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Steady-state spectroscopy of zinc-bacteriopheophytin containing LH1 - an in vitro and in silico study
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By reversible dissociation of the light-harvesting complex 1 (LH1) of Rhodospirillum rubrum it is possible to (partly) exchange the bacteriochlorophyll (BChl) a by zinc-bacteriopheophytin (Zn-BPheo). After reassociation of the protein a complex is formed which can have different percentages of Zn-BPheo bound to the polypeptides [Biochemistry 39 (2000) 1091].Low-temperature absorption spectra show a shift of the absorption maximum from 886 to 863 nm, when the native LH1 complex is compared to a modified complex containing 90% Zn-BPheo, whereas the positions of the absorption maxima of BChl a and Zn-BPheo differ by only 6 nm, when the pigments are bound to the isolated polypeptides. Using an exciton model with static disorder of site energies based on the ring-like structure of LH1 we can describe this shift by assuming a difference in the excitonic coupling originating solely from the different dipole strength of the exchanged Zn-BPheo compared with the original BChl a. We estimate that in LH1 the nearest-neighbour interaction energy of two BChl a molecules is around 400 cm(-1) and the diagonal disorder is around 600 cm-1.Furthermore, we determined if the energy transfer in pigment-modified complexes is similar to native LH1. This can be observed by selectively exciting electronic states depending on their energy and measuring the polarized emission spectra at low temperature. The native complex was compared to a complex containing 70% Zn-BPheo. The fluorescence anisotropy and the shift of the emission maximum in native LH1 and 70%-Zn-BPheo-LH1 depending on the excitation wavelength can generally be described within the same disordered exciton model, extended in a simple way with line shapes. The model proved to be simple and robust when applied to these engineered light-harvesting complexes.
photosynthesis, pigment-protein complex, light harvesting, excitons, modelling, LIGHT-HARVESTING COMPLEX, PHOTOSYNTHETIC REACTION CENTERS, B820 SUBUNIT FORM, RHODOBACTER-SPHAEROIDES, ANTENNA COMPLEX, RHODOSPIRILLUM-RUBRUM, RHODOPSEUDOMONAS-ACIDOPHILA, PURPLE BACTERIA, ENERGY-TRANSFER, PIGMENT ORGANIZATION
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