Femtosecond energy transfer within the LH2 peripheral antenna of the photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas palustris LL

The ultrafast energy transfer among the pigments of the B800-850 membrane antenna pigment-protein complex of the photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas palustris LL has been studied with ≈ 100 fs tunable infrared pulses at room temperature and 77 K. It is shown that the B800→B850 transfer time is similar in both species and occurs with a characteristic time constant of 0.6-0.8 ps at room temperature and 2.4-2.6 ps at 77 K. Measurements of absorption anisotropy in the 800 nm band shows that the depolarizing energy transfer among B800 molecules is slower than the B800→B850 transfer. At room temperature such a transfer occurs with a time constant of 0.8-1.6 ps and at 77 K with a time constant much longer than the excited state lifetime. A fast ≈ 300 fs non-depolarizing, almost temperature-independent, relaxation process is also observed within the B800 band. Energy transfer between a pair of almost parallel B800 molecules and possibly also vibrational relaxation are discussed as the possible origins of this process.