1. Introduction 2

2. Structure of the bacterial PSU 5

2.1 Organization of the bacterial PSU 5

2.2 The crystal structure of the RC 9

2.3 The crystal structures of LH-II 11

2.4 Bacteriochlorophyll pairs in LH-II and the RC 13

2.5 Models of LH-I and the LH-I-RC complex 15

2.6 Model for the PSU 17

3. Excitation transfer in the PSU 18

3.1 Electronic excitations of BChls 22

3.1.1 Individual BChls 22

3.1.2 Rings of BChls 22

3.1.2.1 Exciton states 22

3.1.3 Effective Hamiltonian 24

3.1.4 Optical properties 25

3.1.5 The effect of disorder 26

3.2 Theory of excitation transfer 29

3.2.1 General theory 29

3.2.2 Mechanisms of excitation transfer 32

3.2.3 Approximation for long-range transfer 34

3.2.4 Transfer to exciton states 35

3.3 Rates for transfer processes in the PSU 37

3.3.1 Car→BChl transfer 37

3.3.1.1 Mechanism of Car→BChl transfer 39

3.3.1.2 Pathways of Car→BChl transfer 40

3.3.2 Efficiency of Car→BChl transfer 40

3.3.3 B800-B850 transfer 44

3.3.4 LH-II→LH-II transfer 44

3.3.5 LH-II→LH-I transfer 45

3.3.6 LH-I→RC transfer 45

3.3.7 Excitation migration in the PSU 46

3.3.8 Genetic basis of PSU assembly 49

4. Concluding remarks 53

5. Acknowledgments 55

6. References 55

Life as we know it today exists largely because of photosynthesis, the process through which light energy is converted into chemical energy by plants, algae, and photosynthetic bacteria (Priestley, 1772; Barnes, 1893; Wurmser, 1925; Van Niel, 1941; Clayton & Sistrom, 1978; Blankenship et al. 1995; Ort & Yocum, 1996). Historically, photosynthetic organisms are grouped into two classes. When photosynthesis is carried out in the presence of air it is called oxygenic photosynthesis (Ort & Yocum, 1996). Otherwise, it is anoxygenic (Blankenship et al. 1995). Higher plants, algae and cyanobacteria perform oxygenic photosynthesis, which involves reduction of carbon dioxide to carbohydrate and oxidation of water to produce molecular oxygen. Some photosynthetic bacteria, such as purple bacteria, carry out anoxygenic photosynthesis that involves oxidation of molecules other than water. In spite of these differences, the general principles of energy transduction are the same in anoxygenic and oxygenic photosynthesis (Van Niel, 1931, 1941; Stanier, 1961; Wraight, 1982; Gest, 1993). The primary processes of photosynthesis involve absorption of photons by light-harvesting complexes (LHs), transfer of excitation energy from LHs to the photosynthetic reaction centers (RCs), and the primary charge separation across the photosynthetic membrane (Sauer, 1975; Knox, 1977; Fleming & van Grondelle, 1994; van Grondelle et al. 1994). In this article, we will focus on the anoxygenic photosynthetic process in purple bacteria, since its photosynthetic system is the most studied and best characterized during the past 50 years.