1 Excitons in Intact Cells of Photosynthetic Bacteria 2 Arvi Freiberg,* ,†,‡ Mihkel Pajusalu, † and Margus Ra ̈ tsep † 3 † Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia 4 ‡ Institute of Molecular and Cell Biology, University of Tartu, Riia 23, Tartu 51010, Estonia 5 ABSTRACT: Live cells and regular crystals seem fundamentally 6 incompatible. Still, effects characteristic to ideal crystals, such as coherent 7 sharing of excitation, have been recently used in many studies to explain 8 the behavior of several photosynthetic complexes, especially the inner 9 workings of the light-harvesting apparatus of the oldest known 10 photosynthetic organisms, the purple bacteria. To this date, there has 11 been no concrete evidence that the same effects are instrumental in real 12 living cells, leaving a possibility that this is an artifact of unnatural study 13 conditions, not a real effect relevant to the biological operation of 14 bacteria. Hereby, we demonstrate survival of collective coherent 15 excitations (excitons) in intact cells of photosynthetic purple bacteria. 16 This is done by using excitation anisotropy spectroscopy for tracking the 17 temperature-dependent evolution of exciton bands in light-harvesting 18 systems of increasing structural complexity. The temperature was gradually raised from 4.5 K to ambient temperature, and the 19 complexity of the systems ranged from detergent-isolated complexes to complete bacterial cells. The results provide conclusive 20 evidence that excitons are indeed one of the key elements contributing to the energetic and dynamic properties of photosynthetic 21 organisms. 22 ■ INTRODUCTION 23 Photosynthesis is one of the most important processes in the 24 biosphere, supplying almost all of the life with energy. 25 Fundamentals of this process, however, lie deep within 26 quantum physics and are still poorly understood. 1−3 One of 27 these quantum-mechanical concepts, nowadays widely used for 28 explaining photosynthesis, is the exciton. 3 An exciton involves 29 collective excitation of many pigment molecules simulta- 30 neously, making it possible to transfer energy without any 31 movement of charges. The concept was originally developed 32 theoretically by Frenkel for highly ordered crystals more than 33 eighty years ago. 4 Excitons in the context of photosynthesis 34 have been discussed already since 1938, 5 and exciton effects 35 have been experimentally found in different biological samples 36 that have been cooled to very low temperatures. 3,6 There have 37 been also recent reports about coherent energy pathways in 38 subparts of photosynthetic units at physiological temper- 39 atures, 7−9 proving an old theoretical prediction, 10 but still the 40 evidence for excitons within living photosynthetic organisms 41 has remained elusive. 42 In this paper, we will focus on the presence of excitons within 43 light-harvesting (LH) protein complexes as well as in their 44 functional assemblies from photosynthetic purple bacterium 45 Rhodobacter (Rb.) sphaeroides. This species is known to contain 46 highly regular antenna systems, 11 such as LH1 and LH2 47 complexes, well supporting the exciton idea. The former 48 complexes are naturally found in monomeric and dimeric RC- 49 LH1-PufX configurations consisting of one or two semicircles 50 of bacteriochlorophyll a (Bchl) molecules that produce C- and f1 51 S-shaped B875 antennas around, respectively, one or two 52 f1 reaction centers (RC) 12−14 (see Figure 1 for a dimeric RC- 53 LH1-PufX structure). The LH2 complex consists of two arrays 54 of Bchl molecules in circular arrangement, one of them (B800 55 in Figure 1), close to the cytoplasmic side of the protein, 56 contains nine, and the other (B850), at the periplasmic side, 57 eighteen molecules. 15 Both the RC-LH1-PufX and LH2 58 complexes self-assemble into intracytoplasmic membrane 59 vesicles (chromatophores) within the bacterial cytoplasm. 16,17 60 The solar radiation itself is absorbed by the antenna pigments, 61 and the excitation energy is funneled to the RC, where it is 62 efficiently utilized in a sequence of electron transfer 63 processes. 18,19 To verify generality of the results, parallel 64 measurements were also conducted on samples from 65 Rhodopseudomonas (Rps.) acidophila, another photosynthetic 66 purple bacterium, whose LH complexes are similar to those of 67 Rb. sphaeroides. 15,20 68 The present work was primarily designed to identify the 69 physiological-temperature exciton effects in LH complexes 70 under conditions of different complexity, both in detergent- 71 isolated and in natural, membrane-bound, forms. To system- 72 atically study this problem, we begin by characterizing the 73 spectral properties of the individual complexes in the most 74 easily interpretable cases, which are solidified solutions of 75 detergent-isolated complexes at near absolute-zero temperature Special Issue: Rienk van Grondelle Festschrift Received: October 5, 2012 Revised: January 9, 2013 Article pubs.acs.org/JPCB © XXXX American Chemical Society A dx.doi.org/10.1021/jp3098523 | J. Phys. Chem. B XXXX, XXX, XXX−XXX srh00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.5.i1:3915 | 2.0 alpha 39) 2012/12/04 10:21:00 | PROD-JCA1 | rq_1064924 | 2/13/2013 10:05:10 | 8