1
|
Cupellini L, Qian P, Nguyen-Phan TC, Gardiner AT, Cogdell RJ. Quantum chemical elucidation of a sevenfold symmetric bacterial antenna complex. PHOTOSYNTHESIS RESEARCH 2023; 156:75-87. [PMID: 35672557 PMCID: PMC10070313 DOI: 10.1007/s11120-022-00925-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 04/09/2022] [Accepted: 05/12/2022] [Indexed: 06/15/2023]
Abstract
The light-harvesting complex 2 (LH2) of purple bacteria is one of the most studied photosynthetic antenna complexes. Its symmetric structure and ring-like bacteriochlorophyll arrangement make it an ideal system for theoreticians and spectroscopists. LH2 complexes from most bacterial species are thought to have eightfold or ninefold symmetry, but recently a sevenfold symmetric LH2 structure from the bacterium Mch. purpuratum was solved by Cryo-Electron microscopy. This LH2 also possesses unique near-infrared absorption and circular dichroism (CD) spectral properties. Here we use an atomistic strategy to elucidate the spectral properties of Mch. purpuratum LH2 and understand the differences with the most commonly studied LH2 from Rbl. acidophilus. Our strategy exploits a combination of molecular dynamics simulations, multiscale polarizable quantum mechanics/molecular mechanics calculations, and lineshape simulations. Our calculations reveal that the spectral properties of LH2 complexes are tuned by site energies and exciton couplings, which in turn depend on the structural fluctuations of the bacteriochlorophylls. Our strategy proves effective in reproducing the absorption and CD spectra of the two LH2 complexes, and in uncovering the origin of their differences. This work proves that it is possible to obtain insight into the spectral tuning strategies of purple bacteria by quantitatively simulating the spectral properties of their antenna complexes.
Collapse
Affiliation(s)
- Lorenzo Cupellini
- Department of Chemistry and Industrial Chemistry, University of Pisa, 56124, Pisa, Italy.
| | - Pu Qian
- Materials and Structure Analysis, Thermofisher Scientific, Achtseweg Nordic 5, 5651 GTC, Eindhoven, The Netherlands
| | - Tu C Nguyen-Phan
- Institute of Molecular, Cell and Systems Biology, University of Glasgow, Glasgow, G12 8QQ, UK
| | - Alastair T Gardiner
- Laboratory of Anoxygenic Phototrophs, Centre Algatech, Novohradská 237 - Opatovický mlýn, 379 01, Třeboň, Czech Republic
| | - Richard J Cogdell
- Institute of Molecular, Cell and Systems Biology, University of Glasgow, Glasgow, G12 8QQ, UK
| |
Collapse
|
2
|
Thyrhaug E, Schröter M, Bukartė E, Kühn O, Cogdell R, Hauer J, Zigmantas D. Intraband dynamics and exciton trapping in the LH2 complex of Rhodopseudomonas acidophila. J Chem Phys 2021; 154:045102. [PMID: 33514092 DOI: 10.1063/5.0033802] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Over the last several decades, the light-harvesting protein complexes of purple bacteria have been among the most popular model systems for energy transport in excitonic systems in the weak and intermediate intermolecular coupling regime. Despite this extensive body of scientific work, significant questions regarding the excitonic states and the photo-induced dynamics remain. Here, we address the low-temperature electronic structure and excitation dynamics in the light-harvesting complex 2 of Rhodopseudomonas acidophila by two-dimensional electronic spectroscopy. We find that, although at cryogenic temperature energy relaxation is very rapid, exciton mobility is limited over a significant range of excitation energies. This points to the presence of a sub-200 fs, spatially local energy-relaxation mechanism and suggests that local trapping might contribute substantially more in cryogenic experiments than under physiological conditions where the thermal energy is comparable to or larger than the static disorder.
