Photosynthesis Research

, Volume 86, Issue 1, pp 185–201

Red Chlorophylls in the Exciton Model of Photosystem I

  • Sarunas Vaitekonis
  • Gediminas Trinkunas
  • Leonas Valkunas
Regular Paper

DOI: 10.1007/s11120-005-2747-x

Cite this article as:
Vaitekonis, S., Trinkunas, G. & Valkunas, L. Photosynth Res (2005) 86: 185. doi:10.1007/s11120-005-2747-x


Structural arrangement of pigment molecules of Photosystem I of photosynthetic cyanobacterium Synechococcus elongatus is used for theoretical modeling of the excitation energy spectrum. It is demonstrated that a straightforward application of the exciton theory with the assumption of the same molecular transition energy does not describe the red side of the absorption spectrum. Since the inhomogeneity in the molecular transition energies caused by a dispersive interaction with the molecular surrounding cannot be identified directly from the structural model, the evolutionary search procedure is used for fitting the low temperature absorption and circular dichroism spectra. As a result, one dimer, three trimers and one tetramer of chlorophyll molecules responsible for the red side of the absorption spectrum with their assignment to the spectroscopically established three bands at 708, 714 and 719 nm are determined. All of them are found to be situated not in the very close vicinity of the reaction center but are encircling it almost at the same distance. In order to explain the unusual broadening on the red side of the spectrum the exciton state mixing with the charge transfer (CT) states is considered. It is shown that two effects can be distinguished as caused by mixing of those states: (i) the oscillator strength borrowing by the CT state from the exciton transition and (ii) the borrowing of the high density of the CT state by the exciton state. The intermolecular vibrations between two counter-charged molecules determine the high density in the CT state. From the broad red absorption wing it is concluded that the CT state should be the lowest state in the complexes under consideration. Such mixing effect enables resolving the diversity in the molecular transition energies as determined by different theoretical approaches.


exciton–CT mixing PS I red pigments Synechococcus elongatus 



circular dichroism




charge transfer


excited state absorption


full width at half maximum


linear dichroism


light-harvesting complex I


optical density, absorption


Photosystem I


Photosystem II


reaction center

Copyright information

© Springer 2005

Authors and Affiliations

  • Sarunas Vaitekonis
    • 1
  • Gediminas Trinkunas
    • 1
  • Leonas Valkunas
    • 1
    • 2
  1. 1.Institute of PhysicsVilniusLithuania
  2. 2.Theoretical Physics ChairFaculty of Physics of Vilnius universityVilniusLithuania