INTERBAND AND INTRABAND ENERGY-TRANSFER IN LH2-ANTENNA COMPLEXES OF PURPLE BACTERIA - A FLUORESCENCE LINE-NARROWING AND HOLE-BURNING STUDY

High-resolution site-selection fluorescence- and hole-burning spectros copy were used to study energy transfer in two LH2 light-harvesting co mplexes of purple bacteria: the B800-850 complex of isolated Rb. sphae roides and the B800-820 complex of Rps. acidophila, at 1.2 K. Fluoresc ence spectra, hole widths, and hole depths were measured as a function of excitation wavelength lambda(exc) within the B800 band. For lambda (exc) greater than or equal to 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(exc). In thi s spectral region only interband B800 --> B850 (B820) energy transfer takes place. For 780 nm less than or equal to lambda(exc) less than or equal to 798 nm, the fluorescence bands are bread and the transfer ti me, obtained from hole widths extrapolated to zero burning-flurence de nsity, decreases toward the blue side of B800. In this wavelength regi on competition occurs between B800 --> B850 (B820) and B800 --> B800 ' 'downhill'' energy transfer. For lambda(exc) less than or equal to 780 nm, the broad fluorescence bands, with maxima at lambda(em) similar t o 805 nm, become independent of lambda(exc) and intraband B800 --> B80 0 transfer combined with excited-state vibrational relaxation are the dominant processes. The spectral distribution of the most-red absorbin g pigments within the B800 band, which transfer energy exclusively fro m B800 to B850 (B820), was determined from the depth of the hole versu s lambda(exc). The results indicate that one-third of the B800 pigment s transfer their energy only to B850 (B820), from which it is conclude d that the minimal functional LH2-unit consists of at least three B800 pigments and six B850 pigments, in addition to carotenoids.