Natural Selection of Carotenoid Configurations by the Reaction Center and the Light-Harvesting Complex of Photosynthetic Bacteria

Abstract

Carotenoid is an intrinsic component of the reaction center (RC) and the light-harvesting complex (LH) of photosynthetic bacteria, and is known to have dual functions [1–5]. One is harvesting and transferring light energy; the light energy harvested by carotenoid is transferred finally to the bacteriochlorophyll (BChl) pair in RC, to trigger a series of electron transfer reactions. The other is photo-protection by scavenging harmful singlet oxygen and triplet BChl, which are produced by accumulated excess energy. Carotenoid bound to the light-harvesting complex takes the all-trans configuration [6], while carotenoid bound to the reaction center takes a unique cis configuration, which was first detected by Lutz [7] by means of resonance Raman spectroscopy. Lutz and coworkers have shown that this cis configuration is common among the RC’s of different kinds of photosynthetic bacteria [8], and that the binding of specific carotenoid causes its isomerization from the all-trans to the cis configuration [9].