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

Abstract 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.