Ultrafast dynamics of excitation energy transfer and trapping in Bchl a and Bchl b-containing photosynthetic bacteria

Abstract

Using IR 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 centre. In bacteriochlorophyll (Bchl) a-containing purple bacteria the light-harvesting antenna is highly heterogeneous, consisting of several spectroscopically distinct pigments. Energy transfer between 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 light-harvesting antenna only takes approximately 10 ps (measured at 77 K). This implies that trapping is relatively slow, approximately 35 ps at 77 K, and probably not much faster at room temperature. Energy transfer between neighbouring Bchl molecules within a pigment—protein complex occurs on the 1 ps to subpicosecond time scale. For the Bchl b-containing purple bacterium Rhodopseudomonas viridis the results show that the antenna appears homogeneous at room temperature and 77 K, and that trapping appears to be very fast, and approximately described by a random-walk model using known spectral properties of the antenna and reaction centre also appear to be much more tightly coupled than in the Bchl a-containing purple bacteria.