Abstract
Energy transfer and trapping in the light harvesting antennae of purple photosynthetic bacteria is an ultrafast process, which occurs with a quantum efficiency close to unity. However the mechanisms behind this process have not yet been fully understood. Recently it was proposed that low-lying energy dark states, such as charge transfer states and polaron pairs, play an important role in the dynamics and directionality of energy transfer. However, it is difficult to directly detect those states because of their small transition dipole moment and overlap with the B850/B870 exciton bands. Here we present a new experimental approach, which combines the selectivity of two-dimensional electronic spectroscopy with the availability of genetically modified light harvesting complexes, to reveal the presence of those dark states in both the genetically modified and the wild-type light harvesting 2 complexes of Rhodopseudomonas palustris. We suggest that Nature has used the unavoidable charge transfer processes that occur when LH pigments are concentrated to enhance and direct the flow of energy.
Highlights
Energy transfer in photosynthetic complexes has been the subject of detailed studies during the past few decades[1,7,8,9,10,11], and recently it has been proposed that the coupling between vibronic and charge transfer (CT) states promotes the high efficiency of the first steps of charge separation in the PSII reaction centre at physiological temperatures[12,13]
The energy transfer dynamics of light harvesting complexes in purple bacteria has been the subject of detailed studies over the past decades
The presence and possible role of CT states and/or polaron pairs has been proposed in different theoretical models in order to explain the ultra-fast dynamics of different photosynthetic comple xes[4,11,14,30,34]
Summary
Energy transfer in photosynthetic complexes has been the subject of detailed studies during the past few decades[1,7,8,9,10,11], and recently it has been proposed that the coupling between vibronic and charge transfer (CT) states promotes the high efficiency of the first steps of charge separation in the PSII reaction centre at physiological temperatures[12,13]. One ring consists of weakly coupled BChl ‘monomers’, absorbing at 800 nm (constituting the B800 band), whereas the other ring contains strongly coupled BChl dimers, which typically have an absorption peak at 850 nm (the B850 band) This is the case for the LH2 complex isolated from Rps. palustris grown under high light conditions (wild type or WT-LH2)[22]. It has been possible to obtain a deletion mutant of Rps. palustris that only has one pair of the puc genes left: the pucD α β genes (Southall and Cogdell, unpublished data) This mutant only expresses an LH2 complex when it is grown under low light conditions, and the particularity of this LH2 complex (M-LH2) is that the B850 band is blue-shifted by about 40 nm and merges with the B800 band, resulting in a quasi-single absorption peak in the 800 nm spectral region (the B810 band), as shown in Fig. 1a in blue
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