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Köhler, Jürgen:
Optical Spectroscopy of Individual Light-Harvesting Complexes.
In: Single Molecules.
Bd. 2
(2001)
Heft 4
.
- S. 285-286.
ISSN 1438-5171
DOI: https://doi.org/10.1002/1438-5171(200112)2:4<285::AID-SIMO285>3.0.CO;2-1
Abstract
The initial steps of photosynthesis involve the transfer of energy of absorbed light to the photochemical reaction centre, where a charge separation takes place and the excitation energy becomes available in the form of chemical energy. Pigment-protein complexes surrounding the reaction centre function as highly efficient collectors of light that greatly increase the total absorption cross section of the reaction centre, hence the name light-harvesting complexes [1]. The light-harvesting complexes of photosynthetic purple bacteria have been intensively investigated during the last years but remarkably there is no consensus as yet about the properties of their electronically excited states.Typically these bacteria contain two types of complexes, LH1 and LH2, which both are integral membrane proteins. The reaction centre presumably forms the core of the LH1 complex while the LH2 complexes are arranged around the perimeter of the LH1 ring in a two-dimensional structure. For the LH2 complex of the purple bacterium Rhodopseudomonas acidophila the geometric structure is known in great detail from X-ray crystallography [2]. It comprises 27 bacteriochlorophyll a (BChl a) and (presumably) 9 carotenoid molecules which are non-covalently bound to the protein matrix. The BChl a molecules are organized in two concentric rings. One ring features a group of nine well-separated BChl a molecules (B800) with an absorption band at about 800 nm. The other ring consists of eighteen closely interacting BChl a molecules (B850) absorbing around 850 nm. Interestingly the LH2 complex is highly symmetric with a nine-fold symmetry axis, which coincides with the cylindrical structure of the complex.Important parameters to describe the electronic structure of photosynthetic pigment protein complexes are the strength of the intermolecular interaction V, the spread in V (off-diagonal disorder), the spread in site energy, D, of the individual BChl a molecules (diagonal-disorder), and the strength of the electron-phonon interaction. The ratio V/D determines whether the nature of the energy-transfer process has to be described in terms of incoherent hopping (V/D « 1) or of coherently delocalized excitons (V/D » 1) [3, 4]. The great difficulty to determine the various parameters that play a role in the description of the electronic structure of light-harvesting complexes and the process of energy transfer is the fact that the optical absorption lines are inhomogeneously broadened. A serious problem is that this broadening reflects the contribution of two qualitatively different types of energetic disorder: i) disorder between different antenna complexes (intercomplex disorder) and ii) disorder within the individual complexes (intracomplex disorder). Only the latter contributes to the electronic structure of the exciton manifold, which makes it very difficult to interpret ensemble data in this regard. We have circumvented this problem by studying individual LH2 complexes from Rhodopseudomonas acidophila.In a series of papers we have investigated the electronic structure of the B800 ring as well as that of the B850 assembly. The results show that the properties of the electronically excited states of the two rings of BChl a molecules, absorbing at 800 nm and 850 nm, differ strongly. In contrast to the B800 BChl a molecules, for which the optical absorption results in electronically excited states that are localized on a few BChl a molecules [5-7], the same process leads for the B850 BChl a molecules to electronically excited states that are coherently delocalized over the eighteen BChl a molecules (Frenkel-excitons) [8]. In addition we have found evidence for a symmetry reduction of the LH2 complex that can be envisaged as an elliptical distortion [9, 10].
Weitere Angaben
| Publikationsform: | Artikel in einer Zeitschrift |
|---|---|
| Begutachteter Beitrag: | Ja |
| Institutionen der Universität: | Fakultäten > Fakultät für Mathematik, Physik und Informatik > Physikalisches Institut > Lehrstuhl Experimentalphysik IX - Spektroskopie weicher Materie > Lehrstuhl Experimentalphysik IX - Spektroskopie weicher Materie - Univ.-Prof. Dr. Jürgen Köhler Fakultäten Fakultäten > Fakultät für Mathematik, Physik und Informatik Fakultäten > Fakultät für Mathematik, Physik und Informatik > Physikalisches Institut Fakultäten > Fakultät für Mathematik, Physik und Informatik > Physikalisches Institut > Lehrstuhl Experimentalphysik IX - Spektroskopie weicher Materie |
| Titel an der UBT entstanden: | Ja |
| Themengebiete aus DDC: | 500 Naturwissenschaften und Mathematik > 530 Physik |
| Eingestellt am: | 11 Apr 2018 13:14 |
| Letzte Änderung: | 11 Apr 2018 13:14 |
| URI: | https://eref.uni-bayreuth.de/id/eprint/43336 |