H. Bergström

Excitation-energy transport in the bacteriochlorophyll antenna systems of Rhodospirillum rubrum and Rhodobacter sphaeroides, studied by low-intensity picosecond absorption spectroscopy

Abstract We have studied the energy-transfer dynamics in chromatophores of Rhodospirillum rubrum and Rhodobacter sphaeroides (formerly called Rhodopseudomonas sphaeroides ) under conditions of closed and open reaction centers, using low-intensity picosecond infrared absorption recovery measurements. The possibilities of selective infrared excitation and probing as well as picosecond anisotropy decay measurements offered by this technique have allowed us to obtain a detailed picture of the transfer dynamics. Upon selective excitation of a bacteriochlorophyll (BChl)-protein complex (B800–850, B875 or B880) a very low initial value of the absorption anisotropy ( r (0) ≈ 0.1) is observed for the excited-state decays of BChl 850, BChl 875 and BChl 880. This is consistent with a very fast ( k ≈ 3 · 10 12 s −1 ) initial transfer of energy between similar bacteriochlorophyll molecules. We suggest that this fast transfer occurs within a minimum unit of 6–8 chromophores. Only direct excitation of BChl 800 shows highly polarized, very short-lived (1–2 ps) absorbance changes due to the excited state of BChl 800. On a slower time-scale, 5–10 ps, the energy then migrates between the units of 6–8 bacteriochlorophyll molecules. On a similar time-scale, 35–50 ps, an equilibration of the excitation density occurs between different pigment pools. Thus in Rb. sphaeroides there is an equilibration between B800–850 and B875 ( τ = 37 ± 4 ps) and in R. rubrum there is a similar process between B880 and B896 ( τ = 47 ± 4 ps). These slower processes are associated with a further decay of the anisotropy from the initial value of ≈ 0.1 to very low values (less than 0.01). Our results suggest that the protein-pigment complexes designated B800–850 and B880 are not spectrally homogeneous, but may consist of spectrally slightly different bacteriochlorophyll molecules.

Picosecond kinetics of radiationless relaxations of triphenyl methane dyes. Evidence for a rapid excited-state equilibrium between states of differing geometry

Abstract The photophysics of several triphenylmethane (TPM) dyes has been studied in n -alcohols and glycerol/water solutions. We have been able to measure the rate and characterize the viscosity dependence of several radiationless decay channels. We propose a kinetic scheme to account for our observations. Important points in this model are, the existence of more than one ground-state species (in some dyes), a rapid excited-state equilibrium between states of differing geometry and solvent-induced spectral shifts.

Temperature dependence of energy transfer from the long wavelength antenna BChl-896 to the reaction center in Rhodospirillum rubrum, Rhodobacter sphaeroides (w.t. and M21 mutant) from 77 to 177K, studied by picosecond absorption spectroscopy.

Decay of the bacteriochlorophyll excited state was measured in membranes of the purple bacteria Rhodospirillum (R.) rubrum, Rhodobacter (Rb.) sphaeroides wild type and Rb. sphaeroides mutant M21 using low intensity picosecond absorption spectroscopy. The excitation and probing pulses were chosen in the far red wing of the long wavelength absorption band, such that predominantly the minor antenna species B896 was excited. The decay of B896 was studied between 77 and 177K under conditions that the traps were active. In all species the B896 excited state decay is almost temperature independent between 100 and 177K, and probably between 100 and 300 K. In this temperature range the decay rates for the various species are very similar and close to 40 ps. Below 100 K this rate remains temperature independent in Rb. sphaeroides w. t. and M21, while in R. rubrum a steep decrease sets in. An analysis of this data with the theory of nuclear tunneling indicates an activation energy for the final transfer step from B896 to the special pair of 70cm-1 for R. rubrum and 30cm-1 or less for Rb. sphaeroides.

Energy transfer within the bacteriochlorophyll antenna of purple bacteria at 77 K, studied by picosecond absorption recovery

Abstract The dynamics of energy transfer within the bacteriochlorophyll antenna of Rhodobacter sphaeroides and Rhodospirillum rubrum, with closed reaction centers, has been studied at 77 K using low-intensity infrared picosecond absorption recovery. Measurements of isotropic decay as well as decay of induced anisotropy yielded a detailed picture of the energy transfer pathways in the two antenna systems. The BChl antenna of Rb. sphaeroides seems to consist of at least four different BChl a species: BChl 800, BChl 850, BChl 875, and BChl 896. Upon excitation of the highest-energy pigment, a transfer sequence towards lower energy is initiated. The transfer steps between the different pigment pools are characterized by the following time constants: BChl 800 → BChl 850, т = 2 ± 1 ps ; BChl 850 → BChl 875, т = 40 ± 5 ps ; BChl 875 → BChl 896, т = 20 ± 5 ps . Once the excitations are localized on B896 a slower quenching, т = 190 ± 10 ps , by closed reaction centers (P+) occurs. From measurements of decay of induced anisotropy it is further concluded that efficient transfer between BChl 800 molecules takes place on a time-scale comparable to the BChl 800 → BChl 850 transfer. A marked increase in anisotropy in the red wing of the absorption spectrum offers a clear evidence of the presence of the long-wavelength antenna component B896. The BChl antenna of R. rubrum is composed of two BChl a species, BChl 880, and BChl 896, and the energy transfer kinetics are observed to be very similar to the corresponding part of Rb. sphaeroides. Some evidence of further spectral inhomogeneity (apart from B896) or spectral shifts induced by excitation of the B880 antenna pigment was also obtained. Several possible models are discussed for the origin and organization of the B896 pigment.

