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The ultrafast carotenoid band shift upon excitation of nearby bacteriochlorophyll molecules was studied in three different light harvesting complexes from purple bacteria. The results were analyzed in terms of changes in local electric field of the carotenoids. Time dependent density functional theory calculations based on known and model structures led to good agreement with experimental results, strongly suggesting that the mutual orientation of the pigment molecules rather than the type of the carotenoid molecules determines the extent of the ultrafast band shift. We further estimate that the protein induced local field nearby carotenoid molecule is about 4 or 6 MV/cm, depending on the orientation of the change of the electrical dipole in the carotenoid upon optical transition.

Using near-infrared femtosecond absorption spectroscopy, we have determined the S1 energies of the carotenoids spheroidene and rhodopin glucoside in LH2 complexes of purple bacteria. The S1 energies in the LH2 complexes yield values of 13400 ± 100 cm-1 for spheroidene and 12550 ± 150 cm-1 for rhodopin glucoside, which are very close to the S1 energies obtained for both carotenoids in solution. The 850 cm-1 difference between the S1 energies of these two carotenoids significantly affects the energy transfer pathways within the LH2 complexes. The S1 energy of spheroidene in the LH2 complex of Rhodobacter (Rb.) sphaeroides is high enough to allow efficient energy transfer from the S1 state to bacteriochlorophylls, resulting in a substantial shortening of the spheroidene S1 lifetime in the LH2 complex (1.7 ps) compared with the lifetime in solution (8.5 ps). Rhodopin glucoside, which occurs in Rhodopseudomonas (Rps.) acidophila, has an S1 energy in the LH2 complex too low for efficient S1-mediated energy tran...

LH2 complexes from Rb. sphaeroides were modified genetically so that lycopene, with 11 saturated double bonds, replaced the native carotenoids which contain 10 saturated double bonds. Tuning the S1 level of the carotenoid in LH2 in this way affected the dynamics of energy transfer within LH2, which were investigated using both steady-state and time-resolved techniques. The S1 energy of lycopene in n-hexane was determined to be approximately 12 500 +/- 150 cm(-1), by direct measurement of the S1-S2 transient absorption spectrum using a femtosecond IR-probing technique, thus placing an upper limit on the S1 energy of lycopene in the LH2 complex. Fluorescence emission and excitation spectra demonstrated that energy can be transferred from lycopene to the bacteriochlorophyll molecules within this LH2 complex. The energy-transfer dynamics within the mutant complex were compared to wild-type LH2 from Rb. sphaeroides containing the carotenoid spheroidene and from Rs. molischianum, in which lycopene is the native carotenoid. The results show that the overall efficiency for Crt --> B850 energy transfer is approximately 80% in lyco-LH2 and approximately 95% in WT-LH2 of Rb. sphaeroides. The difference in overall Crt --> BChl transfer efficiency of lyco-LH2 and WT-LH2 mainly relates to the low efficiency of the Crt S(1) --> BChl pathway for complexes containing lycopene, which was 20% in lyco-LH2. These results show that in an LH2 complex where the Crt S1 energy is sufficiently high to provide efficient spectral overlap with both B800 and B850 Q(y) states, energy transfer via the Crt S1 state occurs to both pigments. However, the introduction of lycopene into the Rb. sphaeroides LH2 complex lowers the S1 level of the carotenoid sufficiently to prevent efficient transfer of energy to the B800 Q(y) state, leaving only the Crt S1 --> B850 channel, strongly suggesting that Crt S1 --> BChl energy transfer is controlled by the relative Crt S1 and BChl Q(y) energies.

The multiexciton density matrix theory is utilized to achieve a microscopic description of exciton−exciton annihilation (EEA). We apply the theory to the 18 bacteriochlorophyll (BChl) molecules of the B850 ring of the light-harvesting complex LH2 of Rhodobacter sphaeroides. The simulation of the EEA process reproduces the intensity-dependent transient absorption kinetic experiments very well, and insight is obtained on microscopic parameters such as the internal conversion rate of BChl in LH2. The exciton dynamics and the different relaxation processes are visualized by constructing a multiexciton spectrogram.

