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Título : Hexanol-Induced Order-Disorder Transitions in Lamellar Self-Assembling Aggregates of Bacteriochlorophyll c in Chlorobium tepidum Chlorosomes
Autor : Arellano, Juan B. ; Torkkeli, Mika; Tuma, Roman; Laurinmäki, Pasi A.; Melo, Thor Bernt; Ikonen, Teemu P.; Butcher, Sarah J.; Serimaa, Ritva E.; Psencik, Jakub
Palabras clave : Chlorosomes
Lamellar aggregates
Chlorobium tepidum
Order-disorder transitions
Fecha de publicación : mar-2008
Editor: American Chemical Society
Citación : Langmuir 24(5): 2035–2041 (2008)
Resumen: Chlorosomes are light-harvesting complexes of green photosynthetic bacteria. Chlorosomes contain bacteriochlorophyll (BChl) c, d, or e aggregates that exhibit strong excitonic coupling. The short-range order, which is responsible for the coupling, has been proposed to be augmented by pigment arrangement into undulated lamellar structures with spacing between 2 and 3 nm. Treatment of chlorosomes with hexanol reversibly converts the aggregated chlorosome chlorophylls into a form with spectral properties very similar to that of the monomer. Although this transition has been extensively studied, the structural basis remains unclear due to variability in the obtained morphologies. Here we investigated hexanol-induced structural changes in the lamellar organization of BChl c in chlorosomes from Chlorobium tepidum by a combination of X-ray scattering, electron cryomicroscopy, and optical spectroscopy. At a low hexanol/pigment ratio, the lamellae persisted in the presence of hexanol while the short-range order and exciton interactions between chlorin rings were effectively eliminated, producing a monomer-like absorption. The result suggested that hexanol hydroxyls solvated the chlorin rings while the aliphatic tail partitioned into the hydrophobic part of the lamellar structure. This partitioning extended the chlorosome along its long axis. Further increase of the hexanol/pigment ratio produced round pigment-hexanol droplets, which lost all lamellar order. After hexanol removal the spectral properties were restored. In the samples treated under the high hexanol/pigment ratio, lamellae reassembled in small domains after hexanol removal while the shape and long-range order were irreversibly lost. Thus, all the interactions required for establishing the short-range order by self-assembly are provided by BChl c molecules alone. However, the long-range order and overall shape are imposed by an external structure, e.g., the proteinaceous chlorosome baseplate.
Versión del editor: http://pubs.acs.org/doi/abs/10.1021/la703024e
URI : http://hdl.handle.net/10261/3151
DOI: 10.1021/la703024e
ISSN: 0743-7463
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