively. The equilibrium constants between (all-E) and (13Z) isomers were estimated to be 0.29 in hexane and 0.31 in benzene, respectively, from the point at which the amount of (13Z)-lycopene reached its maximum.

Excited-State Processes in the Carotenoid Zeaxanthin after Excess Energy Excitation

Aiming for better understanding of the large complexity of excited-state processes in carotenoids, we have studied the excitation wavelength dependence of the relaxation dynamics in the carotenoid zeaxanthin. Excitation into the lowest vibrational band of the S2 state at 485 nm, into the 0-3 vibrational band of the S2 state at 400 nm, and into the 2B(u)+ state at 266 nm resulted in different relaxation patterns. While excitation at 485 nm produces the known four-state scheme (S2 --> hot S1 --> S1 --> S0), excess energy excitation led to additional dynamics occurring with a time constant of 2.8 ps (400 nm excitation) and 4.9 ps (266 nm excitation), respectively. This process is ascribed to a conformational relaxation of conformers generated by the excess energy excitation. The zeaxanthin S state was observed regardless of the excitation wavelength, but its population increased after 400 and 266 nm excitation, suggesting that conformers generated by the excess energy excitation are important for directing the population toward the S state. The S2-S1 internal conversion time was shortened from 135 to 70 fs when going from 485 to 400 nm excitation, as a result of competition between the S2-S1 internal conversion from the vibrationally hot S2 state and S2 vibrational relaxation. The S1 lifetime of zeaxanthin was within experimental error the same for all excitation wavelengths, yielding approximately 9 ps. No long-lived species have been observed after excitation by femtosecond pulses regardless of the excitation wavelength, but excitation by nanosecond pulses at 266 nm generated both zeaxanthin triplet state and cation radical.

Trapping of an assembly intermediate of photosynthetic LH1 antenna beyond B820 subunit. Significance for the assembly of photosynthetic LH1 antenna

Most photosynthetic LH1 antennae undergo dissociation into B820 subunits, suggesting their universal character as structural modules. However, dissociation into subunits seems to occur reversibly only in the absence of carotenoids and the subunits were never found to bind carotenoids. The interactions of carotenoids with B820 have been studied in a newly developed reconstitution assay of the LH1 antenna from Rhodospirillum rubrum (Fiedor, L., Akahane, J., and Koyama, Y. (2004) Biochemistry 43, 16487-16496). These model studies show that B820 subunits strongly interact with carotenoids and spontaneously form stable LH1-like complexes with substoichiometric carotenoid content. This is the first experimental evidence that B820 may occur as a short-lived intermediate in the assembly of LH1 in vivo. The resulting complex of B820 subunits with carotenoid, termed iB873, is homogeneous, according to ion exchange chromatography and reproducible pigment composition. The iB873-bound carotenoid is as efficient in energy transfer to bacteriochlorophyll as the one in native antenna. To our knowledge, iB873 is the first complex binding functional carotenoid, with the spectral and biochemical properties intermediate between that of B820 and the fully assembled LH1.

Ultrafast Dynamics of Isolated Model Photoactive Yellow Protein Chromophores: “Chemical Perturbation Theory” in the Laboratory

Pump-probe and pump-dump probe experiments have been performed on several isolated model chromophores of the photoactive yellow protein (PYP). The observed transient absorption spectra are discussed in terms of the spectral signatures ascribed to solvation, excited-state twisting, and vibrational relaxation. It is observed that the protonation state has a profound effect on the excited-state lifetime of p-coumaric acid. Pigments with ester groups on the coumaryl tail end and charged phenolic moieties show dynamics that are significantly different from those of other pigments. Here, an unrelaxed ground-state intermediate could be observed in pump-probe signals. A similar intermediate could be identified in the sinapinic acid and in isomerization-locked chromophores by means of pump-dump probe spectroscopy; however, in these compounds it is less pronounced and could be due to ground-state solvation and/or vibrational relaxation. Because of strong protonation-state dependencies and the effect of electron donor groups, it is argued that charge redistribution upon excitation determines the twisting reaction pathway, possibly through interaction with the environment. It is suggested that the same pathway may be responsible for the initiation of the photocycle in native PYP.

Ultrafast transient lens spectroscopy of various C40 carotenoids: lycopene, β-carotene, (3R,3′R)-zeaxanthin, (3R,3′R,6′R)-lutein, echinenone, canthaxanthin, and astaxanthin

