High pressure studies of energy transfer and strongly coupled bacteriochlorophyll dimers in photosynthetic protein complexes |
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Authors: | N R S Reddy H-M Wu R Jankowiak R Picorel R J Cogdell G J Small |
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Institution: | (1) Ames Laboratory-USDOE, Iowa State University, 50011 Ames, IA, USA;(2) Department of Chemistry, Iowa State University, 50011 Ames, IA, USA;(3) E.E. Aula Dei, CSIC, Apdo. 202, 50080 Zaragoza, Spain;(4) Department of Botany, The University of Glasgow, G12 8QQ Glasgow, UK |
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Abstract: | High pressure is used with hole burning and absorption spectroscopies at low temperatures to study the pressure dependence of the B800 B850 energy transfer rate in the LH2 complex of Rhodobacter sphaeroides and to assess the extent to which pressure can be used to identify and characterize states associated with strongly coupled chlorophyll molecules. Pressure tuning of the B800–B850 gap from 750 cm\s-1 at 0.1 MPa to 900 cm-1 at 680 MPa has no measurable effect on the 2 ps energy transfer rate of the B800–850 complex at 4.2 K. An explanation for this resilience against pressure, which is supported by earlier hole burning studies, is provided. It is based on weak coupling nonadiabatic transfer theory and takes into account the inhomogeneous width of the B800–B850 energy gap, the large homogeneous width of the B850 band from exciton level structure and the Franck-Condon factors of acceptor protein phonons and intramolecular BChl a modes. The model yields reasonable agreement with the 4.2 K energy transfer rate and is consistent with its weak temperature dependence. It is assumed that it is the C9-ring exciton levels which lie within the B850 band that are the key acceptor levels, meaning that BChl a modes are essential to the energy transfer process. These ring exciton levels derive from the strongly allowed lowest energy component of the basic B850 dimer. However, the analysis of B850s linear pressure shift suggests that another Förster pathway may also be important. It is one that involves the ring exciton levels derived from the weakly allowed upper component of the B850 dimer which we estimate to be quasi-degenerate with B800. In the second part of the paper, which is concerned with strong BChl monomer-monomer interactions of dimers, we report that the pressure shifts of B875 (LH2), the primary donor absorption bands of bacterial RC (P870 of Rb. sphaeroides and P960 of Rhodopseudomonas viridis) and B1015 (LH complex of Rps. viridis) are equal and large in value ( -0.4 cm01/MPa at 4.2 K) relative to those of isolated monomers in polymers and proteins (< -0.1 cm01/MPa). The shift rate for B850 at 4.2 K is-0.28 cm–1/MPa. A model is presented which appears to be capable of providing a unified explanation for the pressure shifts.Abbreviations B800
BChl antenna band absorbing (at room temperature) at 800 nm (B850, B875, B1015 are defined similarly)
- CD
circular dichroism
- FC factor
Franck-Condon factor
- FMO comple
Fenna-Matthews-Olson complex
- L-S theory
Laird-Skinner theory
- LH1
core light-harvesting complex of the BChl antenna complexes
- LH2
peripheral light-harvesting complex of the BChl antenna complexes
- NPHB
non-photochemical hole burning
- P960
absorption band of special pair of Rhodopseudomonas viridis absorbing at 960 nm (room temperature). P870 of Rhodobacter sphaeroides is defined similarly
- QM/MM results
quantum mechanical/molecular mechanical results
- RC
reaction center
- ZPH
zero phonon hole |
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Keywords: | hole burning homogeneous broadening inhomogeneous broadening Rhodobacter sphaeroides Rhodopseudomonas acidophila |
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