Excitation-energy transport in the bacteriochlorophyll antenna systems of Rhodospirillum rubrum and Rhodobacter sphaeroides, studied by low-intensity picosecond absorption spectroscopy

Abstract

We have studied the energy-transfer dynamics in chromatophores of Rhodospirillum rubrum and Rhodobacter sphaeroides (formerly called Rhodopseudomonas sphaeroides) under conditions of closed and open reaction centers, using low-intensity picosecond infrared absorption recovery measurements. The possibilities of selective infrared excitation and probing as well as picosecond anisotropy decay measurements offered by this technique have allowed us to obtain a detailed picture of the transfer dynamics. Upon selective excitation of a bacteriochlorophyll (BChl)-protein complex (B800–850, B875 or B880) a very low initial value of the absorption anisotropy ( r(0) ≈ 0.1) is observed for the excited-state decays of BChl 850, BChl 875 and BChl 880. This is consistent with a very fast ( k ≈ 3 · 10 12 s −1) initial transfer of energy between similar bacteriochlorophyll molecules. We suggest that this fast transfer occurs within a minimum unit of 6–8 chromophores. Only direct excitation of BChl 800 shows highly polarized, very short-lived (1–2 ps) absorbance changes due to the excited state of BChl 800. On a slower time-scale, 5–10 ps, the energy then migrates between the units of 6–8 bacteriochlorophyll molecules. On a similar time-scale, 35–50 ps, an equilibration of the excitation density occurs between different pigment pools. Thus in Rb. sphaeroides there is an equilibration between B800–850 and B875 ( τ = 37 ± 4 ps) and in R. rubrum there is a similar process between B880 and B896 ( τ = 47 ± 4 ps). These slower processes are associated with a further decay of the anisotropy from the initial value of ≈ 0.1 to very low values (less than 0.01). Our results suggest that the protein-pigment complexes designated B800–850 and B880 are not spectrally homogeneous, but may consist of spectrally slightly different bacteriochlorophyll molecules.