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Biophysical Journal 84:2483-2491 (2003)
© 2003 The Biophysical Society

The Ring Structure and Organization of Light Harvesting 2 Complexes in a Reconstituted Lipid Bilayer, Resolved by Atomic Force Microscopy

Amalia Stamouli*,{dagger}, Sidig Kafi*, Dionne C. G. Klein{dagger}, Tjerk H. Oosterkamp{dagger}, Joost W. M. Frenken{dagger}, Richard J. Cogdell{ddagger} and Thijs J. Aartsma*

* Department of Biophysics, Huygens Laboratory, Leiden University, 2300 RA Leiden, The Netherlands; {dagger} Department of Interface Physics, Kamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, The Netherlands; and {ddagger} Division of Biochemistry and Molecular Biology, IBLS, University of Glasgow, Glasgow G12 8QQ, UK

Correspondence: Address reprint requests to Dr. Amalia Stamouli, Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands. Tel.: +31-71-5275969; Fax: +31-71-5275819; E-mail: stamouli{at}biophys.leidenuniv.nl.

The main function of the transmembrane light-harvesting complexes in photosynthetic organisms is the absorption of a light quantum and its subsequent rapid transfer to a reaction center where a charge separation occurs. A combination of freeze-thaw and dialysis methods were used to reconstitute the detergent-solubilized Light Harvesting 2 complex (LH2) of the purple bacterium Rhodopseudomonas acidophila strain 10050 into preformed egg phosphatidylcholine liposomes, without the need for extra chemical agents. The LH2-containing liposomes opened up to a flat bilayer, which were imaged with tapping and contact mode atomic force microscopy under ambient and physiological conditions, respectively. The LH2 complexes were packed in quasicrystalline domains. The endoplasmic and periplasmic sides of the LH2 complexes could be distinguished by the difference in height of the protrusions from the lipid bilayer. The results indicate that the complexes entered in intact liposomes. In addition, it was observed that the most hydrophilic side, the periplasmic, enters first in the membrane. In contact mode the molecular structure of the periplasmic side of the transmembrane pigment-protein complex was observed. Using Föster's theory for describing the distance dependent energy transfer, we estimate the dipole strength for energy transfer between two neighboring LH2s, based on the architecture of the imaged unit cell.




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