Light-Harvesting Function of Carotenoids in Photo-synthesis: The Roles of the Newly Found 1 1 B u ⴚ State Yasushi Koyama Ferdy S. Rondonuwu Ritsuko Fujii Yasutaka Watanabe Faculty of Science and Technology, KwanseiGakuin University, Sanda, Hyogo 669-1337, Japan Received 1 September 2003; accepted 28 November 2003 Published online 19 April 2004 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/bip.20034 Abstract: This minireview article highlights the energetics and the dynamics of the 1 1 B u ⫺ and 3 1 A g ⫺ states of carotenoids discovered very recently. Those “hidden” covalent states have been revealed by measurements of resonance-Raman excitation profiles of crystalline carotenoids. The depen- dence of the energies of the low-lying singlet states, including the 1 1 B u ⫹ ,3 1 A g ⫺ ,1 1 B u ⫺ , and 2 1 A g ⫺ states, on the number of conjugated double bonds (n) is in agreement with the extrapolation of those state energies calculated by Tavan and Schulten for shorter polyenes (P. Tavan and K. Schulten, Journalof Chemical Physics, 1986,vol.85,pp.6602– 6609). It has also been shown that the internal-conversion processes among those singlet statestake place in accord with the state ordering, i.e., 1 1 B u ⫹ 3 1 1 B u ⫺ 3 2 1 A g ⫺ 3 1 1 A g ⫺ (the ground state) for carotenoids having n ⫽ 9 and 10, whereas 1 1 B u ⫹ 3 3 1 A g ⫺ 3 1 1 B u ⫺ 3 2 1 A g ⫺ 3 1 1 A g ⫺ for carotenoids having n ⫽ 11–13. Radiative transitions of 1 1 B u ⫹ 3 2 1 A g ⫺ and 1 1 B u ⫺ 3 2 1 A g ⫺ as well as a branching into the triplet manifold of 1 1 B u ⫺ 3 1 3 A g 3 1 3 B u have also been found. Those low-lying singlet states of all-trans carotenoids can facilitate multiple channels of singlet-energy transfer to bacteriochlorophyll in the LH2 antenna complexes of purple photosynthetic bacteria. Thus,the newly found 1 1 B u ⫺ and 3 1 A g ⫺ states of carotenoids need to be incorporated into the picture of carotenoid-to-bacteriochlorophyll singlet- energy transfer. © 2004 Wiley Periodicals, Inc.Biopolymers 74: 2–18, 2004 Keywords: carotenoids; photosynthesis; excited states (1 1 B u ⫺ and 3 1 A g ⫺ ); light-harvesting function INTRODUCTION The structures of the LH2 antenna complexes have been determined by x-ray crystallography, and the detailed arrangements of the carotenoid and bacterio- chlorophyll (BChl)molecules are presented. 1,2 The results provided us with a strong basis in studying the mechanisms of the light-harvesting function of caro- tenoids in bacterial photosynthesis, i.e., the absorption of photon by the carotenoid molecule, and subsequent transfer of its singlet energy to the BChl molecule. The conjugated chain of all-trans carotenoids in the antenna complexes has approximate C 2h symmetry, and it gives rise to low-lying singlet states with four differentsymmetries includingthe k 1 A g ⫺ , l 1 B u ⫺ , m 1 A g ⫹ , and n 1 B u ⫹ states. Here, k, l, m, and n are the labeling ofeach type ofelectronic state from the lowestto the higherenergies, and the superscript Correspondence to: YasushiKoyama;email: ykoyama@ kwansei.ac.jp Biopolymers, Vol.74,2–18 (2004) © 2004 Wiley Periodicals, Inc. 2