Energy transfer dynamics of isolated B800–850 and B800–820 pigment-protein complexes of Rhodobacter sphaeroides and Rhodopseudomonas acidophila

We have compared the energy-transfer dynamics of different B800–850 and B800–820 isolated pigment-protein complexes. Picosecond absorption spectroscopy under annihilation-free conditions has been used to measure the energy-transfer rate from BChl 800 to BChl 850/820 at 296 and 77 K. It is found that at room temperature the BChl 800 → BChl 850/820 transfer is very fast (< 1 ps in all studied complexes). At 77 K in the type 1 B800–850 complex of Rhodobacter sphaeroides and Rhodopseudomonas acidophila the BChl 800 to BChl 850 transfer is about 1–2 ps, whereas in the other complexes the decay of the BChl 800 excited state occurs within less than 1 ps, also at 77 K. The observed subtle differences between the different complexes give insight into the fine details of energy transfer. The excited state of BChl 850/820 has a long lifetime (≈ 1 ns), but measurements of induced anisotropy reveal very fast energy transfer between more or less identical BChl 850 molecules.

Characterization of excitation energy trapping in photosynthetic purple bacteria at 77 K

We have studied the energy‐transfer dynamics in chromatophores of Rhodobacter sphaeroides and Rhodospirillum rubrum at 77 K, with functional charge separation. Using low‐intensity picosecond absorption recovery, we determined that transfer between the energetically low‐lying antenna component BChl896 and the special pair of the reaction center occurs with a time constant of 37 ps in Rb. sphaeroides and 75 ps in R. rubrum. Assuming that a Förster energy‐transfer mechanism applies to the process, this allows us to estimate the distance between BChl896 in the B875 complex and the special pair P870 in the reaction center to range between 26 and 39 Å in Rb. sphaeroides. Such a distance indicates that the BChl896 pigment and the special pair of the reaction center are at the minimum separation allowed by the size and shape of the reaction center and the light‐harvesting polypeptides.

Energy transfer within the isolated B875 light-harvesting pigment-protein complex of Rhodobacter sphaeroides at 77 K studied by picosecond absorption spectroscopy

The energy transfer dynamics at 77 K within detergent solubilized and purified preparations of the B875 pigment‐protein complex of Rhodobacter sphaeroides have been investigated by picosecond absorption spectroscopy. Isotropic absorption recovery and decay of induced absorption anisotropy provide clear evidence that B875 is inhomogeneous in these preparations. We interpret the results as fast (τ = 15 ± 5 ps) energy transfer from the major BChl 875 pigments to a minor pigment pool, B896. The excited state of B896 decays with a time constant of 650 ± 50 ps. We suggest that B896 is intrinsic to B875 complexes and exists in a highly organized state, close to the reaction center. In the intact membrane, B896 may concentrate excitation energy in the vicinity of the reaction center special pair, thereby increasing the efficiency of the final energy transfer step.

Energy-transfer dynamics in three light-harvesting mutants of Rhodobacter sphaeroides: a picosecond spectroscopy study.

Picosecond absorption spectroscopy has been used to investigate energy-transfer dynamics within the LH1 and LH2 light-harvesting complexes of three mutants of Rhodobacter sphaeroides. We demonstrate that both complexes are inhomogeneous; each contains a specialized pigment pool which absorbs maximally at a longer wavelength. Within LH2 (mutant NF57), Bchl850 transfers energy to Bchl870 in 39 +/- 9 ps; within LH1 (mutants M21 and M2192), energy is transferred from Bchl875 to Bchl896 in 22 +/- 4 and 35 +/- 5 ps, respectively. Examination of the decay of induced absorption anisotropy indicates that each of these specialized pools exists in a state which is highly organized with respect to the remainder of the pigments. Such an arrangement may facilitate and direct energy migration toward the reaction center.

Energy transfer within the isolated light-harvesting B800-850 pigment-protein complex of Rhodobacter sphaeroides

We have used picosecond absorption spectroscopy with low intensity (5 · 1011–5 · 1012 photons · pulse−1 · cm−2) continuously tunable infrared (800–900 nm) pulses to study the energy transfer dynamics in the isolated B800–850 pigment-protein complex of Rhodobacter sphaeroides. Our results suggest the following picture of the energy transfer dynamics: (i) a fast transfer, within approx. 1 ps, from BChl 800 to BChl 850; (ii) transfer among different BChl 800s with a rate which is at the most of the same order of magnitude as that of BChl 800 → BChl 850 transfer; (iii) very fast transfer (k > 1 · 1012 s−1) between BChl 850 molecules. Assuming Forster type of energy transfer maximum distances of about 22 and 15 A are obtained for the BChl 800–BChl 850 and BChl 850–BChl 850 separations, respectively.

Picosecond absorption studies of intermolecular electronic energy transfer in micellar systems. II. Polarization-dependent studies of energy migration and energy trapping at low donor excitation densities

Abstract A picosecond excite-and-probe beam investigation on intermolecular electronic energy transfer (EET) between dye molecules in one-component systems (electronic energy migration - EM) and two-component systems (electronic energy trapping - ET) is presented and compared with existing theories for EET in restricted volumes. The experiments were carried out on a transient absorption spectrometer based on synchronously pumped dye laser (pulse duration 5 ps). Donor excitation density was less than 1%. In the case of ET good agreement has been obtained with the Forster theory, in which in addition an inhomogeneous spatial distribution of the dye molecules has been considered (part I of this study). In the case of EM direct evidence for the theory of Ediger and Fayer has been obtained by measuring the polarization anisotropy at early times after excitation.