TCB Publications - Abstract

Xiche Hu, Thorsten Ritz, Ana Damjanović, and Klaus Schulten. Pigment organization and transfer of electronic excitation in the purple bacteria. Journal of Physical Chemistry B, 101:3854-3871, 1997.

HU97 Absorption of light by light harvesting complexes and transfer of electronic excitation to the photosynthetic reaction center (RC) constitutes the primary light harvesting process of photosynthesis. This process is investigated on the basis of an atomic level structure of the so-called photosynthetic unit of the photosynthetic bacterium Rb. sphaeroides. The photosynthetic unit combines in the intracytoplasmic membrane a nanometric assembly of three protein complexes: (i) the photosynthetic reaction center, (ii) a ring-shaped light harvesting complex LH-I, and (iii) multiple copies of a similar complex, LH-II. The unit has been modeled using the known structure of (i) and for (ii) a model, recently obtained and complexed appropriately with (i); for (iii) the structure of LH-II of Rs. molischianum is substituted. The model describes in detail the organization of chromophores involved in primary light absorption and excitation transfer: a hierarchy of ring-shaped chlorophyll aggregates which surround four centrally located chlorophylls of the photosynthetic reaction center. The chlorophylls involved in the overall transfer are found in a co-planar arrangements. On the basis of the modeled structure a quantum-mechanical description of the entire light harvesting process is developed. For this purpose an effective Hamiltonian is established a priori and then employed to describe the LH-II $\rightarrow$ LH-II $\rightarrow$ LH-I $\rightarrow$ RC cascade of excitation transfer. The transfer times calculated are in agreement with measured transfer times. The results suggest that excitons are the key carriers of the excitation transferred, i.e., electronic excitations are coherently delocalized in the photosynthetic unit. This suggestion is corroborated by an investigation of the effect of inhomogeneous broadening on the predicted excitons in LH-II and LH-I, and effect, which is found to be significant, but small. A particularly important role is played by the lowest energy excitons to which the circular arrangement of chlorophylls imparts vanishing oscillator strength. Despite the lack of oscillator strength the low energy excitons are well suited for exciton transfer on a sub-picosecond and picosecond time scale. The accessory chlorophylls of the photosynthetic reaction center are found to be critical for the LH-I $\rightarrow$ RC transfer which would take several hundred picoseconds without these chlorophylls.

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