Published: March 15, 2011 r2011 American Chemical Society 4816 dx.doi.org/10.1021/la104532b | Langmuir 2011, 27, 4816–4828 ARTICLE pubs.acs.org/Langmuir Temperature and Ionic Strength Effects on the Chlorosome Light-Harvesting Antenna Complex Kuo-Hsiang Tang, ‡ Liying Zhu, § Volker S. Urban, ^ Aaron M. Collins, †,‡ Pratim Biswas, § and Robert E. Blankenship ‡, * ‡ Department of Biology and Department of Chemistry, Campus Box 1137, Washington University in St. Louis, St. Louis, Missouri 63130, United States § Aerosol and Air Quality Research Laboratory, Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States ^ Center for Structural Molecular Biology, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States b S Supporting Information ’ INTRODUCTION Photosynthetic organisms such as bacteria, algae, and plants use light-harvesting (LH) antenna systems to capture solar energy and transfer the excitation energy to reaction centers (RCs) where the electron transfer for photochemistry takes place. LH antenna complexes are diverse structures that are highly specialized and optimized to allow photosynthetic organ- isms to capture the maximum light energy available in their environment. 1,2 Two types of LH antenna complexes have been identified: one is the proteinpigment complex, commonly found in most photosynthetic organisms, and the other is the pigmentpigment complex, that is chlorosomes. 3,4 Both types of LH antenna complexes are produced by the moderately thermo- philic filamentous anoxygenic phototrophic (FAP) bacterium Chloroflexus aurantiacus, and function in light-harvesting and energy transfer. C. aurantiacus is able to absorb solar energy and convert it into chemical energy under both low and high light conditions, making it potentially of interest for the development of artificial photosynthetic systems. The proposed energy transfer pathway in the photosynthetic machinery of C. aurantiacus is shown in Figure 1. In C. aurantia- cus, the light energy is first absorbed by chlorosomes, large complexes attached to the cytoplasmic side of the inner cell membrane. After photon absorption by chlorosome pigments, an ultrafast energy transfer takes place to proteinpigment com- plexes, first the baseplate, then to the integral-membrane light- harvesting B808866 complex, 58 and finally to the reaction center (RC). The B808866 complex of C. aurantiacus has been proposed to function similarly as the B880 light-harvesting complex I (LH1) of purple photosynthetic bacteria, 4 and the structural information of the B808866 complex and RC of C. aurantiacus has been investigated recently by SANS. 9 In contrast to most types of LH antenna complexes, chloro- somes, located on the cytoplasmic side of the inner membrane in Received: November 14, 2010 Revised: February 27, 2011 ABSTRACT: Chlorosomes, the peripheral light-harvesting antenna complex from green photosynthetic bacteria, are the largest and one of the most efficient light-harvesting antenna complexes found in nature. In contrast to other light-harvesting antennas, chlorosomes are constructed from more than 150 000 self-assembled bacteriochlorophylls (BChls) and contain rela- tively few proteins that play secondary roles. These unique properties have led to chlorosomes as an attractive candidate for developing biohybrid solar cell devices. In this article, we investigate the temperature and ionic strength effects on the viability of chlorosomes from the photosynthetic green bacterium Chloroflexus aurantiacus using small-angle neutron scattering and dynamic light scattering. Our studies indicate that chlorosomes remain intact up to 75 °C and that salt induces the formation of large aggregates of chlorosomes. No internal structural changes are observed for the aggregates. The salt-induced aggregation, which is a reversible process, is more efficient with divalent metal ions than with monovalent metal ions. Moreover, with treatment at 98 °C for 2 min, the bulk of the chlorosome pigments are undamaged, while the baseplate is destroyed. Chlorosomes without the baseplate remain rodlike in shape and are 3040% smaller than with the baseplate attached. Further, chlorosomes are stable from pH 5.5 to 11.0. Together, this is the first time such a range of characterization tools have been used for chlorosomes, and this has enabled elucidation of properties that are not only important to understanding their functionality but also may be useful in biohybrid devices for effective light harvesting.