Resolving the excited state equilibrium of peridinin in solution

Optical spectroscopic studies of light-harvesting by pigment-reconstituted peridinin-chlorophyll-proteins at cryogenic temperatures

Low temperature, steady-state, optical spectroscopic methods were used to study the spectral features of peridinin-chlorophyll-protein (PCP) complexes in which recombinant apoprotein has been refolded in the presence of peridinin and either chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll d (Chl d), 3-acetyl-chlorophyll a (3-acetyl-Chl a) or bacteriochlorophyll a (BChl a). Absorption spectra taken at 10 K provide better resolution of the spectroscopic bands than seen at room temperature and reveal specific pigment-protein interactions responsible for the positions of the Qy bands of the chlorophylls. The study reveals that the functional groups attached to Ring I of the two protein-bound chlorophylls modulate the Qy and Soret transition energies. Fluorescence excitation spectra were used to compute energy transfer efficiencies of the various complexes at room temperature and these were correlated with previously reported ultrafast, time-resolved optical spectroscopic dynamics data. The results illustrate the robust nature and value of the PCP complex, which maintains a high efficiency of antenna function even in the presence of non-native chlorophyll species, as an effective tool for elucidating the molecular details of photosynthetic light-harvesting.

Use of ultrafast dispersed pump-dump-probe and pump-repump-probe spectroscopies to explore the light-induced dynamics of peridinin in solution

Optical pump-induced dynamics of the highly asymmetric carotenoid peridinin in methanol was studied by dispersed pump-probe, pump-dump-probe, and pump-repump-probe transient absorption spectroscopy in the visible region. Dispersed pump-probe measurements show that the decay of the initially excited S2 state populates two excited states, the S1 and the intramolecular charge-transfer (ICT) state, at a ratio determined by the excitation wavelength. The ensuing spectral evolution occurs on the time scale of a few picoseconds and suggests the equilibration of these states. Dumping the stimulated emission of the ICT state with an additional 800-nm pulse after 400- and 530-nm excitation preferentially removes the ICT state contribution from the broad excited-state absorption, allowing for its spectral characterization. At the same time, an unrelaxed ground-state species, which has a subpicosecond lifetime, is populated. The application of the 800-nm pulse at early times, when the S2 state is still populated, led to direct generation of the peridinin cation, observed for the first time in a transient absorption experiment. The excited and ground electronic states manifold of peridinin has been reconstructed using target analysis; this approach combined with the measured multipulse spectroscopic data allows us to estimate the spectra and time scales of the corresponding transient states.

A simple artificial light-harvesting dyad as a model for excess energy dissipation in oxygenic photosynthesis

Under excess illumination, plant photosystem II dissipates excess energy through the quenching of chlorophyll fluorescence, a process known as nonphotochemical quenching. Activation of nonphotochemical quenching has been linked to the conversion of a carotenoid with a conjugation length of nine double bonds (violaxanthin) into an 11-double-bond carotenoid (zeaxanthin). It has been suggested that the increase in the conjugation length turns the carotenoid from a nonquencher into a quencher of chlorophyll singlet excited states, but unequivocal evidence is lacking. Here, we present a transient absorption spectroscopic study on a model system made up of a zinc phthalocyanine (Pc) molecule covalently linked to carotenoids with 9, 10, or 11 conjugated carbon-carbon double bonds. We show that a carotenoid can act as an acceptor of Pc excitation energy, thereby shortening its singlet excited-state lifetime. The conjugation length of the carotenoid is critical to the quenching process. Remarkably, the addition of only one double bond can turn the carotenoid from a nonquencher into a very strong quencher. By studying the solvent polarity dependence of the quenching using target analysis of the time-resolved data, we show that the quenching proceeds through energy transfer from the excited Pc to the optically forbidden S(1) state of the carotenoid, coupled to an intramolecular charge-transfer state. The mechanism for excess energy dissipation in photosystem II is discussed in view of the insights obtained on this simple model system.

