Abstract

The purple bacterium Rps. viridis is a Bchl-b containing species that contains only one type of antenna, the LH-1 complex. Electron crystallography has revealed that the photosynthetic membrane of this bacterium contains a highly ordered array of core complexes each consisting of a reaction centre surrounded by a ring of Bchl-b binding antenna subunints [1]. For Bchl-a containing photosynthetic membranes it has been shown using sub-picosecond spectroscopy that both transfer between different antenna complexes and very fast transfer between the pigments of the same complex occurs. This transfer is expressed as a shift of the observed bleaching spectrum [2,3] and depolarisation of the spontaneous emission signal due to transfer between differently oriented pigments [4]. Although there are large homologies between the Bcl-a containing bacteria and Rps. viridis the spectral properties of their LH-1 complexes are rather different [5]. At least at low temperature the LH-1 antenna of Rps. viridis is heterogeneous. These deviating spectral properties make this antenna system interesting to study with sub-picosecond spectroscopy. The timeresolved experiments presented were performed with an amplified Ti:sapphire laser system driving an optical parametric amplifier (OPA). The regenerative amplifier (REGA) produces 4 to 5 μJ pulses of about 180 fs FWHM with a repetition rate of 200 kHz. 25% of the output is used to generate white light in a thin sapphire slab. After selecting the proper wavelength range with bandpass filters, part of the continuum is used as a probe pulse. The remaining 75% of the REGA output is doubled and pumps a two pass OPA that is seeded with the remaining white light continuum. For our experiments the idler frequency was selected from the output of the OPA and used to excite the sample. The detection wavelength was selected with a monochromator behind the sample (bandwidth 5 nm). The cross correlation of the pump and probe pulse was about 100 fs. Membrane fragments were prepared as described before [5].

© 1996 Optical Society of America