Collapse
Affiliation(s)
- Erling Thyrhaug
- Dynamical Spectroscopy, Department of Chemistry, Technical University of Munich, Lichtenbergstraße 4, 85748 Garching b. Munich, Germany
| | - Marco Schröter
- Chemical Physics, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden
| | - Eglė Bukartė
- Chemical Physics, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden
| | - Oliver Kühn
- Institute of Physics, University of Rostock, Albert Einstein Straße 23-24, 18059 Rostock, Germany
| | - Richard Cogdell
- Institute of Molecular, Cell and Systems Biology, University of Glasgow, Room 402 Davidson Building, Glasgow G12 8QQ, Scotland
| | - Jürgen Hauer
- Dynamical Spectroscopy, Department of Chemistry, Technical University of Munich, Lichtenbergstraße 4, 85748 Garching b. Munich, Germany
| | - Donatas Zigmantas
- Chemical Physics, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden
| |
Collapse
|
3
|
Cupellini L, Lipparini F, Cao J. Absorption and Circular Dichroism Spectra of Molecular Aggregates With the Full Cumulant Expansion. J Phys Chem B 2020; 124:8610-8617. [PMID: 32901476 PMCID: PMC7901647 DOI: 10.1021/acs.jpcb.0c05180] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
![]()
The exciton Hamiltonian of multichromophoric aggregates can be probed by spectroscopic
techniques such as linear absorption and circular dichroism. To compare calculated
Hamiltonians to experiments, a lineshape theory is needed, which takes into account the
coupling of the excitons with inter- and intramolecular vibrations. This coupling is
normally introduced in a perturbative way through the cumulant expansion formalism and
further approximated by assuming a Markovian exciton dynamics, for example with the
modified Redfield theory. Here, we present the implementation of the full cumulant
expansion (FCE) formalism (J. Chem.
Phys.142, 2015, 09410625747060) to
efficiently compute absorption and circular dichroism spectra of molecular aggregates
beyond the Markov approximation, without restrictions on the form of
exciton–phonon coupling. By employing the LH2 system of purple bacteria as a
challenging test case, we compare the FCE lineshapes with the Markovian lineshapes
obtained with the modified Redfield theory, showing that the latter presents a less
satisfying agreement with experiments. The FCE approach instead accurately describes the
lineshapes, especially in the vibronic sideband of the B800 peak. We envision that the
FCE approach will become a valuable tool for accurately comparing model exciton
Hamiltonians with optical spectroscopy experiments.
Collapse
Affiliation(s)
- Lorenzo Cupellini
- Dipartimento di Chimica e Chimica Industriale, University of Pisa, via G. Moruzzi 13, 56124 Pisa, Italy
| | - Filippo Lipparini
- Dipartimento di Chimica e Chimica Industriale, University of Pisa, via G. Moruzzi 13, 56124 Pisa, Italy
| | - Jianshu Cao
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
| |
Collapse
|
4
|
Gellings E, Cogdell RJ, van Hulst NF. Room-Temperature Excitation-Emission Spectra of Single LH2 Complexes Show Remarkably Little Variation. J Phys Chem Lett 2020; 11:2430-2435. [PMID: 32142282 DOI: 10.1021/acs.jpclett.0c00375] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Excitation spectroscopy gives direct insight into the excited state manifold, energy transfer, transient intermediates, vibrations, and so on. Unfortunately, excitation spectroscopy of single molecules under ambient conditions has remained challenging. Here we present excitation spectra alongside emission spectra of the same individual light-harvesting complex LH2 of the purple bacteria Rps. acidophila. The acquisition of both the excited and ground state spectra allows us to quantify disorder and interband correlations, which are key variables for the interpretation of observed long-lasting coherences. We have overcome the low photostability and small fluorescence quantum yield that are inherent to many biologically relevant systems by combining single-molecule Fourier transform spectroscopy, low excitation intensities, and effective data analysis. We find that LH2 complexes show little spectral variation (130-170 cm-1), that their two absorption bands (B800-B850) are uncorrelated, and that the Stokes shift is not constant. The low amount of spectral disorder underlines the protective role of the protein scaffold, benefiting the efficient energy transport throughout the light-harvesting membrane.
