Broadband 2D Electronic Spectroscopy Reveals a Carotenoid Dark State in Purple Bacteria

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Science  05 Apr 2013:
Vol. 340, Issue 6128, pp. 52-56
DOI: 10.1126/science.1230106

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A Light in the Dark

Carotenoids enhance the efficiency of photosynthesis by absorbing light at wavelengths inaccessible to chlorophyll. However, the mechanism underlying transfer of the absorbed energy from these pigments to the site of chemical catalysis is unclear. Studies have debated the involvement of a “dark” state in the carotenoid electronic structure—a state inaccessible by direct absorption populated by partial relaxation of the initial excited state. Ostroumov et al. (p. 52) used two-dimensional electronic spectroscopy to provide evidence of this state.


Although the energy transfer processes in natural light-harvesting systems have been intensively studied for the past 60 years, certain details of the underlying mechanisms remain controversial. We performed broadband two-dimensional (2D) electronic spectroscopy measurements on light-harvesting proteins from purple bacteria and isolated carotenoids in order to characterize in more detail the excited-state manifold of carotenoids, which channel energy to bacteriochlorophyll molecules. The data revealed a well-resolved signal consistent with a previously postulated carotenoid dark state, the presence of which was confirmed by global kinetic analysis. The results point to this state’s role in mediating energy flow from carotenoid to bacteriochlorophyll.

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