Excited-State Dynamics of Carotenoids in Light-Harvesting Complexes. 2. Dissecting Pulse Structures from Optimal Control Experiments

Dispersed multipump-probe (PPP) spectroscopy was used to explore the role of saturation, annihilation, and structured pulses in recent coherent control experiments on the light-harvesting 2 complex from Rhodopseudomonas acidophila (Herek et al. Nature 2002, 417, 533). We discuss the complimentary aspects between the PPP technique and coherent control studies, in particular the ability to dissect complicated pulse structures and the utility in exploring incoherent mechanisms. With the aid of a simple multistate model involving only population dynamics, we illustrate how the optimized structured pulses may be explained in terms of an interplay between excited-state populations, saturation, and annihilation. Furthermore, we discuss the experimental conditions that are required for incoherent effects to contribute to control experimental signals, with particular emphasis on pulse intensities, and show that the optimization of a ratio of conservative signals (i.e., not modulated by external dynamics) is required to exclude saturation effects from coherent control studies.

Excited-State Dynamics of Carotenoids in Light-Harvesting Complexes. 1. Exploring the Relationship between the S1 and S* States

Dispersed transient absorption spectra collected at variable excitation intensities in combination with time-resolved signals were used to explore the underlying connectivity of the electronic excited-state manifold of the carotenoid rhodopin glucoside in the light-harvesting 2 complex isolated from Rhodopseudomonas acidophila. We find that the S state, which was recently identified as an excited state in carotenoids bound in bacterial light-harvesting complexes, exhibits a different response to the increase of excitation intensity than the S(1) state, which suggests that the models used so far to describe the excited states of carotenoids are incomplete. We propose two new models that can describe both the time-resolved and the intensity-dependent data; the first postulates that S(1) and S* are not populated in parallel after the decay of the initially excited S(2) state but instead result from the excitation of distinct ground-state subpopulations. The second model introduces a resonantly enhanced light-induced transition during excitation, which promotes population to higher-lying excited states that favors the formation of S* over S(1). Multiwavelength target analysis of the time-resolved and excitation-intensity dependence measurements were used to characterize the involved states and their responses. We show that both proposed models adequately fit the measured data, although it is not possible to determine which model is most apt. The physical origins and implications of both models are explored.

Spectroscopic characterization of the excitation energy transfer in the fucoxanthin-chlorophyll protein of diatoms

We characterized the energy transfer pathways in the fucoxanthin-chlorophyll protein (FCP) complex of the diatom Cyclotella meneghiniana by conducting ultrafast transient absorption measurements. This light harvesting antenna has a distinct pigment composition and binds chlorophyll a (Chl-a), fucoxanthin and chlorophyll c (Chl-c) molecules in a 4:4:1 ratio. We find that upon excitation of fucoxanthin to its S2 state, a significant amount of excitation energy is transferred rapidly to Chl-a. The ensuing dynamics illustrate the presence of a complex energy transfer network that also involves energy transfer from the unrelaxed or 'hot' intermediates. Chl-c to Chl-a energy transfer occurs on a timescale of a 100 fs. We observe no significant spectral evolution in the Chl-a region of the spectrum. We have applied global and target analysis to model the measured excited state dynamics and estimate the spectra of the states involved; the energy transfer network is discussed in relation to the pigment organization of the FCP complex.

