Direct visualization of exciton reequilibration in the LH1 and LH2 complexes of Rhodobacter sphaeroides by multipulse spectroscopy

Thomas A Cohen Stuart, Mikas Vengris, Vladimir I Novoderezhkin, Richard J Cogdell, C Neil Hunter, Rienk van Grondelle
Biophysical Journal 2011 May 4, 100 (9): 2226-33
The dynamics of the excited states of the light-harvesting complexes LH1 and LH2 of Rhodobacter sphaeroides are governed, mainly, by the excitonic nature of these ring-systems. In a pump-dump-probe experiment, the first pulse promotes LH1 or LH2 to its excited state and the second pulse dumps a portion of the excited state. By selective dumping, we can disentangle the dynamics normally hidden in the excited-state manifold. We find that by using this multiple-excitation technique we can visualize a 400-fs reequilibration reflecting relaxation between the two lowest exciton states that cannot be directly explored by conventional pump-probe. An oscillatory feature is observed within the exciton reequilibration, which is attributed to a coherent motion of a vibrational wavepacket with a period of ∼150 fs. Our disordered exciton model allows a quantitative interpretation of the observed reequilibration processes occurring in these antennas.

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