SPECTRAL EQUILIBRATION AND PRIMARY PHOTOCHEMISTRY IN HELIOBACILLUS-MOBILIS AT CRYOGENIC TEMPERATURE

We performed multicolor femtosecond transient absorption measurements on membranes of the photosynthetic bacterium Heliobacillus mobilis at 20 K, by selective excitation at either the red or the blue extreme of the bacteriochlorophyll g Q(y) band, which is split in three spectral forms (Bchl g 778, 793, and 808) at low temperature. In contrast to r oom temperature, there is no observable uphill energy transfer upon ex citation at the red extreme. This provides a direct experimental confi rmation of the expected strong temperature dependence of uphill energy transfer in multichromophore systems. Upon excitation at the blue edg e, downhill energy transfer is observed on time ranges varying over 2 orders of magnitude and is discussed in terms of four distinct energy transfer processes: Bchl g 778 --> Bchl g 793* (similar to 50 fs);Bch l g 778 --> Bchl g 808* (similar to 400 fs); Bchl g 793* --> Bchl g 8 08 (similar to 1.4 ps); and within Bchl g 808* (similar to 7 ps). Sur prisingly, the amount of oxidized primary donor P798(+) formed on the time scale of picoseconds and tens of picoseconds was found to depend on the excitation conditions: trapping occurs mainly in similar to 80 ps and slower from directly excited Bchl g 808 and can additionally o ccur in a few picoseconds from Bchl g 778 and Bchl g 793* upon blue e xcitation. This finding implies that spectral equilibration is not com plete prior to charge separation and furthermore is inconsistent with a funnel model, in which P798 is surrounded by long-wavelength pigment s. More generally, we discuss to what extent our data bring constraint s on the spatial distribution of the different spectral forms of the p igments.