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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.

Carotenoids are important biomolecules that are ubiquitous in nature and find widespread application in medicine. In photosynthesis, they have a large role in light harvesting (LH) and photoprotection. They exert their LH function by donating their excited singlet state to nearby (bacterio)chlorophyll molecules. In photosynthetic bacteria, the efficiency of this energy transfer process can be as low as 30%. Here, we present evidence that an unusual pathway of excited state relaxation in carotenoids underlies this poor LH function, by which carotenoid triplet states are generated directly from carotenoid singlet states. This pathway, operative on a femtosecond and picosecond timescale, involves an intermediate state, which we identify as a new, hitherto uncharacterized carotenoid singlet excited state. In LH complex-bound carotenoids, this state is the precursor on the reaction pathway to the triplet state, whereas in extracted carotenoids in solution, this state returns to the singlet ground state without forming any triplets. We discuss the possible identity of this excited state and argue that fission of the singlet state into a pair of triplet states on individual carotenoid molecules constitutes the mechanism by which the triplets are generated. This is, to our knowledge, the first ever direct observation of a singlet-to-triplet conversion process on an ultrafast timescale in a photosynthetic antenna. C arotenoids (Cars) serve a variety of functions in biological systems. In photosynthesis, they act as light-harvesting (LH) pigments by absorbing sunlight in the blue and green parts of the solar spectrum and transferring the excited state energy to nearby (bacterio)chlorophylls (BChl) (1, 2). The BChl molecules subsequently transfer this energy to a photochemical energyconverting protein known as the reaction center (RC), where the excited state energy is fixed by means of a series of electron transfer reactions (3). During these energy-and electrontransfer processes, which may take up to hundreds of picoseconds, the singlet excited and charge-separated states of BChl are subject to intersystem crossing to the triplet state, which occurs on a timescale of several nanoseconds. Although produced with a small probability, these BChl triplet states are potentially harmful to the organism because they can promote molecular oxygen to its singlet excited state, which is a highly reactive and damaging species. Cars can efficiently accept and safely dissipate BChl triplet and singlet oxygen states, and this photoprotective quality is utilized by essentially all photosynthetic organisms (1, 2).The first singlet excited state of Cars, S 1 , carries gerade symmetry (with respect to inversion) as does the ground state, S 0 , and is therefore dipole-forbidden. The second excited singlet state, S 2 , carries ungerade symmetry and is dipole-allowed (1). The specific strong Car absorption in the blue and green regions of the visible part of the electromagnetic spectrum is caused by the transition to this second excited state,...

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