Polarized fluorescence spectroscopy of oriented isolated spinach Photosystem I particles |
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Authors: | Andreeva Atanaska Velitchkova Maya |
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Institution: | (1) Faculty of Physics, Department of Condensed Matter of Physics, Sofia University, 5, J. Bourchier blvd, BG-1164 Sofia, Bulgaria;(2) lnstitute of Biophysics, Bulgarian Academy of Sciences, Acad G. Bonchev str. bl.21, 1113 Sofia, Bulgaria |
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Abstract: | The fluorescence anisotropy of Photosystem I (PS I) particles, isolated from spinach chloroplasts and containing approximately
200 chlorophyll molecules per reaction center, is investigated at low temperatures. The particles are oriented by squeezing
in polyacrylamid gels with different macroscopic deformation parameters. Fluorescence anisotropy is measured upon steady state
excitation with a laser line at 632.8 nm. A formula for the fluorescence anisotropy in oriented Photosystem I particles is
applied for a different polarization of the linearly polarized exciting light. Our calculations are based on the consideration
of the Photosystem I complex as a triple-chromophore complex: the absorbing chlorophyll molecules (chl), belonging to the
light-harvesting complex of PS I (LHC), and two fluorophores, emitting at 720 nm (F720) and at 735 nm (F735), respectively.
Using polarized fluorescence spectroscopy with a different polarization of the linearly polarized exciting light, the experimental
dependence of the fluorescence anisotropy on this polarization is obtained. Based on this dependence and applying the derived
formula, as a first approximation, both the orientation of the photosynthetic pigments with respect to the membrane and their
mutual orientation are determined in PS I particles. As the most probable average orientational angles in PS I particles,
we obtained the values 35°÷ 50°, 50°÷ 60°, and 65°÷ 67° for the absorbing dipoles of chl and for the emission dipoles of F720
and F735, respectively, with the normal of the plane of the membrane. For their mutual orientation, the following limits are
determined: 10°÷ 20°, 40 ± 2°, 20°÷ 30° for the angles between chl and F720, chl and F735; and F720 and F735, correspondingly.
Of course, the values of the angles estimated as a result of our study are an average value of all angles of the excited transitions
and must be considered as their first approximation valid for the idealized case when all PS I particles are oriented in gel.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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Keywords: | anisotropy long wavelength pigments pigment orientation |
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