Showing papers by "Rienk van Grondelle published in 2010"

Physical origins and models of energy transfer in photosynthetic light-harvesting

Abstract: We perform a quantitative comparison of different energy transfer theories, i.e. modified Redfield, standard and generalized Forster theories, as well as combined Redfield-Forster approach. Physical limitations of these approaches are illustrated and critical values of the key parameters indicating their validity are found. We model at a quantitative level the spectra and dynamics in two photosynthetic antenna complexes: in phycoerythrin 545 from cryptophyte algae and in trimeric LHCII complex from higher plants. These two examples show how the structural organization determines a directed energy transfer and how equilibration within antenna subunits and migration between subunits are superimposed.

Revisiting the optical properties of the FMO protein

Abstract: We review the optical properties of the FMO complex as found by spectroscopic studies of the Qy band over the last two decades. This article emphasizes the different methods used, both experimental and theoretical, to elucidate the excitonic structure and dynamics of this pigment–protein complex.

Two different charge separation pathways in photosystem II.

Abstract: Charge separation is an essential step in the conversion of solar energy into chemical energy in photosynthesis. To investigate this process, we performed transient absorption experiments at 77 K with various excitation conditions on the isolated Photosystem II reaction center preparations from spinach. The results have been analyzed by global and target analysis and demonstrate that at least two different excited states, (Chl D1PheD1)* and (PD1PD2ChlD1)*, give rise to two different pathways for ultrafast charge sepa- ration. We propose that the disorder produced by slow protein motions causes energetic differentiation among reaction center complexes, leading to different charge separation pathways. Because of the low tem- perature, two excitation energy trap states are also present, generating charge-separated states on long time scales. We conclude that these slow trap states are the same as the excited states that lead to ultrafast charge separation, indicating that at 77 K charge separation can be either activation-less and fast or activated and slow.

Identification of excited-state energy transfer and relaxation pathways in the peridinin–chlorophyll complex: an ultrafast mid-infrared study

Abstract: The peridinin chlorophyll-a protein (PCP) is a water–soluble, trimeric light harvesting complex found in marine dinoflagellates that binds peridinin and Chl-a in an unusual stoichiometric ratio of 4 : 1. In this paper, the pathways of excited-state energy transfer and relaxation in PCP were identified by means of femtosecond visible-pump, mid-infrared probe spectroscopy. In addition, excited-state relaxation of peridinin dissolved in organic solvent (CHCl3 and MeOH) was investigated. For peridinin in solution, the transient IR signatures of the low-lying S1 and intramolecular charge transfer (ICT) states were similar, in line with a previous ultrafast IR study. In PCP, excitation of the optically allowed S2 state of peridinin results in ultrafast energy transfer to Chl-a, in competition with internal conversion to low-lying optically forbidden states of peridinin. After vibrational relaxation of the peridinin hot S1 state in 150 fs, two separate low-lying peridinin singlet excited states are distinguished, assigned to an ICT state and to a slowly transferring, vibrationally relaxed S1 state. These states exhibit different lactone bleaches, indicating that the ICT and S1 states localize on distinct peridinins. Energy transfer from the peridinin ICT state to Chl-a constitutes the dominant energy transfer channel and occurs with a time constant of 2 ps. The peridinin S1 state mainly decays to the ground state through internal conversion, in competition with slow energy transfer to Chl-a. The singlet excited state of Chl-a undergoes intersystem crossing (ISC) to the triplet state on the nanosecond timescale, followed by rapid triplet excitation energy transfer (TEET) from Chl-a to peridinin, whereby no Chl-a triplet is observed but rather a direct rise of the peridinin triplet. The latter contains some Chl-a features due to excitonic coupling of the pigments. The peridinin triplet state shows a lactone bleach mode at 1748 cm−1, while that of the peridinin ICT state is located at 1745 cm−1, indicating that the main channels of singlet and triplet energy transfer in PCP proceed through distinct peridinins. Our results are consistent with an energy transfer scheme where the ICT state mainly localizes on Per621/611 and Per623/613, the S1 state on Per622/612 and the triplet state on Per624/614.

Photosynthesis: Quantum design for a light trap.

Abstract: The photosynthetic apparatus of cryptophyte algae is odd — its pigments are farther apart than is expected for efficient functioning. A study into how this apparatus works so well finds quantum effects at play.

Protochlorophyllide excited-state dynamics in organic solvents studied by time-resolved visible and mid-infrared spectroscopy.

