Energy and electron transfer upon selective femtosecond excitation of pigments in membranes of Heliobacillus mobilis

Excitation energy transfer steps in membranes of Heliobacillus mobilis were directly monitored by transient absorption spectroscopy with a time resolution of 30 fs under selective excitation within the inhomogeneously broadened bacteriochlorophyll g Qy band. The initial anisotropy was found to be > 0.4, indicating that the pigments are excitonically coupled. After initial decay of this anisotropy in < 50 fs, major sub-picosecond components associated with spectral equilibration were identified, corresponding to uphill energy transfer with a 300 fs time constant (812 nm excitation) and downhill energy transfer with 100 and 500 fs components (770 nm excitation). These equilibrations are ascribed predominantly to single excitation transfer steps, as anisotropy measurements showed that equilibration within spectrally similar pigments occurs on the same time scale as spectral equilibration, a situation which contrasts with that in photosystem I. Downhill energy transfer occurs to a significant extent directly to an energetically heterogeneous population of excited states as well as in a sequential way via gradually lower-lying pools of bacteriochlorophyll g. This finding supports a description in which all pigments, including the blue-most absorbing, are spatially organized in a random way rather than in clusters of spectrally similar species. Spectral equilibration is not entirely completed prior to formation of the primary radical pair P798(+)A(0)(-), which was found to proceed in a multiexponential way (time constants of 5 and 30 ps). No indication for the formation of radical species other than P798(+)A(0)(-) on the time scale up to 100 ps was found.