Dark States in the Light-Harvesting complex 2 Revealed by Two-dimensional Electronic Spectroscopy

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.

Model of Vibrons in Quantum Photosynthesis as an Analog of a Model of Laser

“…For inversion free lasers the system should contain quantum "dark" states. Dark states in quantum photosynthesis were discussed in the literature [4], [6], [7] which supports the considered in the present paper approach.…”

Revealing and Characterizing Dark Excitons through Coherent Multidimensional Spectroscopy

“…For organic photovoltaics the extraction of energy necessarily involves a charge-transfer state in which the electron and hole are spatially separated and thus interact weakly with light [15][16][17][18]. A similar charge transfer state is crucial for charge separation in photosynthesis while recent proposals have suggested a role for other symmetry-forbidden dark states [19,20]. Our ability to gain a detailed understanding of these states and their roles is hampered by their weak interaction with light.…”

Stability and properties of quasi-stable conformational states in the LH2 light-harvesting complex of Rbl. acidophilus bacteria formed by hexacoordination of bacteriochlorophyll a magnesium atom

“…40 Due to the differences in protein surroundings and pigment-pigment interactions these two rings give rise to two separate bands in the absorption spectrum with the maxima near 800 and 850 nm (hence the names of the rings). The interaction of the BCLs between and within the rings leads to rapid excitation energy transfer on a sub-and picosecond scale within the complex [33,34,35]. 45 Similar structures, with C 9 and C 8 symmetries, respectively, were found for the LH2 complexes of Rh.…”

“…acidophila [29]) was determined by X-ray crystallography [30,31,32]. 35 This ring-like complex has C 9 symmetry and comprises 9 α,β-heterodimers of protein molecules connected to the pigments: a pair of closely located bacteriochlorophylls a (BCLs), one more BCL and a carotenoid, rhodopin glucoside (RG). Due to the symmetry of the complex BCLs form two rings: B850 ring, consisting of 18 closely packed BCLs, and a more sparce B800 ring (see Fig.…”