Tuning energy transfer in the peridinin-chlorophyll complex by reconstitution with different chlorophylls

In vitro studies of the carotenoid peridinin, which is the primary pigment from the peridinin chlorophyll-a protein (PCP) light harvesting complex, showed a strong dependence on the lifetime of the peridinin lowest singlet excited state on solvent polarity. This dependence was attributed to the presence of an intramolecular charge transfer (ICT) state in the peridinin excited state manifold. The ICT state was also suggested to be a crucial factor in efficient peridinin to Chl-a energy transfer in the PCP complex. Here we extend our studies of peridinin dynamics to reconstituted PCP complexes, in which Chl-a was replaced by different chlorophyll species (Chl-b, acetyl Chl-a, Chl-d and BChl-a). Reconstitution of PCP with different Chl species maintains the energy transfer pathways within the complex, but the efficiency depends on the chlorophyll species. In the native PCP complex, the peridinin S1/ICT state has a lifetime of 2.7 ps, whereas in reconstituted PCP complexes it is 5.9 ps (Chl-b) 2.9 ps (Chl-a), 2.2 ps (acetyl Chl-a), 1.9 ps (Chl-d), and 0.45 ps (BChl-a). Calculation of energy transfer rates using the Förster equation explains the differences in energy transfer efficiency in terms of changing spectral overlap between the peridinin emission and the absorption spectrum of the acceptor. It is proposed that the lowest excited state of peridinin is a strongly coupled S1/ICT state, which is the energy donor for the major energy transfer channel. The significant ICT character of the S1/ICT state in PCP enhances the transition dipole moment of the S1/ICT state, facilitating energy transfer to chlorophyll via the Förster mechanism. In addition to energy transfer via the S1/ICT, there is also energy transfer via the S2 and hot S1/ICT states to chlorophyll in all reconstituted PCP complexes.

The Charge-Transfer Character of the S0 → S2 Transition in the Carotenoid Peridinin Is Revealed by Stark Spectroscopy

Peridinin, the carotenoid in the peridinin chlorophyll a protein (PCP), was studied by Stark (electroabsorption) spectroscopy to determine the change in electrostatic properties produced on excitation within the absorption band, in methyl tetrahydrofuran (MeTHF) versus ethylene glycol (EG), at 77 K. Strikingly, a large change in the permanent dipole moment (|Deltamu|) was found between the ground state, S(0) (1(1)A(g)(-)), and the Franck-Condon region of the S(2) (1(1)B(u)(+)) excited state, in both MeTHF (22 D) and EG (approximately 27 D), thus revealing the previously unknown charge transfer (CT) character of this pi-pi transition in peridinin. Such a large |Deltamu| produced on excitation, we suggest, facilitates the bending of the lactone moiety, toward which charge transfer occurs, and the subsequent formation of the previously identified intramolecular CT (ICT) state at lower energy. This unexpectedly large S(2) dipole moment, which has not been predicted even from high-level electronic structure calculations, is supported by calculating the shift of the peridinin absorption band as a function of solvent polarity, using the experimentally derived result. Overall, the photoinduced charge transfer uncovered here is expected to affect the excited-state reactivity of peridinin and, within the protein, be important for efficient energy transfer from the carotenoid S(2) and S(1)/ICT states to the chlorophylls in PCP.

Femtosecond Time-Resolved Absorption Spectroscopy of Astaxanthin in Solution and in α-Crustacyanin

Steady-state absorption and femtosecond time-resolved spectroscopic studies have been carried out on astaxanthin dissolved in CS2, methanol, and acetonitrile, and in purified alpha-crustacyanin. The spectra of the S0 --> S2 and S1 --> S(n) transitions were found to be similarly dependent on solvent environment. The dynamics of the excited-state decay processes were analyzed with both single wavelength and global fitting procedures. In solution, the S1 lifetime of astaxanthin was found to be approximately 5 ps and independent of solvent. In alpha-crustacyanin, the lifetime was noticeably shorter at approximately 1.8 ps. Both fitting procedures led to the conclusion that the lifetime of the S2 state was either comparable to or shorter than the instrument response time. The data support the idea that dimerization of astaxanthin in alpha-crustacyanin is the primary molecular basis for the bathochromic shift of the S0 --> S2 and S1 --> S(n) transitions. Planarization of the astaxanthin molecule, which leads to a longer effective pi-electron conjugated chain and a lower S1 energy, accounts for the shorter tau1 in the protein.