Femtosecond and Hole-Burning Studies of B800's Excitation Energy Relaxation Dynamics in the LH2 Antenna Complex ofRhodopseudomonas acidophila(Strain 10050)

Abstract

One- and two-color pump/probe femtosecond and hole-burning data are reported for the isolated B800−850 (LH2) antenna complex of Rhodopseudomonas acidophila (strain 10050). The two-color profiles are interpretable in terms of essentially monophasic B800→B850 energy transfer with kinetics ranging from 1.6 to 1.1 ps between 19 and 130 K for excitation at or to the red of the B800 absorption maximum. The B800 zero-phonon hole profiles obtained at 4.2 K with burn frequencies located near or to the red of this maximum yielded a transfer time of 1.8 ps. B800 hole-burning data (4.2 K) are also reported for chromatophores at ambient pressure and pressures of 270 and 375 MPa. At ambient pressure the B800−B850 energy gap is 950 cm-1, while at 270 and 375 MPa it is close to 1000 and 1050 cm-1, respectively. However, no dependence of the B800→B850 transfer time on pressure was observed, consistent with data for the B800−850 complex of Rhodobacter sphaeroides. The resilience of the transfer rate to pressure-induced changes in the energy gap and the weak temperature dependence of the rate are consistent with the model that has the spectral overlap (of Förster theory) provided by the B800 fluorescence origin band and weak vibronic absorption bands of B850. However, both the time domain and hole-burning data establish that there is an additional relaxation channel for B800, which is observed when excitation is located to the blue of the B800 absorption maximum. Several explanations for this faster channel are considered, including that it is due to intra-B800 energy transfer or a manifestation of coupling of B800 with quasi-degenerate upper exciton levels of the B850 molecules. The data indicate that it is not due to vibrational relaxation.