Collapse
Affiliation(s)
- Esther Gellings
- ICFO - Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain
| | - Richard J Cogdell
- Davidson Building, Institute of Molecular, Cell and Systems Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom
| | - Niek F van Hulst
- ICFO - Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain
- ICREA - Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
| |
Collapse
|
5
|
Pieper J, Artene P, Rätsep M, Pajusalu M, Freiberg A. Evaluation of Electron–Phonon Coupling and Spectral Densities of Pigment–Protein Complexes by Line-Narrowed Optical Spectroscopy. J Phys Chem B 2018; 122:9289-9301. [DOI: 10.1021/acs.jpcb.8b05220] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
|
6
|
Stability and properties of quasi-stable conformational states in the LH2 light-harvesting complex of Rbl. acidophilus bacteria formed by hexacoordination of bacteriochlorophyll a magnesium atom. Chem Phys 2017. [DOI: 10.1016/j.chemphys.2017.04.010] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
|
7
|
Kondo T, Chen WJ, Schlau-Cohen GS. Single-Molecule Fluorescence Spectroscopy of Photosynthetic Systems. Chem Rev 2017; 117:860-898. [DOI: 10.1021/acs.chemrev.6b00195] [Citation(s) in RCA: 70] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Affiliation(s)
- Toru Kondo
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge Massachusetts 02139, United States
| | - Wei Jia Chen
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge Massachusetts 02139, United States
| | - Gabriela S. Schlau-Cohen
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge Massachusetts 02139, United States
| |
Collapse
|
8
|
Sisto A, Stross C, van der Kamp MW, O’Connor M, McIntosh-Smith S, Johnson GT, Hohenstein EG, Manby FR, Glowacki DR, Martinez TJ. Atomistic non-adiabatic dynamics of the LH2 complex with a GPU-accelerated ab initio exciton model. Phys Chem Chem Phys 2017; 19:14924-14936. [DOI: 10.1039/c7cp00492c] [Citation(s) in RCA: 50] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
We present GPU-accelerated ab initio molecular dynamics simulations of nonadiabatic dynamics in the LH2 complex in full atomistic detail.
Collapse
Affiliation(s)
- Aaron Sisto
- PULSE Institute and Department of Chemistry
- Stanford University
- Stanford
- USA
- SLAC National Accelerator Laboratory
| | - Clem Stross
- School of Chemistry
- University of Bristol
- Bristol
- UK
| | | | - Michael O’Connor
- School of Chemistry
- University of Bristol
- Bristol
- UK
- Department of Computer Science
| | | | - Graham T. Johnson
- California Institute for Quantitative Biosciences (QB3)
- University of California
- San Francisco
- USA
- Department of Bioengineering and Therapeutic Sciences
| | | | | | - David R. Glowacki
- School of Chemistry
- University of Bristol
- Bristol
- UK
- Department of Computer Science
| | - Todd J. Martinez
- PULSE Institute and Department of Chemistry
- Stanford University
- Stanford
- USA
- SLAC National Accelerator Laboratory
| |
Collapse
|
9
|
Adolphs J, Berrer M, Renger T. Hole-Burning Spectroscopy on Excitonically Coupled Pigments in Proteins: Theory Meets Experiment. J Am Chem Soc 2016; 138:2993-3001. [PMID: 26811003 PMCID: PMC4786881 DOI: 10.1021/jacs.5b08246] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
![]()
A theory for the calculation of resonant
and nonresonant hole-burning
(HB) spectra of pigment–protein complexes is presented and
applied to the water-soluble chlorophyll-binding protein (WSCP) from
cauliflower. The theory is based on a non-Markovian line shape theory
(Renger and MarcusJ. Chem. Phys.2002, 116, 9997) and includes exciton delocalization, vibrational
sidebands, and lifetime broadening. An earlier approach by Reppert
(J. Phys. Chem. Lett.2011, 2, 2716) is found to describe nonresonant HB spectra only. Here we present
a theory that can be used for a quantitative description of HB data
for both nonresonant and resonant burning conditions. We find that
it is important to take into account the excess energy of the excitation
in the HB process. Whereas excitation of the zero-phonon transition
of the lowest exciton state, that is, resonant burning allows the
protein to access only its conformational substates in the neighborhood
of the preburn state, any higher excitation gives the protein full
access to all conformations present in the original inhomogeneous
ensemble. Application of the theory to recombinant WSCP from cauliflower,
reconstituted with chlorophyll a or chlorophyll b, gives excellent agreement with experimental data by Pieper
et al. (J. Phys. Chem. B2011, 115, 405321417356) and allows us to obtain an upper bound of the lifetime of the upper
exciton state directly from the HB experiments in agreement with lifetimes
measured recently in time domain 2D experiments by Alster et al. (J. Phys. Chem. B2014, 118, 352424627983).
