Inter- and Intraband Energy Transfer in LH2-Antenna Complexes of Purple Bacteria. A Fluorescence Line-Narrowing and Hole-Burning Study

Abstract

High-resolution site-selection fluorescence- and hole-burning spectroscopy were used to study energy transfer in two LH2 light-harvesting complexes of purple bacteria: the B800-850 complex of isolated Rb. sphaeroides and the B800-820 complex of Rps. acidophila, at 1.2 K. Fluorescence spectra, hole widths, and hole depths were measured as a function of excitation wavelength [lambda][sub exc] within the B800 band. For [lambda][sub exc] [>=] 798 nm, fluorescence line-narrowing is observed and the energy-transfer times ([tau] = 2.5 and 2.0 ps for B800-850 and B800-820, respectively) are independent of [lambda][sub exc]. In this spectral region only interband B800 [yields] B850 (B820) energy transfer takes place. For 780 nm [<=] [lambda][sub exc] [<=] 798 nm, the fluorescence bands are broad and the transfer time, obtained from hole widths extrapolated to zero burning-fluorence density, decreases toward the blue side of B800. In this wavelength region competition occurs between B800 [yields] B850 (B820) and B800 [yields] B800 [open quotes]downhill[close quotes] energy transfer. 36 refs., 5 figs.