دانلود رایگان مقاله اثرات طیفی و جنبشی همراه با مونتاژ مجتمع هسته رودوباکتر اسفروئیدیس

Abstract

In the present work, spectral and kinetic changes accompanying the assembly of the light-harvesting 1 (LH1) complex with the reaction center (RC) complex into monomeric RC-LH1 and dimeric RC-LH1-PufX core complexes of the photosynthetic purple bacterium Rhodobacter sphaeroides are systematically studied over the temperature range of 4.5–300 K. The samples were interrogated with a combination of optical absorption, hole burning, fluorescence excitation, steady state and picosecond time resolved fluorescence spectroscopy. Fair additivity of the LH1 and RC absorption spectra suggests rather weak electronic coupling between them. A low-energy tail revealed at cryogenic temperatures in the absorption spectra of both monomeric and dimeric core complexes is proved to be due to the special pair of the RC. At selected excitation intensity and temperature, the fluorescence decay time of core complexes is shown to be a function of multiple factors, most importantly of the presence/absence of RCs, the supramolecular architecture (monomeric or dimeric) of the complexes, and whether the complexes were studied in a native membrane environment or in a detergent - purified state

4. Conclusions

In the present work, we systematically studied and interpreted the spectral and kinetic changes accompanying the assembly of the LH1 complex with the RC complex into core complexes of the photosynthetic purple bacterium Rba. sphaeroides. One of the prime motivations of this work was that the many literature data on the excitation energy transfer and trapping in photosynthetic bacterial membranes appear fragmented, irregular, and sometimes even controversial. This unsatisfactory situation is frequently vaguely assigned to “biological variability”. The present work was set to make an order in at least some of the data by systematically studying a range of complexes from one and the same species, and using similar, carefully selected and controlled experimental conditions. As a result, the multiple factors that influence most the fluorescence lifetime of the core complexes, both in detergent-isolated and membrane-embedded form, were established, evaluated, and discussed, providing a more comprehensive understanding of the field. In the context of the current debate about the origin and the role of low-energy states in photosynthesis [51] we discovered a low-energy absorption tail present in the core complexes complete with the LH1 and RC complexes but absent in lone LH1 complexes. This tail, being accessible only at low temperatures, was identified as the special pair absorption band of the RC. We further showed that dimeric core complexes are more efficient quenchers of antenna exciton polarons than monomeric cores, confirming the recent discovery on full mutant chromatophore membranes of Rba. sphaeroides.