Fluorescence spectroscopy of 13′‐cis‐spheroidene at 170 K: A reason for the natural selection of the all‐trans configuration for the light‐harvesting function

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Abstract
As a follow‐up to fluorescence and fluorescence‐excitation spectroscopy at 170 K of all‐trans‐spheroidene free in solutions Y. Watanabe, T. Kameyama, Y. Miki, M. Kuki, and Y. Koyama, “The 2′Ag− state and two additional low‐lying electronic states of spheroidene newly identified by fluorescence and fluorescence‐excitation spectroscopy at 170K,” Chem. Phys. Lett. 206, 62–68 (1993). and bound to the light‐harvesting complex of Rhodobacter sphaeroides 2.4.1, Y. Koyama, Y. Miki, T. Kameyama, R. J. Cogdell, and Y. Watanabe, “Low‐lying electronic levels of spheroidene bound to the light‐harvesting (LH2) complex of Rhodobacter sphaeroides 2.4.1. as determined by fluorescence and fluorescence‐excitation spectroscopy at 170K,” Chem. Phys. Lett. 208, 479–485 (1993). 13′‐cis‐spheroidene in n‐hexane solution has been examined for comparison. All‐trans‐spheroidene exhibits efficient internal conversion from the 3A to the 2A state and fluorescence from both the 2A and B states, while 13‐‐cis‐spheroidene exhibits internal conversion from the “3Ag‐” state to the “B” state and fluorescence only from the “B” state. Thus, all‐trans‐spheroidene can provide two channels of carotenoid‐to‐bacteriochlorophyll singlet‐energy transfer, while 13′‐cis‐spheroidene can provide only one. This must be a reason for the natural selection of the all‐trans configuration for the light‐harvesting function. © 1995 John Wiley & Sons, Inc.

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