Color-tuning is a critical survival mechanism for photosynthetic organisms. Calcium ions are believed to enhance both spectral tuning and thermostability in obligatory calcium-containing sulfur purple bacteria. This study examined the thermo- and piezo stability of the LH1-RC complexes from two calcium-containing sulfur purple bacteria notable for their extreme red-shifted spectra. The results generally show limited reversibility of both temperature and pressure effects related to the malleability of calcium-binding sites. While the pressure-induced decomposition product closely resembles the calcium-depleted form of the chromoproteins, the thermally induced products reveal monomeric B777 and dimeric B820 forms of bacteriochlorophyll a, similar to those seen in non-sulfur purple bacteria treated with detergent. The study further found nearly unison melting of the protein tertiary and secondary structures. Overall, our findings do not support a direct link between color adjustment and thermodynamic stability in light-harvesting chromoproteins.

The structure of the exciton manifold in the core bacteriochlorophyll antenna complexes from the purple photosynthetic bacterium Rhodobacter sphaeroides has been studied at 5K using conventional and polarized fluorescence excitation spectroscopy supported with model simulations. Exciton bandwidths in the range of 2100–2200cm−1, depending on sample integrity, have been determined in correlation with the largest mean coupling energy between the nearest-neighbor bacteriochlorophyll molecules close to 600cm−1. These numbers are considerably bigger than those in any other native antenna complex known. Another noteworthy feature of these complexes is dominance of structural (off-diagonal) static disorder over the energetic (diagonal) disorder.

A dual nature of fluorescence from LH2 pigment–protein complexes, which is a part of the light harvesting system of purple bacteria, is confirmed by fluorescence-lifetime dependence on recording wavelength and spectrally selective spectroscopy. An analysis based on the Holstein molecular crystal model, modified by allowing diagonal disorder, suggests coexistence of large- and small-radius self-trapped excitons, which serve as the origin of the dual fluorescence.

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.

The bandwidth of the exciton manifold in LH2 peripheral antenna complexes from the photosynthetic bacterium Rhodobacter sphaeroides has been determined using polarized fluorescence excitation spectroscopy and model simulations. It is shown that the fluorescence anisotropy spectra reveal a hidden structure of the exciton band that is correlated with its boundaries. The estimated exciton coupling energy and exciton bandwidth in the native complex is ∼360 and ∼1620 cm−1, respectively. The corresponding numbers in a mutant with the B800 bacteriochlorophyll molecules absent are somewhat larger (∼420 and ∼1890 cm−1), probably due to tightening of the protein structure.

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.

Fluorescence line narrowing and hole-burning spectroscopic studies of excitons in the LH2 pigment-protein complex, which is a part of the light harvesting system of purple bacteria, are combined with straightforward numerical simulations of the emission spectra based on exciton polaron model. The analysis provides evidence for self-trapping of all the excitons, except the lowest one.

The absorption and emission spectra of excitons in LH2 antenna complexes from the photosynthetic purple bacterium Rhodobacter sphaeroides have been studied under hydrostatic pressure. The measurements made between ambient pressure and 6kbar over a broad temperature range reveal largely different rates of the pressure-induced shifts for the absorption and emission bands. Numerical calculations based on exciton polaron model provide evidence for the exciton self-trapping at ambient pressure as well as for the pressure stabilization of the self-trapped exciton states responsible for the emission, whereas the light absorbing states belong to nearly free excitons over the whole pressure and temperature ranges studied.

Experimental evidence is presented showing that excitons in circular antenna complexes from photosynthetic bacteria are dynamically self trapped in about 200 fs by coupling to nuclear vibrations. The induced deformation covers similar to 20% of the complex circumference at low temperature. This self trapping, the first of its kind observed in biological systems, results in a broad fluorescence spectrum and considerably improves energy resonance between heterogeneous antenna complexes. Exciton self trapping may thus be a part of nature's strategy, increasing the speed and efficiency of energy transfer in photosynthesis.

