Excitation energy transfer in the light-harvesting antenna of Rhodospirillum rubrum was studied at room temperature using sub-picosecond transient absorption measurements. Upon excitation of Rs. rubrum membranes with a 200 fs, 600 nm laser flash in the Qx transition of the bacteriochlorophyll-a (BChl-a) absorption, the induced transient absorption changes in the Qy region were monitored. In Rs. rubrum membranes the observed delta OD spectrum exhibits ground state bleaching, excited state absorption and stimulated emission. Fast Qx --> Qy relaxation occurs in approximately 100-200 fs as reflected by the building up of stimulated emission. An important observation is that the zero-crossing of the transient difference absorption (delta OD) spectrum exhibits a dynamic redshift from 863 to 875 nm that can be described with by a single exponential with 325 fs time constant. The shape of the transient difference spectrum observed in a purified subunit of the core light-harvesting antenna, B820, consisting of only a single interacting pair of BChl-as, is similar to the spectrum observed in Rs. rubrum membranes and clearly different from the spectrum of BChl-a in a protein/detergent mixture. In the B820 and monomeric BChl-a preparations the 100-200 fs Qx --> Qy relaxation is still observed, but the dynamic redshift of the delta OD spectrum is absent. The spectral kinetics observed in the Rs. rubrum membranes are interpreted in terms of the dynamics of excitation equilibration among the antenna subunits that constitute the inhomogeneously broadened antenna. A simulation of this process using a set of reasonable physical parameters is consistent with an average hopping time in the core light harvesting of 220-270 fs, resulting in an average single-site excitation lifetime of 50-70 fs. The observed rate of this equilibration process is in reasonable agreement with earlier estimations for the hopping time from more indirect measurements. The implications of the findings for the process of excitation trapping by reaction centers will be discussed.
The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray¯uorescence from very dilute systems, weak uorescence transitions or X-ray Raman scattering. Transition-metal K¯uorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weaker features of the K spectrum resulting from valence-level and`interatomic' ligand to metal transitions contain detailed information on the ligandatom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive lowenergy (<600 eV) X-ray absorption ®ne structure (XAFS) spectra using hard ($10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption ®ne structure (EXAFS) features. XRS ®nds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated complex systems, reactive compounds and samples under extreme conditions (pressure, temperature). Recent results are discussed.
Using pump-probe spectroscopy, the dynamics of energy transfer within the inhomogeneously broadened light-harvesting antenna of LH-1-only mutants of the photosynthetic bacterium Rhodobacter sphaeroides was studied at room temperature and 4.2 K. In both cases, the transient difference spectra shift approximately 140 cm -1 to lower energy, with most of the shift occurring within the first picosecond after excitation. Employing an inhomogeneous distribution for the excited-state energies of the subunits in the LH-1 antenna and using a weak coupling (Fo ¨rster-type) energy-transfer mechanism, the observations can well be simulated with a transfer time between optimally overlapping antenna subunits at a single lattice distance of approximately 0.28 ps at room temperature (RT) and 0.40 ps at 4.2 K. We find that the fwhm width of the inhomogeneous distribution function decreases from 400 cm -1 at RT to 200 cm -1 at 4.2 K.
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