Ultrafast dynamics of excitation energy transfer and trapping in bacterial photosynthesis

Abstract

With infrared transient absorption spectroscopy we have studied how energy migrates through the light-harvesting antenna of photosynthetic purple bacteria, and how the energy is trapped by the reaction center. In Bchl a-containing purple bacteria the light-harvesting (LH) antenna is highly heterogeneous, consisting of several spectroscopically distinct pigments. Energy transfer among these pigments occurs on several different time-scales. The overall trapping time at room temperature is about 60 ps, while energy transfer from the high-energy to the low-energy pigments of the lightharvesting antenna only takes 10 ps (measured at 77 K). This implies that trapping is relatively slow, 35 ps at 77 K and probably not much faster at room temperature. Energy transfer among neighboring Bchl molecules within a pigment-protein complex occurs on the 1 ps to subpicosecond time scale. For the Bchl b-containing purple bacterium Rps.viridis the results show that the antenna is homogeneous, and trapping can approximately be described by a randomwalk model, provided that the light-harvesting antenna and reaction center are more tightly coupled than in the Bchl a containingpurple bacteria.