Oellerich, S., Ketelaars, M., Segura, J.-M., Margis, G., Ruijter, W. de, Köhler, J., Schmidt, J., and Aartsma, T.J.
The initial event in bacterial photosynthesis is the absorption of sunlight by an array of pigment-protein complexes, the so called light-harvesting complexes (LH). The absorbed light energy is then efficiently transferred to the reaction centre (RC), where the charge separation and thus the primary conversion into chemical energy takes place. The photosynthetic purple bacterium Rhodopseudomonas (Rps.) acidophila usually contains two types of light-harvesting complexes, light-harvesting complex 1 (LH1) and light-harvesting complex 2 (LH2). When grown under low-light and/or low temperature conditions, an additional spectroscopic variant of LH2 is expressed: the light-harvesting complex 3 (LH3) [1, 2]. The high-resolution crystal structure of LH3, which is also denoted as B800-820, has recently been resolved and revealed a C9-symmetry similar to that of LH2, thus with the bacteriochlorophyll a (BChl a) pigments arranged in two concentric rings. The outer ring of LH3 has nine well-separated BChl a pigments, that absorb at around 800 nm (B800 ring), while the eighteen closely interacting BChl a pigments in the inner ring absorb at around 820 nm (B820 ring). The B800 and B820 pigments are arranged with their molecular plane perpendicular and parallel to the symmetry axis of the complex, respectively. Since the overall structures of LH2 and LH3 are very similar, the interesting question to address is what causes the shift of the long-wavelength absorption band from 850 nm in LH2 down to 820 nm in LH3. It has previously been demonstrated that optical single-molecule spectroscopy at low temperature provides direct insight into relevant parameters determining the electronic structure of LH2 and LH1 complexes [5-8]. Measuring fluorescence-excitation spectra of single LH complexes avoids ensemble averaging and resolves bands otherwise masked by inhomogeneous line broadening. By applying this technique to individual LH3 complexes of Rps. acidophila (str. 7750), we were able to gain more insight into the electronic structure of LH3 as well as in the possible origin of the spectral shift from 850 to 820 nm. The spectra revealed a clear difference in the spectral features of the B800 and B820 band. In the B800 band several, relatively narrow, lines with bandwidths ranging from 2 to 13 cm-1 full width half maximum (FWHM) were observed, whereas in the B820 band only a limited number of broad bands with spectral widths of 60 to 150 cm-1 FWHM were present. These results clearly indicate a difference in electronic structure and dynamics of the bands. In the B800 band the excitations are mainly localized on individual pigments with picosecond dynamics within the band. In the B820 band the excitations are strongly delocalized over the ring with exciton relaxation in the femtosecond regime. Furthermore, studying the polarization dependence of the spectra revealed that about 60 % of the studied complexes show two broad, orthogonally polarized bands in the 820 nm region. These two bands were assigned to the kcirc= ±1 states of a circular exciton which have their degeneracy lifted by δE ± 1 = 160 cm-1. The overall spectral behaviour of the studied LH3 complexes and the previously studied LH2 complexes [5-7] is very similar, even though LH3 exhibits more spectral heterogeneity than LH2. Taking these spectral similarities into account, especially with respect to the excitonic behaviour of the B820 (LH3) and B850 (LH2) bands, it is concluded that the spectral shift from 850 to 820 nm is not caused by changes in the interaction energy. Instead, the spectral shift appears to be induced by changes in the site energies of the pigments.