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Title: Triplet–triplet energy transfer in artificial and natural photosynthetic antennas

Abstract

In photosynthetic organisms, protection against photo-oxidative 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, while it is ultrafast in the oxygen-rich chloroplasts of oxygen evolving photosynthetic organisms. In order 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 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 frommore » oxygenic (LHCII) and anoxygenic organisms (LH2). In conclusion, both DFT and EPR analysis further indicates that upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.« less

Authors:
 [1];  [2];  [3];  [4];  [4];  [4];  [5];  [5];  [4];  [4];  [4];  [6]; ORCiD logo [2]
  1. Yale Univ., New Haven, CT (United States); Institute of High Performance Computing (Singapore)
  2. Univ. Paris Sud, Gif sur Yvette (France)
  3. Yale Univ., New Haven, CT (United States); Univ. de Puerto Rico en Cayey, Cayey (Puerto Rico)
  4. Arizona State Univ., Tempe, AZ (United States)
  5. Argonne National Lab. (ANL), Argonne, IL (United States)
  6. Yale Univ., New Haven, CT (United States)
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
European Research Council (ERC); Agence Nationale de la recherche (ANR); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22), Chemical Sciences, Geosciences, and Biosciences Division; USDOE
OSTI Identifier:
1366576
Alternate Identifier(s):
OSTI ID: 1374200
Grant/Contract Number:
AC02-06CH11357; FG02-03ER15393; SC0001059
Resource Type:
Journal Article: Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 114; Journal Issue: 28; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 59 BASIC BIOLOGICAL SCIENCES; artificial photosynthesis; DFT; EPR; QM/MM; carotenoid; natural bond orbital analysis; photoprotection; phthalocyanine; resonance Raman; transient absorption spectroscopy; triplet-triplet coupling; triplet-triplet energy transfer

Citation Formats

Ho, Junming, Kish, Elizabeth, Méndez-Hernandez, Dalvin D., WongCarter, Katherine, Pillai, Smitha, Kodis, Gerdenis, Niklas, Jens, Poluektov, Oleg G., Gust, Devens, Moore, Thomas A., Moore, Ana L., Batista, Victor S., and Robert, Bruno. Triplet–triplet energy transfer in artificial and natural photosynthetic antennas. United States: N. p., 2017. Web. doi:10.1073/pnas.1614857114.
Ho, Junming, Kish, Elizabeth, Méndez-Hernandez, Dalvin D., WongCarter, Katherine, Pillai, Smitha, Kodis, Gerdenis, Niklas, Jens, Poluektov, Oleg G., Gust, Devens, Moore, Thomas A., Moore, Ana L., Batista, Victor S., & Robert, Bruno. Triplet–triplet energy transfer in artificial and natural photosynthetic antennas. United States. doi:10.1073/pnas.1614857114.
Ho, Junming, Kish, Elizabeth, Méndez-Hernandez, Dalvin D., WongCarter, Katherine, Pillai, Smitha, Kodis, Gerdenis, Niklas, Jens, Poluektov, Oleg G., Gust, Devens, Moore, Thomas A., Moore, Ana L., Batista, Victor S., and Robert, Bruno. Mon . "Triplet–triplet energy transfer in artificial and natural photosynthetic antennas". United States. doi:10.1073/pnas.1614857114.
@article{osti_1366576,
title = {Triplet–triplet energy transfer in artificial and natural photosynthetic antennas},
author = {Ho, Junming and Kish, Elizabeth and Méndez-Hernandez, Dalvin D. and WongCarter, Katherine and Pillai, Smitha and Kodis, Gerdenis and Niklas, Jens and Poluektov, Oleg G. and Gust, Devens and Moore, Thomas A. and Moore, Ana L. and Batista, Victor S. and Robert, Bruno},
abstractNote = {In photosynthetic organisms, protection against photo-oxidative 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, while it is ultrafast in the oxygen-rich chloroplasts of oxygen evolving photosynthetic organisms. In order 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 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). In conclusion, both DFT and EPR analysis further indicates that upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.},
doi = {10.1073/pnas.1614857114},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 28,
volume = 114,
place = {United States},
year = {Mon Jun 26 00:00:00 EDT 2017},
month = {Mon Jun 26 00:00:00 EDT 2017}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1073/pnas.1614857114