Collapse
Affiliation(s)
- Julian Adolphs
- Institut für Theoretische Physik, Johannes Kepler Universität Linz , Altenberger Str. 69, 4040 Linz, Austria
| | - Manuel Berrer
- Institut für Theoretische Physik, Johannes Kepler Universität Linz , Altenberger Str. 69, 4040 Linz, Austria
| | - Thomas Renger
- Institut für Theoretische Physik, Johannes Kepler Universität Linz , Altenberger Str. 69, 4040 Linz, Austria
| |
Collapse
|
10
|
Piatkowski L, Gellings E, van Hulst NF. Broadband single-molecule excitation spectroscopy. Nat Commun 2016; 7:10411. [PMID: 26794035 PMCID: PMC4735816 DOI: 10.1038/ncomms10411] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2015] [Accepted: 12/09/2015] [Indexed: 01/31/2023] Open
Abstract
Over the past 25 years, single-molecule spectroscopy has developed into a widely used tool in multiple disciplines of science. The diversity of routinely recorded emission spectra does underpin the strength of the single-molecule approach in resolving the heterogeneity and dynamics, otherwise hidden in the ensemble. In early cryogenic studies single molecules were identified by their distinct excitation spectra, yet measuring excitation spectra at room temperature remains challenging. Here we present a broadband Fourier approach that allows rapid recording of excitation spectra of individual molecules under ambient conditions and that is robust against blinking and bleaching. Applying the method we show that the excitation spectra of individual molecules exhibit an extreme distribution of solvatochromic shifts and distinct spectral shapes. Importantly, we demonstrate that the sensitivity and speed of the broadband technique is comparable to that of emission spectroscopy putting both techniques side-by-side in single-molecule spectroscopy.
Collapse
Affiliation(s)
- Lukasz Piatkowski
- ICFO—Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
| | - Esther Gellings
- ICFO—Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
| | - Niek F. van Hulst
- ICFO—Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
- ICREA—Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
| |
Collapse
|
11
|
Camacho R, Tubasum S, Southall J, Cogdell RJ, Sforazzini G, Anderson HL, Pullerits T, Scheblykin IG. Fluorescence polarization measures energy funneling in single light-harvesting antennas--LH2 vs conjugated polymers. Sci Rep 2015; 5:15080. [PMID: 26478272 PMCID: PMC4609963 DOI: 10.1038/srep15080] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2015] [Accepted: 09/09/2015] [Indexed: 01/15/2023] Open
Abstract
Numerous approaches have been proposed to mimic natural photosynthesis using artificial antenna systems, such as conjugated polymers (CPs), dendrimers, and J-aggregates. As a result, there is a need to characterize and compare the excitation energy transfer (EET) properties of various natural and artificial antennas. Here we experimentally show that EET in single antennas can be characterized by 2D polarization imaging using the single funnel approximation. This methodology addresses the ability of an individual antenna to transfer its absorbed energy towards a single pool of emissive states, using a single parameter called energy funneling efficiency (ε). We studied individual peripheral antennas of purple bacteria (LH2) and single CP chains of 20 nm length. As expected from a perfect antenna, LH2s showed funneling efficiencies close to unity. In contrast, CPs showed lower average funneling efficiencies, greatly varying from molecule to molecule. Cyclodextrin insulation of the conjugated backbone improves EET, increasing the fraction of CPs possessing ε = 1. Comparison between LH2s and CPs shows the importance of the protection systems and the protein scaffold of LH2, which keep the chromophores in functional form and at such geometrical arrangement that ensures excellent EET.
Collapse
Affiliation(s)
- Rafael Camacho
- Chemical Physics, Lund University, PO Box 124, Lund, SE-22100, Sweden
| | - Sumera Tubasum
- Chemical Physics, Lund University, PO Box 124, Lund, SE-22100, Sweden
| | - June Southall
- Glasgow Biomedical Research Centre, University of Glasgow, G12 8QQ, United Kingdom
| | - Richard J Cogdell
- Glasgow Biomedical Research Centre, University of Glasgow, G12 8QQ, United Kingdom
| | - Giuseppe Sforazzini
- Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, United Kingdom
| | - Harry L Anderson
- Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, United Kingdom
| | - Tõnu Pullerits
- Chemical Physics, Lund University, PO Box 124, Lund, SE-22100, Sweden
| | - Ivan G Scheblykin
- Chemical Physics, Lund University, PO Box 124, Lund, SE-22100, Sweden
| |
Collapse
|
12
|
Rätsep M, Pajusalu M, Linnanto JM, Freiberg A. Subtle spectral effects accompanying the assembly of bacteriochlorophylls into cyclic light harvesting complexes revealed by high-resolution fluorescence spectroscopy. J Chem Phys 2015; 141:155102. [PMID: 25338912 DOI: 10.1063/1.4897637] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We have observed that an assembly of the bacteriochloropyll a molecules into B850 and B875 groups of cyclic bacterial light-harvesting complexes LH2 and LH1, respectively, results an almost total loss of the intra-molecular vibronic structure in the fluorescence spectrum, and simultaneously, an essential enhancement of its phonon sideband due to electron-phonon coupling. While the suppression of the vibronic coupling in delocalized (excitonic) molecular systems is predictable, as also confirmed by our model calculations, a boost of the electron-phonon coupling is rather unexpected. The latter phenomenon is explained by exciton self-trapping, promoted by mixing the molecular exciton states with charge transfer states between the adjacent chromophores in the tightly packed B850 and B875 arrangements. Similar, although less dramatic trends were noted for the light-harvesting complexes containing chlorophyll pigments.