Abstract: Protochlorophyllide (PChlide) is a precursor in the biosynthesis of chlorophyll. Complexed with NADPH to the enzyme protochlorophyllide oxidoreductase (POR), it is reduced to chlorophyllide, a process that occurs via a set of spectroscopically distinct intermediate states and is initiated from the excited state of PChlide. To obtain a better understanding of these catalytic events, we characterized the excited state dynamics of PChlide in the solvents tetrahydrofuran (THF), methanol, and Tris/Triton buffer using ultrafast transient absorption in the visible and mid-infrared spectral regions and time-resolved fluorescence emission experiments. For comparison, we present time-resolved transient absorption measurements of chlorophyll a in THF. From the combined analysis of these experiments, we derive that during the 2-3 ns excited state lifetime an extensive multiphasic quenching of the emission occurs due to solvation of the excited state, which is in agreement with the previously proposed internal charge transfer (ICT) character of the S1 state ( Zhao , G. J. ; Han , K. L. Biophys. J. 2008 , 94 , 38 ). The solvation process in methanol occurs in conjunction with a strengthening of a hydrogen bond to the Pchlide keto carbonyl group. We demonstrate that the internal conversion from the S2 to S1 excited states is remarkably slow and stretches out on to the 700 fs time scale, causing a rise of blue-shifted signals in the transient absorption and a gain of emission in the time-resolved fluorescence. A triplet state is populated on the nanosecond time scale with a maximal yield of approximately 23%. The consequences of these observations for the catalytic pathway and the role of the triplet and ICT state in activation of the enzyme are discussed.

Electronic and protein structural dynamics of a photosensory LOV histidine kinase

Abstract: The bacterium Caulobacter crescentus encodes a two-component signaling protein, LovK, that contains an N-terminal photosensory LOV domain coupled to a C-terminal histidine kinase. LovK binds a flavin cofactor, undergoes a reversible photocycle, and displays regulated ATPase and autophosphorylation activity in response to visible light. Femtosecond to nanosecond visible absorption spectroscopy demonstrates congruence between full-length LovK and isolated LOV domains in the mechanism and kinetics of light-dependent cysteinyl-C4(a) adduct formation and rupture, while steady-state absorption and fluorescence line narrowing (FLN) spectroscopies reveal unique features in the electronic structure of the LovK flavin cofactor. In agreement with other sensor histidine kinases, ATP binds specifically to LovK with micromolar affinity. However, ATP binding to the histidine kinase domain of LovK has no apparent effect on global protein structure as assessed by differential Fourier transform infrared (FTIR) spectroscopy...

An investigation of slow charge separation in a Tyrosine M210 to Tryptophan mutant of the Rhodobacter sphaeroides reaction center by femtosecond mid-infrared spectroscopy

Abstract: Energy and electron transfer in a tyrosine M210 to tryptophan (YM210W) mutant of the Rhodobacter sphaeroides reaction center (RC) were investigated through time-resolved visible pump/mid-infrared (mid-IR) probe spectroscopy at room temperature, with the aim to further characterize the primary charge separated states in the RC. This mutant is known to display slow and multi-exponential charge separation, and was used in earlier work to prove the existence of an alternative route for charge separation starting from the accessory bacteriochlorophyll in the active branch, BL. The mutant RCs were excited at 860 nm (direct excitation of the primary donor (P) BChls (PL/PM)), 600 nm (unselective excitation), 805 nm (direct excitation of both accessory bacteriochlorophyll cofactors BL and BM) and 795 nm (direct excitation of BL). Absorption changes associated with carbonyl (CO) stretch vibrational modes of the cofactors and protein were recorded in the region between 1600 and 1775 cm−1, and both a sequential analysis and simultaneous target analysis of the data were performed. The decay of P* in the YM210W mutant was multi-exponential with lifetimes of 29 and 63.5 ps. The decay of P+BL− state was ∼10 times longer in the YM210W RC than in the R-26 RC (∼7 ps vs. ∼0.7 ps), and in the mid-IR difference absorption spectrum of P+BL− the stretching frequency of the 9-keto CO group of BL in the ground state was located around 1675–1680 cm−1, consistent with the presence of a hydrogen bond donated by an adjacent water molecule. Excitation at 795 nm produced a small amount of BL*-driven charge separation, as assessed from the excitation wavelength dependence of the raw difference spectra recorded during the first few ps after excitation. This process led to the formation of P+BL−. Only the relaxed form of the P+HL− radical pair was observed in the YM210W mutant, and the mid-IR difference absorption spectra of P+HL− and P+BL− showed a change in the relative amplitude of the PL+ and PM+ bands when compared to equivalent spectra for the R-26 RC. This indicates that the YM210W mutation causes an increased localization of the electron hole on the PM half of the dimer. The absorbance difference spectrum of P+HL− in the R-26 RC contains a feature attributable to a Stark shift of one or more amide CO oscillators. This feature was shifted to lower frequency by ∼5 cm−1 in the YM210W RC, and consideration of the limited structural changes in this RC indicates that this feature arises from an amide CO group in the immediate vicinity of the M210 residue, most probably that of the adjacent M209 amino acid.