The Core Antenna System

Independently of interactions with different peripheral antenna proteins, PSI in cyanobacteria, green algae, and higher plants always harbor a core antenna system that, in the cyanobacterium T. elongatus, consists of 90 chlorophyll a and 22 carotenoid molecules, according to the crystal structure (Figure 2.4a and b) [17]. The function of these pigments is to absorb light and to transfer the excitation energy to the chlorophylls in the center of the complex, often called the reaction center, where the energy is used to initiate transmembrane charge separation. At room temperature, the core antenna of PSI is remarkably efficient in transferring excitation energy to the reaction center, which is expressed by a very low fluorescence yield of less than 1% [163] and a quantum yield of charge separation greater than 95% [164].

Figure 2.4 The PSI core antenna system. (a) Arrangement of the cofactors in a PSI monomer, shown in the same orientation as in Figure 2.1c. The antenna chlorophylls which have been mentioned in the text are highlighted by larger stick radii and labeled. The chlorophylls highlighted in red are assumed to belong to the "red chlorophylls"; the chlorophylls A40 and B39 shown in yellow are the "connecting chlorophylls" which are located closest to the electron

Figure 2.4 The PSI core antenna system. (a) Arrangement of the cofactors in a PSI monomer, shown in the same orientation as in Figure 2.1c. The antenna chlorophylls which have been mentioned in the text are highlighted by larger stick radii and labeled. The chlorophylls highlighted in red are assumed to belong to the "red chlorophylls"; the chlorophylls A40 and B39 shown in yellow are the "connecting chlorophylls" which are located closest to the electron transfer chain. The large ellipse centered at the pseudo-C2 axis roughly indicates the boundary of the central domain of the PSI core antenna. (b) Same as in (a), but rotated 90° about the horizontal axis. The vertical broken lines indicate the boundary of the central antenna domain. Most of the chlorophylls in the peripheral domains are arranged in two layers, one at the lumenal and one at the stromal side.

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