Purple Bacteria Antenna Systems

The antenna system is thus a way for nature to increase the cross section for light harvesting of sunlight for the same RC. One single RC can take care of excitations from a large number of antenna chromophores, and carry out charge separation. 

FIGURE 15.5 The significance of distance in the transfer of electrons (PIET) and excitations (PIEET) between identical chromophores. Charge separation (PIET) has a lower energy, the shorter the ET distance. PIEET has the same average energy for the final state, independent of distance. 

FIGURE 15.6 Part of the antenna system of purple bacteria. The figure on the right shows how the transition moments are coupled in the ground state. 

In PIEET, the average final state energy is independent of the distance. The solvent or protein contribution to the Stokes shift is quite negligible. A high reaction rate may be obtained if = -AG. If we have PIEET between equal molecules (AG = 0), a small Stokes shift is beneficial for a high reaction rate. PIET, on the other hand, has a large solvent shift and is possible only at short transfer distance. 

Another task of the storage ring may be to prevent the transfer of triplet states to the RC. Triplets cannot be transferred over long distances. If decay to spin triplet states takes place, the excitation may be considered as lost. 

---

Wavepacket motion is now routinely observed in systems ranging from the very simple to the very complex. In the latter category, we note that coherent vibrational motion on functionally significant time scales has been observed in the photosynthetic reaction center [15], bacteriorhodopsin [16], rhodopsin [17], and light-harvesting antenna of purple bacteria (LH1) [18-20]. Particularly striking are the results of Zadoyan et al. [21] on the... 

According to the fossil records [13], the early photosensitizers were purple bacteria and green sulfur bacteria. Purple bacteria have been the subject of many structural [14] and spectroscopic studies. [15-17] Particular attention was focused on their antenna complexes (LH1, LH2), which represent the best understood system in terms of light collection and energy transfer. Recently, some interest has also been directed to green bacteria containing chlorosomes,... 

Fig. 2. Spectral range (absorption in nm) of the antenna system of oxygenic (plants, algae, cyanobacteria) and anoxygenic (green and purple bacteria) photosynthetic organisms. The antenna systems (antenna complexes) are located either within (intramembrane) or on the surface (extramembrane) of the photosynthetic membrane.

Purple bacteria with one type of antenna system... 

Purple bacteria with two types of antenna systems... 

The first antenna system (or light-harvesting complex) to be structurally determined belongs to purple bacteria (Cogdell et al., 1995). The remarkable fact with this antenna system is that it has evolved into circular shapes containing a certain number of identical chromophores. 

FIGURE 15.7 Antenna system of purple bacteria, determined by x-ray crystallography by R. J. Cogdell et al. The light-harvesting smaller circles (LHl) donate the excitation to the inner ring (LH2). LH2 donates the excitation to the special pair of the reaction center. 

Figure 7 Antenna system in purple bacteria. (Reproduced with permission from Ref. 28. National Academy of Sciences, 1998.)...

Artificial antenna complexes that resemble biological counterparts in organization and light harvesting were published. These systems mimic the circular arrangement of pigments in the LH-II complex of purple bacteria. One such example is shown in Figure 9 12 porphyrins are noncovalently assembled to form a circular array, where the maximum separation, namely, radius of antenna, is 4.1 nm. ... 

One way in which purple bacteria have managed to economize the biosynthetic and genetic effort required for building large antenna systems was to assemble three BChl a and one carotenoid with two short a-hehcal polypeptides and then to self-assemble these so-called protomers into much larger supramolecular structures [23]. 

---