Using two-color pump−probe femtosecond spectroscopy, the temperature dependence of the energy transfer rate within the peripheral light-harvesting antenna (LH2) of the photosynthetic bacterium Rhodobacter sphaeroides has been measured. The energy transfer time from B800 to B850 is determined to be 0.7, 1.2, and 1.5 ps at 300, 77, and 4.2 K, respectively. These data, combined with earlier results, have been analyzed with regard to the crystal structure and spectroscopic properties of the purple bacterial LH2 complex. We conclude that the transfer within B800 occurs mainly via the incoherent Forster hopping mechanism. For B800 to B850 transfer, estimates based on the Forster formula yield considerably slower transfer times than experimentally observed, suggesting that an additional mechanism may be involved in enhancing the transfer rate. We suggest two possibilities: transfer via the upper excitonic component of B850 band and/or transfer mediated by a carotenoid molecule.

A new channel of excitation energy deactivation in bacterial light harvesting was recently discovered, which leads to carotenoid triplet population on an ultrafast timescale. Here we show that this mechanism is also active in LH2 of Rhodopseudomonas acidophila through analysis of transient absorption data with an evolutionary target analysis. The algorithm offers flexible testing of kinetic network models with low a priori knowledge requirements. It applies universally to the simultaneous fitting of target state spectra and rate constants to time-wavelength-resolved data. Our best-fit model reproduces correctly the well-known cooling and decay behavior in the S1 band, but necessitates an additional, clearly distinct singlet state that does not exchange with S1, promotes ultrafast triplet population and participates in photosynthetic energy transfer.

Singlet-singlet annihilation is used to study exciton delocalization in the light harvesting antenna complex LH2 (B800-B850) from the photosynthetic purple bacterium Rhodobacter sphaeroides. The characteristic femtosecond decay constants of the high intensity isotropic and the low intensity anisotropy kinetics of the B850 ring are related to the hopping time tau(h) and the coherence length N(coh) of the exciton. Our analysis yields N(coh) = 2.8+/-0.4 and tau(h) = 0.27+/-0.05 ps. This approach can be seen as an extension to the concept of the spectroscopic ruler.

The organization of bacteriochlorphyll a (BChl) and carotenoid pigments within the peripheral light harvesting complex of purple bacteria, LH2, has been partially generalized using the 2.5 A resolution crystal structure obtained for Rps. acidophila (1) and is illustrated in Figure 1. Two types of BChl molecules, distinguished by their binding sites and separated by ~20 A, are labeled according to their peak absorption wavelengths as B850 and B800. Carotenoid pigments are observed to “snake” between the BChl molecules. The first, fully resolved in the crystallographic structure, spans the membrane on the inside of the ring with the B800 BChl located at approximately the center of the conjugated region. The head of this carotenoid lies near the N terminus of the protein, where several polar residues can be found. The second carotenoid has been only partially resolved, with the head located on the opposite end of the membrane below the B850 BChls.

The primary events in bacterial photosynthesis involve the capture of light energy by light harvesting (LH) antenna complexes, followed by rapid and efficient energy transfer to a reaction centre (RC), where the absorbed energy is “trapped”. In most species of purple bacteria, there are two types of light harvesting complex. The first type, LH1, is intimately associated with the RC to form a “core” complex. Arranged more peripherally to this and present in variable amounts is the second type of complex called LH2. Both types of LH complex are built on a similar modular principle, in which the light-absorbing pigments bacteriochlorophyll a (Bchla) and carotenoid molecules are non-covalently bound to two low Mr hydrophobic apoproteins α and β. The native complexes are oligomers of these monomeric units. The crystal structure of the LH2 complex from Rps. acidophila 10050 has recently been determined to a resolution of 2.5 A° by McDermott et al1. The complex was observed to be an α9β9 oligomer, with each monomeric unit containing an α and β apoprotein, 3 Bchla molecules and 1 (or 2?) carotenoid (rhodopin-glucoside) molecules. The 9 α apoproteins form a hollow cylinder with the 9 β apoproteins arranged radially outside them. Eighteen Bchla molecules are sandwiched between the α and β apoprotein helices and form a continous overlapping ring. These molecules absorb at approximately 850 nm (B850) and have their bacteriochlorin rings perpendicular to the membrane surface. A further nine Bchla molecules are positioned more peripherally in the complex, situated between the β apoproteins.

Transient absorption spectra and kinetics are used for detailed studies of energy transfer dynamics within the B850 ring of the LH2 antenna of the purple bacterium Rb. sphaeroides. The initial exciton relaxation was found to occur within the first 100 fs and no temperature dependence of this process was observed. Transient absorption spectra of the LH2 antenna complex at 6 K revealed a new spectral band around 880 nm as a result of energy distribution of the lowest exciton states of B850 rings at cryogenic temperatures.