We report studies ofenergy transfer from the 800-nm absorbing pigment (B800) to the 850-nm absorbing pigment (B850) of the LH2 peripheral antenna complex and from LH2 to the core antenna complex (LH1) in Rhodobacter (Rb.) sphaeroides. The B800 to B850 process was studied in membranes from a LH2-reaction center (no LH1) mutant of Rb. sphaeroides and the LH2 to LH1 transfer was studied in both the wild-type species and in LH2 mutants with blueshifted B850. The measurements were performed by using '100-fs pulses to probe the formation of acceptor excitations in a two-color pump-probe measurement. Our experiments reveal a B800 to B850 transfer time of -0.7 ps at 296 K and energy transfer from LH2 to LH1 is characterized by a time constant of -3 ps at 296 K and -5 ps at 77 K. In the blue-shifted B850 mutants, the transfer time from B850 to LH1 becomes gradually longer with increasing blue-shift of the B850 band as a result of the decreasing spectral overlap between the antennae. The results have been used to produce a model for the association between the ring-like structures that are characteristic of both the LH2 and LH1 antennae.Organization and function of the core antenna complex (LH1) and the peripheral antenna complex (LH2) of purple bacteria have been extensively studied in the past (1, 2, 29). A major step forward in this work was taken very recently when the threedimensional structure of a complex of the peripheral antenna absorbing at 800 nm (B800) and at 850 nm (B850) from Rhodopseudomonas (Rps.) acidophila was solved to high resolution showing a ninefold circular symmetry of a13 pairs (3). From this structure of LH2 and the similar circular structure of LH1 (4), it has become clear that the pigment density in these light harvesting pigments is very high, leading to short intermolecular distances and strong dipole-dipole interactions. Presently, two modes of energy transfer are considered, incoherent F6rster transfer or exciton state relaxation (5-8), and it is a challenge for future research to establish the levels of organization at which the two modes of energy transfer are operative. Until experiments and theory have produced a unified description of the energy transfer dynamics, we have chosen to describe the energy transfer steps within LH2 and LH1 and between the complexes as incoherent Forster hopping. Early work on energy transfer dynamics in photosynthetic purple bacteria (9-16) yielded information about the overall exciton lifetime in the antenna and provided a time scale for energy equilibration within individual complexes (9, 10) and over the whole antenna (11,16). In particular, the LH2 antenna is probably the most extensively studied complex. Picosecond absorption and fluorescence studies performed at room and low temperature on the LH2 pigmentprotein complex of Rhodobacter (Rb.) sphaeroides revealed that B800 -> B850 energy transfer is a very fast and temperature-The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereb...

Hole-burned absorption and line-narrowed fluorescence spectra along with nanosecond fluorescence decay kinetics have been studied at 5 K in core LH1 and peripheral LH2 antenna complexes isolated from the photosynthetic purple bacterium Rhodobacter sphaeroides. A dual nature for the respective emission bands has been confirmed in both complexes and has been assigned to the nearly free excitons weakly coupled to lattice vibrations and to the strongly coupled self-trapped excitons. The apparent phonon structure of quasi-free excitons has been analyzed resulting in a total Huang−Rhys factor, a characteristic of the electron−phonon coupling strength, equal to S = 0.85 ± 0.10 in LH1 and to S = 1.05 ± 0.10 in LH2. An estimate for self-trapped excitons is a few times larger. Excitonic polarons are thus proper excitations in LH1 and LH2 complexes, as the electron−phonon coupling cannot be ignored.

High-spectral-resolution hole-burning and fluorescence line-narrowing spectra of excitons in LH2 complexes from the photosynthetic purple bacterium Rhodobacter sphaeroides have been investigated together with conventional broadband fluorescence spectra and their temperature dependence. The steady-state spectroscopy has been complemented by fluorescence lifetime measurements. The experimental results are discussed on the basis of the adiabatic Holstein exciton polaron model, modified by including diagonal disorder. As a result, a new interpretation for the LH2 antenna optical spectra is provided. The exciton when optically excited becomes localized after relaxation. The LH2 fluorescence is mainly due to self-trapped excitons not only at low temperature, as previously suggested (Timpmann, K.; Katiliene, Z.; Woodbury, N. W.; Freiberg, A. J. Phys. Chem. B 2001, 105, 12223), but also over the whole temperature range up to physiological temperatures because the self-trapped exciton binding energy is of the same order as the thermal excitation energy at ambient temperature. The conclusion is made that direct self-trapping relaxation dominates the common energy relaxation between exciton states and that the main factor limiting the relaxed exciton size is dynamic rather than static disorder. The coexistence of large and small exciton polarons at low temperatures has been confirmed. Exciton self-trapping also essentially modifies the long-wavelength tail of the absorption spectrum of LH2 complexes. The fraction of the absorption spectrum that is subject to hole burning is due to large-radius self-trapped excitons that are weakly coupled to the lattice. The rest of this spectrum that survives hole burning belongs to the strongly coupled self-trapped excitons/excimers. Implications of these results on the interpretation of Stark spectroscopy experiments as well as on photosynthetic energy transfer and trapping are discussed.