title: The origin of the split B800 absorption peak in the LH2 complexes from Allochromatium vinosum.
abstract: The absorption spectrum of the high-light peripheral light-harvesting (LH) complex from the photosynthetic purple bacterium Allochromatium vinosum features two strong absorptions around 800 and 850 nm. For the LH2 complexes from the species Rhodopseudomonas acidophila and Rhodospirillum molischianum, where high-resolution X-ray structures are available, similar bands have been observed and were assigned to two pigment pools of BChl a molecules that are arranged in two concentric rings (B800 and B850) with nine (acidophila) or eight (molischianum) repeat units, respectively. However, for the high-light peripheral LH complex from Alc. vinosum, the intruiging feature is that the B800 band is split into two components. We have studied this pigment-protein complex by ensemble CD spectroscopy and polarisation-resolved single-molecule spectroscopy. Assuming that the high-light peripheral LH complex in Alc. vinosum is constructed on the same modular principle as described for LH2 from Rps. acidophila and Rsp. molischianum, we used those repeat units as a starting point for simulating the spectra. We find the best agreement between simulation and experiment for a ring-like oligomer of 12 repeat units, where the mutual arrangement of the B800 and B850 rings resembles those from Rsp. molischianum. The splitting of the B800 band can be reproduced if both an excitonic coupling between dimers of B800 molecules and their interaction with the B850 manifold are taken into account. Such dimers predict an interesting apoprotein organisation as discussed below.
authors.affiliation: Experimental Physics IV and Bayreuth Institute for Macromolecular Research (BIMF), University of Bayreuth, Bayreuth, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Löhner
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Carey
authors.forename: Kirsty
authors.initials: K
authors.lastname: Hacking
authors.forename: Nichola
authors.initials: N
authors.lastname: Picken
authors.forename: Sharon
authors.initials: S
authors.lastname: Kelly
authors.forename: Richard
authors.initials: R
authors.lastname: Cogdell
authors.forename: Jürgen
authors.initials: J
authors.lastname: Köhler
mesh_terms: D002942:Circular Dichroism; D020563:Gammaproteobacteria / Q000502:physiology*; D045342:Light-Harvesting Protein Complexes / Q000502:physiology*; D008954:Models, Biological; D009010:Monte Carlo Method; D010860:Pigments, Biological / Q000502:physiology; D011487:Protein Conformation; D013054:Spectrophotometry, Atomic
chemical_list: D045342:Light-Harvesting Protein Complexes; D010860:Pigments, Biological