The LH1 and LH2 of Purple Bacteria
This short outline of the LH1 and LH2 of purple bacteria is meant to give an impression of a typical complex where the energy pathway is mainly formed by pigment-pigment interaction, i.e. excitonic coupling and FRET. In the LH complexes of purple bacteria, the excitonic coupling of pigments results in fast equilibration of electronic excitation along ringshaped structures in the organized pigments. The absorption of these pigment-rings exhibits maxima at 800 nm and 850 nm and therefore they are denoted as B800 and B850. Figure 23 shows the pigment arrangement. For the nomenclature see also Figure 26. The bacteriochlorophyll
Figure 21. Absorption spectrum of LHC trimers in micelles formed in buffer with Beta-DM. The main absorption bands are directly denoted in the graph and assigned to Chl a, Chl b or carotenoids and proteins with the absorption band of tryptophan (Tryp) at 267 nm.
Figure 22. Molecular structure of the Chls: Chl a, Chl b and Chl d (see inset).
Figure 23. Ring shaped BChl arrangement of the LH complexes found in purple bacteria according to (Schulten, 1999). The LH2 complexes contain an inner ring structure, which is slightly smaller, and an outer ring structure, which is slightly bigger. Similar organisation is found in the bigger LH1 structure surrounding the RC. For more details see text and Figure 24. Image reproduced with permission.
(BChl) molecules B800 in the bigger ring structure of LH2 are less densely packed than those of the smaller structure B850. On account of the shorter distance between neighbouring dipole moments and the steric orientation of the BChl molecules, the intermolecular interaction is much stronger in B850 than in B800. This leads to a stronger coupling and larger excitonic splitting in B850 in comparison with B800. The lowest excitonic state in B850 exhibits a markedly red-shifted absorption maximum at 850 nm (Ketelaars et al., 2001; Ritz et al., 2001). Therefore electronic excitation energy migrates from the outer to the inner ring according to FRET (Freer et al., 1996).
Figure 23 shows the BChl molecules in green and cyan. The proteins are shown in blue and purple (LH1 and LH2) and red and yellow for the L and M subunits of the RC. The BChl organisation in the LH2 is shown in Figure 24. One can see that the BChl molecules in the B850 are more densely packed and orientated in a tilted “sandwich configuration” while the distance of the BChls in B800 is larger and all molecules are lying in the x-y-plane. AFM pictures of the ring shaped LH2 structures of Rubrivivax gelatinosus are shown in Figure 25 (Scheuring et al., 2001). The LH2 shows a typical 9-fold rotational symmetry.
The structural arrangement of the BChls in LH2 (see Figure 24) has strong impact on the pathway of energy transfer. Excitons formed in the
Figure 24. Organisation of the 27 BChl molecules found in the LH2 antenna as published in (Ketelaars et al., 2001). The left side (view in x-z plane) shows the organisation of the 18 molecules of the smaller inner ring (B850) placed in z-direction above the 9 BChls of the bigger outer ring (B800) while the right side shows the view from the top (as in Figure 23) along z-direction on the x-y-plane. Image reproduced with permission.
Figure 25. AFM pictures of the LH2 of Rubrivivax gelatinosus as published in (Scheuring et al., 2001) (The pictures A, C, E and G show the original AFM data while B, D, F and H represent averaged pictures of A, C, E and G, respectively). Scale bars represent 10 nm in the raw data and 2 nm in the averages. Image reproduced with permission.
B800 are efficiently transferred to the B850 for energetic reasons. Detailed quantum chemical calculations performed by Renger and his coworkers revealed that the excitons equilibrate with a time constant of 500 fs along the B800 BChl while the equilibration in the B850 structure occurs with a faster time constant of 100 fs (Renger and Kuhn, 2007). The energy transfer from the B800 to B850 is nearly irreversible and takes
1.2 ps. With a time constant of 3-5 ps the excited states localize in the LH1 and, after 35 ps, in the RC (see Figure 26).
Figure 26. Time constants for the excited state transfer intra and inter the LH2 and LH1 complexes according to (Renger and Kuhn, 2007). Image reproduced with permission.