Energy transfer and clustering of photosynthetic light-harvesting complexes in reconstituted lipid membranes Takehisa Dewa a,b,⇑ , Ayumi Sumino a , Natsuko Watanabe a , Tomoyasu Noji a , Mamoru Nango a,⇑ a Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Japan Science and Technology, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan article info Article history: Available online 7 January 2013 Keywords: Photosynthetic light-harvesting complex LH2 LH1-RC Molecular assembly Energy transfer Reconstitution AFM abstract In purple photosynthetic bacteria, light-harvesting complex 2 (LH2) and light harvesting/reaction centre core complex (LH1-RC) play the key roles of capturing and transferring light energy and subsequent charge separation. These photosynthetic apparatuses form a supramolecular assembly; however, how the assembly influences the efficiency of energy conversion is not yet clear. We addressed this issue by evaluating the energy transfer in reconstituted photosynthetic protein complexes LH2 and LH1-RC and studying the structures and the membrane environment of the LH2/LH1-RC assemblies, which had been embedded into various lipid bilayers. Thus, LH2 and LH1-RC from Rhodopseudomonas palustris 2.1.6 were reconstituted in phosphatidylglycerol (PG), phosphatidylcholine (PC), and phosphatidyletha- nolamine (PE)/PG/cardiolipin (CL). Efficient energy transfer from LH2 to LH1-RC was observed in the PC and PE/PG/CL membranes. Atomic force microscopy revealed that LH2 and LH1-RC were heterogeneously distributed to form clusters in the PC and PE/PG/CL membranes. The results indicated that the phospho- lipid species influenced the cluster formation of LH2 and LH1-RC as well as the energy transfer efficiency. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The light-harvesting (LH) complexes present in photosynthetic membranes are densely packed in lipid bilayers [1,2]. Such a clus- tered LH assembly is necessary for efficient light harvesting. In membranes of purple photosynthetic bacteria, light-harvesting complex 2 (LH2) and light-harvesting 1 (LH1)/reaction centre core complex (LH1-RC) are known to play important roles of capturing light and charge separation [3]. These photosynthetic apparatuses, composed of proteins and pigments (bacteriochlorophylls [BChls] and carotenoids [Car]), form a densely packed supramolecular assembly in the membrane. The LH2 in Rhodopseudomonas (Rps.) acidophila 10050 consists of 9 ab pairs of a-helical polypeptides, 27 BChla (9 B800 and 18 B850 chromophores), and 18 Car, forming a 9-membered cylindri- cal complex [4]. The LH1 in Rps. palustris possesses a 16-membered ellipsoidal structure consisting of 15 ab pairs of a-helical polypep- tides, W polypeptide, 30 BChla (forming B880 chromophore), and Car. The LH1 is known to encircle the reaction centre complex (RC) to form a LH1-RC complex [5]. On absorbing light by B800 of LH2, an intramolecular energy transfer to B850 occurs in 1 ps, and generates excitonic 1 B850 / . The excitation energy is subsequently transferred between neighboring B850 of LH2 mole- cules [6] and further efficiently transferred to LH1 at 3–5 ps [7] and to RC at 35 ps [8,9], where charge separation takes place. Inciden- tally, if excess excited species ( 1 B850 / and 3 Car / ) occur in the LH system, effective quenching mechanisms operate. These are re- ferred to as singlet–singlet and singlet–triplet annihilations [10], and work during excitation energy transfers between B850 of LH2 complexes assembled into a densely packed cluster [11,12]. Recently, Pflock et al. reported that time-resolved fluorescence spectroscopy and dynamic Monte Carlo simulations for LH2 recon- stituted into lipid bilayers, showing a relationship between energy transfer and annihilation [11,12]. Recently, atomic force microscopy (AFM) has revealed supra- molecular organization of photosynthetic membranes, indicating the presence of polymorphic LH2/LH1-RC assemblies in photosyn- thetic bacteria [1,2,13]. Dimerized LH1-RC arrays are known to be connected to LH2 domains in Rhodobacter sphaeroides [1]. Likewise, hexagonally packed, paracrystalline clusters of LH2 and LH1-RC have been described in Rhodospirillum photometricum [2] and Rps. palustris [13], respectively. However, the role of such varied supra- molecular organization in the function of the LH complexes re- mains unclear [14–17]. In addition, little is known about the effect of phospholipid molecules on such a LH2/LH1-RC assembly, its tendency to form clusters, packing density, and its miscibility in phospholipid bilayers. The understanding of lipid–protein 0301-0104/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemphys.2012.12.039 ⇑ Corresponding authors. Address: Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan (T. Dewa). E-mail addresses: takedewa@nitech.ac.jp (T. Dewa), nango@nitech.ac.jp (M. Nango). Chemical Physics 419 (2013) 200–204 Contents lists available at SciVerse ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys