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 room temperature, there is no observable uphill energy transfer upon excitation at the red extreme. This provides a direct experimental confirmation of the expected strong temperature dependence of uphill energy transfer in multichromophore systems. Upon excitation at the blue edge, 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* (approximately 50 fs); Bchl g 778* --> Bchl g 808* (approximately 400 fs); Bchl g 793* --> Bchl g 808* (approximately 1.4 ps); and within Bchl g 808* (approximately 7 ps). Surprisingly, 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 approximately 80 ps and slower from directly excited Bchl g 808* and can additionally occur in a few picoseconds from Bchl g 778* and Bchl g 793* upon blue excitation. This finding implies that spectral equilibration is not complete prior to charge separation and furthermore is inconsistent with a funnel model, in which P798 is surrounded by long-wavelength pigments. More generally, we discuss to what extent our data bring constraints on the spatial distribution of the different spectral forms of the pigments.