The effects of high hydrostatic pressure (up to 8 kbar) on bacteriochlorophyll a Q(y) electronic absorption bands of LH2 photosynthetic antenna complexes have been studied at ambient temperature. A variety of samples were studied, including intact membranes and isolated complexes from wild type and mutant photosynthetic bacteria Rhodobacter sphaeroides, Rhodopseudomonas acidophila, and Rhodospirillum molischianum. The spectra of the complexes universally red shift and broaden under elastic compression, while the variations of the integrated intensity remain within the experimental uncertainty. A qualitatively different slope and variation of the slope of the pressure-induced shift is observed for the B800 and B850 absorption bands of LH2 complexes belonging to quasi-monomer and aggregated pigments, respectively. For the complexes from Rhodobacter sphaeroides, e.g., the corresponding slopes are -28 +/- 2 and -65 +/- 2 cm(-1)/kbar. The shift rate of the B800 band declines with pressure, while the opposite is observed for the B850 band. The shifts show little if any correlation with hydrogen bonds. Using simple phenomenological arguments and numerical simulations of molecular exciton spectra, it is shown that the shift of the B800 band is governed by pigment-protein interactions, while in addition to that, interpigment couplings (including long-range dipolar and short-range orbital overlap interactions) are instrumental for the B850 band shift. The compressibility of the B800 bacteriochlorophyll binding sites deduced from the B800 band shift at ambient pressure is similar to0.02 kbar(-1), and it decreases nonlinearly with pressure. Inter-pigment couplings are responsible for approximately one-third of both the total ambient-pressure solvent shift of the B850 absorption band and its pressure-induced growth. A slight increase with pressure of the B850 band shift due to orbital overlap couplings is predicted.

A long-standing uncertainty has been resolved concerning the early time evolution of light excitations in LH2 antennas of purple photosynthetic bacteria. Both isolated and native membrane-embedded LH2 antenna complexes were comprehensively studied using femtosecond time-resolved transient absorption spectroscopy together with conventional absorption and fluorescence emission spectroscopies at low temperatures. The results show that exciton relaxation within isolated LH2 complexes is characterized by subpicosecond time constants, while spectral evolution associated with energy transfer between the antenna complexes in photosynthetic membranes is characterized by time constants greater than or equal to 1 ps.

The objective of this work is two-fold. First, the effects of static diagonal disorder on the linear and nonlinear absorption spectra of excitons in circular molecular aggregates are studied by computer modeling. Second, it is demonstrated that this simplified model successfully reproduces the main features of both the ground-state absorption and initial pump-probe absorption difference spectra of LH2 antenna proteins from photosynthetic bacteria measured upon spectrally selective population of excitons at low temperature. Of the usual first-order approximations in the Frenkel exciton theory, our model exploits only two: the two-state and the zero electron-vibrational coupling approximations. In our model, the molecules of the aggregate are allowed to have different site energies. The coupling between all aggregate molecules is taken into account. An important difference between our study and previous work is that the exciton state selective spectra are calculated corresponding to the recently performed spectrally selective ultrashort pulse excitation experiment. We investigate the behavior of excitons as a function of disorder separately in the B850 and B800 ring aggregates of LH2. Usually, excitations in the B800 ring have been considered completely localized. The present study reinforces the importance of static diagonal disorder in describing the spectral properties of excitons in the LH2 antenna complex at low temperatures. Moreover, it has been demonstrated that two types of spectral disorder govern the inhomogeneously broadened exciton spectra of antenna complexes embedded into the photosynthetic membrane rather than a single source of disorder. From the comparison of simulated and experimental linear absorption spectra, we suggest that the peculiar asymmetry of the B800 band as well as some of the high-energy sideband structures are due to weak coupling of excitons in the B850 and B800 ring aggregates with intramolecular vibrations of bacteriochlorophyll a molecules.

In photosynthesis, light energy is converted into chemical energy via a sequence of ultrafast charge separating electron transfer (ET) processes in a reaction centre (RC). In photosynthetic bacteria the RC pigment system consists of a dimer of bacteriochlorophyll molecules (the primary donor of electrons, P), two accessory bacteriochlorophyll molecules (BL and BM), two molecules of bacteriopheophytin (HL and HM, and QB). The pigments are arranged in two approximately symmetric branches, termed L and M, that span the membrane, but only the L branch, involving BL, HL and QA, is photochemically active under normal conditions.

