Efficiency of excitation trapping by reaction centres of complex B890 from Chromatium minutissimum

Abstract

The formation and decay of excited states of light-harvesting bacteriochlorophyll (BChl) molecules and primary charge separation in reaction centres (RCs) of pigment-protein complex B890 of the purple bacterium Chromatium minutissimum were studied as a function of excitation wavelength by means of picosecond absorbance difference spectroscopy. Selective excitation in the RC absorption band (at 800 nm) induced rapid bleaching of the absorption band of the RC BChl dimer (τ ∼ 1 ps) without absorption changes due to excited light-harvesting BChl. It is concluded that excitation energy transfer from the BChl dimer of the RC to light-harvesting BChl molecules is considerably slower than RC charge separation. Therefore, energy transfer from the light-harvesting antenna to the RC in bacterial photosynthesis may be described as a ‘migration-limited model’.