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

Marco Ferrettia, Ruud Hendrikxa, Elisabet Romeroa, June Southallb, Richard J. Cogdellb, Vladimir I. Novoderezhkinc, Gregory D. Scholesd, Rienk van Grondellea
aDepartment of Physics and Astronomy, VU University, 1081 HV Amsterdam, The Netherlands; bDivision of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK; cA. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, 119991 Moscow, Russia; dDepartment of Chemistry, Princeton University, Washington Rd, Princeton NJ 08544, USA

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.