1. Introduction 229

2. Structures 234

2.1 The structure of LH2 234

2.2 Natural variants of peripheral antenna complexes 242

2.3 RC–LH1 complexes 242

3. Spectroscopy 249

3.1 Steady-state spectroscopy 249

3.2 Factors which affect the position of the Qy absorption band of Bchla 249

4. Regulation of biosynthesis and assembly 257

4.1 Regulation 257

4.1.1 Oxygen 257

4.1.2 Light 258

4.1.2.1 AppA: blue-light-mediated regulation 259

4.1.2.2 Bacteriophytochromes 259

4.1.3 From the RC to the mature PSU 261

4.2 Assembly 261

4.2.1 LH1 262

4.2.2 LH2 263

5. Frenkel excitons 265

5.1 General 265

5.2 B800 267

5.3 B850 267

5.4 B850 delocalization 273

6. Energy-transfer pathways: experimental results 274

6.1 Theoretical background 274

6.2 ‘Follow the excitation energy’ 276

6.2.1 Bchla→Bchla energy transfer 277

6.2.1.1 B800→B800 277

6.2.1.2 B800→B850 278

6.2.1.3 B850→B850 279

6.2.1.4 B850→B875 280

6.2.1.5 B875→RC 280

6.2.2 Car[harr ]Bchla energy transfer 281

7. Single-molecule spectroscopy 284

7.1 Introduction to single-molecule spectroscopy 284

7.2 Single-molecule spectroscopy on LH2 285

7.2.1 Overview 285

7.2.2 B800 286

7.2.2.1 General 286

7.2.2.2 Intra- and intercomplex disorder of site energies 287

7.2.2.3 Electron-phonon coupling 289

7.2.2.4 B800→B800 energy transfer revisited 290

7.2.3 B850 293

8. Quantum mechanics and the purple bacteria LH system 298

9. Appendix 299

9.1 A crash course on quantum mechanics 299

9.2 Interacting dimers 305

10. Acknowledgements 306

11. References 307

This review describes the structures of the two major integral membrane pigment complexes, the RC–LH1 ‘core’ and LH2 complexes, which together make up the light-harvesting system present in typical purple photosynthetic bacteria. The antenna complexes serve to absorb incident solar radiation and to transfer it to the reaction centres, where it is used to ‘power’ the photosynthetic redox reaction and ultimately leads to the synthesis of ATP. Our current understanding of the biosynthesis and assembly of the LH and RC complexes is described, with special emphasis on the roles of the newly described bacteriophytochromes. Using both the structural information and that obtained from a wide variety of biophysical techniques, the details of each of the different energy-transfer reactions that occur, between the absorption of a photon and the charge separation in the RC, are described. Special emphasis is given to show how the use of single-molecule spectroscopy has provided a more detailed understanding of the molecular mechanisms involved in the energy-transfer processes. We have tried, with the help of an Appendix, to make the details of the quantum mechanics that are required to appreciate these molecular mechanisms, accessible to mathematically illiterate biologists. The elegance of the purple bacterial light-harvesting system lies in the way in which it has cleverly exploited quantum mechanics.