Collapse
Affiliation(s)
- Margus Rätsep
- Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia
| | - Mihkel Pajusalu
- Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia
| | | | - Arvi Freiberg
- Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia and Institute of Molecular and Cell Biology, University of Tartu, Riia 23, 51010 Tartu, Estonia
| |
Collapse
|
13
|
van der Vegte CP, Prajapati JD, Kleinekathöfer U, Knoester J, Jansen TLC. Atomistic Modeling of Two-Dimensional Electronic Spectra and Excited-State Dynamics for a Light Harvesting 2 Complex. J Phys Chem B 2015; 119:1302-13. [DOI: 10.1021/jp509247p] [Citation(s) in RCA: 54] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- C. P. van der Vegte
- Zernike
Institute for Advanced Materials, University of Groningen, Nijenborgh
4, 9747 AG Groningen, The Netherlands
| | - J. D. Prajapati
- School
of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
| | - U. Kleinekathöfer
- School
of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
| | - J. Knoester
- Zernike
Institute for Advanced Materials, University of Groningen, Nijenborgh
4, 9747 AG Groningen, The Netherlands
| | - T. L. C. Jansen
- Zernike
Institute for Advanced Materials, University of Groningen, Nijenborgh
4, 9747 AG Groningen, The Netherlands
| |
Collapse
|
14
|
Pajusalu M, Kunz R, Rätsep M, Timpmann K, Köhler J, Freiberg A. Unified analysis of ensemble and single-complex optical spectral data from light-harvesting complex-2 chromoproteins for gaining deeper insight into bacterial photosynthesis. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 92:052709. [PMID: 26651725 DOI: 10.1103/physreve.92.052709] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/29/2015] [Indexed: 05/15/2023]
Abstract
Bacterial light-harvesting pigment-protein complexes are very efficient at converting photons into excitons and transferring them to reaction centers, where the energy is stored in a chemical form. Optical properties of the complexes are known to change significantly in time and also vary from one complex to another; therefore, a detailed understanding of the variations on the level of single complexes and how they accumulate into effects that can be seen on the macroscopic scale is required. While experimental and theoretical methods exist to study the spectral properties of light-harvesting complexes on both individual complex and bulk ensemble levels, they have been developed largely independently of each other. To fill this gap, we simultaneously analyze experimental low-temperature single-complex and bulk ensemble optical spectra of the light-harvesting complex-2 (LH2) chromoproteins from the photosynthetic bacterium Rhodopseudomonas acidophila in order to find a unique theoretical model consistent with both experimental situations. The model, which satisfies most of the observations, combines strong exciton-phonon coupling with significant disorder, characteristic of the proteins. We establish a detailed disorder model that, in addition to containing a C_{2}-symmetrical modulation of the site energies, distinguishes between static intercomplex and slow conformational intracomplex disorders. The model evaluations also verify that, despite best efforts, the single-LH2-complex measurements performed so far may be biased toward complexes with higher Huang-Rhys factors.
Collapse
Affiliation(s)
- Mihkel Pajusalu
- Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia
| | - Ralf Kunz
- Experimental Physics IV and Bayreuth Institute for Macromolecular Research, University of Bayreuth, 95440 Bayreuth, Germany
| | - Margus Rätsep
- Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia
| | - Kõu Timpmann
- Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia
| | - Jürgen Köhler
- Experimental Physics IV and Bayreuth Institute for Macromolecular Research, University of Bayreuth, 95440 Bayreuth, Germany
| | - Arvi Freiberg
- Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia
- Institute of Molecular and Cell Biology, University of Tartu, Riia 23, 51010 Tartu, Estonia
| |
Collapse
|