The ultrafast internal conversion (IC) dynamics of seven C(40) carotenoids have been investigated at room temperature in a variety of solvents using two-color transient lens (TL) pump-probe spectroscopy. We provide comprehensive data sets for the carbonyl carotenoids canthaxanthin, astaxanthin, and-for the first time-echinenone, as well as new data for lycopene, beta-carotene, (3R,3'R)-zeaxanthin and (3R,3'R,6'R)-lutein in solvents which have not yet been investigated in the literature. Measurements were carried out to determine, how the IC processes are influenced by the conjugation length of the carotenoids, additional substituents and the polarity of the solvent. TL signals were recorded at 800 nm following excitation into the high energy edge of the carotenoid S2 band at 400 nm. For the S2 lifetime solvent-independent upper limits on the order of 100-200 fs are estimated for all carotenoids studied. The S1 lifetimes are in the picosecond range and increase systematically with decreasing conjugation length. For instance, in the sequence canthaxanthin/echinenone/beta-carotene (13/12/11 double bonds) one finds tau1 approximately 5, 7.7 and 9 ps for the S1-->S0 IC process, respectively. Hydroxyl groups not attached to the conjugated system have no apparent influence on tau1, as observed for canthaxanthin/astaxanthin (tau1 approximately 5 ps in both cases). For all carotenoids studied, tau1 is found to be insensitive to the solvent polarity. This is particularly interesting in the case of echinenone, canthaxanthin and astaxanthin, because earlier measurements for other carbonyl carotenoids like, e.g., peridinin partly showed dramatic differences. The likely presence of an intramolecular charge transfer state in the excited state manifold of C40 carbonyl carotenoids, which is stabilized in polar solvents, has obviously no influence on the measured tau1.

Light-harvesting function of carotenoids in photo-synthesis: The roles of the newly found 11Bu? state

This minireview article highlights the energetics and the dynamics of the 1(1)B(u)(-) and 3(1)A(g)(-) states of carotenoids discovered very recently. Those "hidden" covalent states have been revealed by measurements of resonance-Raman excitation profiles of crystalline carotenoids. The dependence of the energies of the low-lying singlet states, including the 1(1)B(u)(+), 3(1)A(g)(-), 1(1)B(u)(-), and 2(1)A(g)(-) states, on the number of conjugated double bonds (n) is in agreement with the extrapolation of those state energies calculated by Tavan and Schulten for shorter polyenes (P. Tavan and K. Schulten, Journal of Chemical Physics, 1986, vol. 85, pp. 6602-6609). It has also been shown that the internal-conversion processes among those singlet states take place in accord with the state ordering, i.e., 1(1)B(u)(+) --> 1(1)B(u)(-) --> 2(1)A(g)(-) --> 1(1)A(g)(-) (the ground state) for carotenoids having n = 9 and 10, whereas 1(1)B(u)(+) --> 3(1)A(g)(-) --> 1(1)B(u) (-) --> 2(1)A(g)(-) --> 1(1)A(g)(-) for carotenoids having n = 11-13. Radiative transitions of 1(1)B(u)(+) --> 2(1)A(g)(-) and 1(1)B(u)(-) --> 2(1)A(g)(-) as well as a branching into the triplet manifold of 1(1)B(u)(-) --> 1(3)A(g) --> 1(3)B(u) have also been found. Those low-lying singlet states of all-trans carotenoids can facilitate multiple channels of singlet-energy transfer to bacteriochlorophyll in the LH2 antenna complexes of purple photosynthetic bacteria. Thus, the newly found 1(1)B(u)(-) and 3(1)A(g)(-) states of carotenoids need to be incorporated into the picture of carotenoid-to-bacteriochlorophyll singlet-energy transfer.

Vibrational Relaxation in beta-Carotene Probed by Picosecond Stokes and Anti-Stokes Resonance Raman Spectroscopy

Picosecond time-resolved Stokes and anti-Stokes resonance Raman spectra of all-trans-beta-carotene are obtained and analyzed to reveal the dynamics of excited-state (S(1)) population and decay, as well as ground-state vibrational relaxation. Time-resolved Stokes spectra show that the ground state recovers with a 12.6 ps time constant, in agreement with the observed decay of the unique S(1) Stokes bands. The anti-Stokes spectra exhibit no peaks attributable to the S(1) (2A(g) (-)) state, indicating that vibrational relaxation in S(1) must be nearly complete within 2 ps. After photoexcitation there is a large increase in anti-Stokes scattering from ground-state modes that are vibrationally excited through internal conversion. The anti-Stokes data are fit to a kinetic scheme in which the C=C mode relaxes in 0.7 ps, the C-C mode relaxes in 5.4 ps and the C-CH(3) mode relaxes in 12.1 ps. These results are consistent with a model for S(1)-S(0) internal conversion in which the C=C mode is the primary acceptor, the C-C mode is a minor acceptor, and the C-CH(3) mode is excited via intramolecular vibrational energy redistribution.

Transient absorption from the 1Bu+ state of all-trans-beta-carotene newly identified in the near-infrared region

We have attempted subpicosecond time-resolved absorption spectroscopy of all-trans-beta-carotene in organic solvents in the 820-1060 nm region and found novel transient absorption features which lived in subpicosecond time scales. A first component that appeared immediately after excitation showed a lifetime of 190 +/- 10 fs in n-hexane in agreement with the 1Bu+ lifetime that had been determined by fluorescence upconversion spectroscopy (195 +/- 10 fs). (Kandori et al. [1994] J. Am. Chem. Soc. 116, 2671-2672.) Therefore, this component is assigned to a transient absorption from the 1Bu+ state.