Citation Metrics:
Cited by: 2works
Citation information provided by
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  • In photosynthetic organisms, protection against photo-oxidative 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, while it is ultrafast in the oxygen-rich chloroplasts of oxygen evolving photosynthetic organisms. In order 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 isomericmore » 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 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). In conclusion, both DFT and EPR analysis further indicates that upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.« less
    Cited by 2
  • The efficiency of triplet energy transfer from the special pair (P) to the carotenoid (C) in photosynthetic reaction centers (RCs) from a large family of mutant strains has been investigated. The mutants carry substitutions at positions L181 and/or M208 near chlorophyll-based cofactors on the inactive and active sides of the complex, respectively. Light-modulated electron paramagnetic resonance at 10 K, where triplet energy transfer is thermally prohibited, reveals that the mutations do not perturb the electronic distribution of P. At temperatures > or = 70 K, we observe reduced signals from the carotenoid in most of the RCs with L181 substitutions.more » In particular, triplet transfer efficiency is reduced in all RCs in which a lysine at L181 donates a sixth ligand to the monomeric bacteriochlorophyll B(B). Replacement of the native Tyr at M208 on the active side of the complex with several polar residues increased transfer efficiency. The difference in the efficiencies of transfer in the RCs demonstrates the ability of the protein environment to influence the electronic overlap of the chromophores and thus the thermal barrier for triplet energy transfer.« less
  • In protein-cofactor reaction center (RC) complexes of purple photosynthetic bacteria, the major role of the bound carotenoid (C) is to quench the triplet state formed on the primary electron donor (P) before its sensitization of the excited singlet state of molecular oxygen from its ground triplet state. This triplet energy is transferred from P to C via the bacteriochlorophyll monomer B{sub B}. Using time-resolved electron paramagnetic resonance (TREPR), we have examined the temperature dependence of the rates of this triplet energy transfer reaction in the RC of three wild-type species of purple nonsulfur bacteria. Species-specific differences in the rate ofmore » transfer were observed. Wild-type Rhodobacter capsulatus RCs were less efficient at the triplet transfer reaction than Rhodobacter sphaeroides RCs, but were more efficient than Rhodospirillum rubrum RCs. In addition, RCs from three mutant strains of R. capsulatus carrying substitutions of amino acids near P and B{sub B} were examined. Two of the mutant RCs showed decreased triplet transfer rates compared with wild-type RCs, whereas one of the mutant RCs demonstrated a slight increase in triplet transfer rate at low temperatures. The results show that site-specific changes within the RC of R. capsulatus can mimic interspecies differences in the rates of triplet energy transfer. This application of TREPR was instrumental in defining critical energetic and coupling factors that dictate the efficiency of this photoprotective process.« less
  • In the reaction centers of photosynthetic organisms, chlorophyll triplet states are sometimes formed by recombination of charge-separated intermediates. These triplets are excellent sensitizers for singlet oxygen formation. Carotenoid polyenes can provide photoprotection from singlet oxygen generation by rapidly quenching chlorophyll triplet states via triplet-triplet energy transfer. Because in bacteria the reaction center carotenoid is not located adjacent to the bacteriochlorophyll special pair, which is the origin of the charge separation, it has been postulated that quenching may occur via a triplet relay involving an intermediate chlorophyll monomer. We now report the synthesis and spectroscopic study of a covalently linked carotenoidmore » (C)-porphyrin (P)-pyropheophorbide (Ppd) triad molecule which mimics this triplet relay. The pyropheophorbide singlet-state C-P-[sup 1]Ppd (generated by direct excitation or energy transfer from the attached porphyrin) undergoes intersystem crossing to the triplet C-P-[sup 3]Ppd. In oxygen-free solutions, this triplet decays to [sup 3]C-p-Ppd through a triplet-transfer relay involving an intermediate C-[sup 3]P-Ppd species. In aerated solutions, quenching of C-P-[sup 3]Ppd by the attached carotenoid competes with singlet oxygen sensitization and thus provides a degree of photoprotection. In a similar traid containing a zinc porphyrin moiety, triplet transfer is slow due to the higher energy of the C-[sup 3]P[sub Zn]-Ppd intermediate, and photoprotection via the relay is nonexistent. The triplet relay ceases to function at low temperatures in both the natural and biomimetic cases due to the endergonicity of the first step. 37 refs., 6 figs., 1 tab.« less
  • A series of phthalocyanine-carotenoid dyads in which a phenylamino group links a phthalocyanine to carotenoids having 8-11 backbone double bonds were examined by visible and near-infrared femtosecond pump-probe spectroscopy combined with global fitting analysis. The series of molecules has permitted investigation of the role of carotenoids in the quenching of excited states of cyclic tetrapyrroles. The transient behavior varied dramatically with the length of the carotenoid and the solvent environment. Clear spectroscopic signatures of radical species revealed photoinduced electron transfer as the main quenching mechanism for all dyads dissolved in a polar solvent (THF), and the quenching rate was almostmore » independent of carotenoid length. However, in a nonpolar solvent (toluene), quenching rates displayed a strong dependence on the conjugation length of the carotenoid and the mechanism did not include charge separation. The lack of any rise time components of a carotenoid S 1 signature in all experiments in toluene suggests that an excitonic coupling between the carotenoid S 1 state and phthalocyanine Q state, rather than a conventional energy transfer process, is the major mechanism of quenching. A pronounced inhomogeneity of the system was observed and attributed to the presence of a phenyl-amino linker between phthalocyanine and carotenoids. On the basis of accumulated work on various caroteno-phthalocyanine dyads and triads, we have now identified three mechanisms of tetrapyrrole singlet excited state quenching by carotenoids in artificial systems: (i) Car-Pc electron transfer and recombination; (ii) 1Pc to Car S 1 energy transfer and fast internal conversion to the Car ground state; (iii) excitonic coupling between 1Pc and Car S 1 and ensuing internal conversion to the ground state of the carotenoid. The dominant mechanism depends upon the exact molecular architecture and solvent environment. These synthetic systems are providing a deeper understanding of structural and environmental effects on the interactions between carotenoids and tetrapyrroles and thereby better defining their role in controlling natural photosynthetic systems.« less