The following question has been addressed in the present work. How external high (up to 8 kbar) hydrostatic pressure acts on photoinduced intramolecular electron transfer and on exciton relaxation processes? Unlike phenomena, as they are, have been studied in different systems: electron transfer in an artificial Zn-porphyrin-pyromellitimide (ZnP-PM) supramolecular electron donor-acceptor complex dissolved in toluene measured at room temperature; exciton relaxation in a natural photosynthetic antenna protein called FMO protein measured at low temperatures, between 4 and 100 K. Spectrally selective picosecond time-resolved emission technique has been used to detect pressure-induced changes in the systems. The following conclusions have been drawn from the electron transfer study: (i) External pressure may serve as a potential and sensitive tool not only to study, but also to control and tune elementary chemical reactions in solvents; (ii) Depending on the system parameters, pressure can both accelerate and inhibit electron transfer reactions; (iii) If competing pathways of the reaction are available, pressure can probably change the branching ratio between the pathways; (iv) The classical nonadiabatic electron transfer theory describes well the phenomena in the ZnP-PM complex, assuming that the driving force or/and reorganisation energy depend linearly on pressure; (v) A decrease in the ZnP-PM donor-acceptor distance under pressure exerts a minor effect on the electron transfer rate. The effect of pressure on the FMO protein exciton relaxation dynamics at low temperatures has been found marginal. This may probably be explained by a unique structure of the protein [D.E. Trondrud, M.F. Schmid, B.W. Matthews, J. Mol. Biol. 188 (1986) p. 443; Y.-F. Li, W. Zhou, E. Blankenship, J.P. Allen, J. Mol. Biol., submitted]. A barrel made of low compressibility beta-sheets may, like a diving bell, effectively screen internal bacteriochlorophyll a molecules from external influence of high pressure. The origin of the observed slow pico = and subnanosecond dynamics of the excitons at the exciton band bottom remains open. The phenomenon may be due to weak coupling of phonons to the exciton states or/and to low density of the relevant low-frequency ( approximately 50 cm(-1)) phonons. Exciton solvation in the surrounding protein and water-glycerol matrix may also contribute to this effect. Drastic changes of spectral, kinetic and dynamic properties have been observed due to protein denaturation, if the protein was compressed at room temperature and then cooled down, as compared to the samples, first cooled and then pressurised.

A time response over almost 5 decades (from 10(-13) to about 10(-8) s) to a (sub)picosecond laser pulse excitation has been observed in the Fenna, Matthews, and Olson (FMO) antenna protein trimer. The FMO protein is unique in having a fine-structured bacteriochiorophyll a Qy exciton absorption spectrum over the whole investigated temperature range between 6 and 160 K. As measured by a two-color pump-probe differential absorption, the population decay of the exciton states of seven strongly coupled bacteriochlorophyll a molecules in a protein monomer is the dominant dynamical process in the subpicosecond time domain. The through-band scattering takes a few picoseconds and depends only weakly on temperature, probably because of a low density of exciton states. At low temperatures, evidence for a slow pico-nanosecond relaxation process has also been obtained via time-dependent red-shift and broadening of the exciton emission spectrum. Two nonexclusive tentative interpretations to this effect have been provided. The phenomenon may be due to exciton solvation in the surrounding protein and water-glycerol matrix or/and due to slow scattering of closely spaced bacteriochlorophyll a exciton states in a protein trimer. The shape of the excited-state absorption spectrum (arising from transitions between singly and doubly excited exciton states) and its oscillator strength has been roughly estimated from the analysis of the pump-probe spectrum. The spectrum peaks at around 805 nm and is less featured compared to the ground-state absorption spectrum. Both spectra have comparable strength.

A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of approximately 2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.

We report studies of energy 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 blue-shifted B850. The measurements were performed by using approximately 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 approximately 0.7 ps at 296 K and energy transfer from LH2 to LH1 is characterized by a time constant of approximately 3 ps at 296 K and approximately 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.

The efficiency of energy back-transfer from the reaction center to the antenna in chromatophores of the photosynthetic purple bacterium Rhodopseudomonas viridis was measured at room temperature by means of picosecond time-resolved fluorescence spectroscopy. It was found that 20 ± 5% of the excitation energy selectively absorbed by the reaction center pigments in the wavelength region around 830 nm is transferred back to the antenna and gives rise to antenna fluorescence. The measured yield of energy detrapping enabled calculation of the energy detrapping (10–13 ps)−1 and trapping (40–50 ps)−1 rates. These results confirm that in Rps. viridis, like in bacteriochlorophyll a-containing purple bacteria, the energy transfer step from antenna pigments to the reaction center is a rate limiting step in the overall energy trapping by the reaction center. We suggest that this situation is termed ‘transfer-to-trap-limited’ dynamics, to distinguish it from the situation where the charge separation is the rate limiting step (trap limited) or the overall energy diffusion through the antenna is limiting (diffusion limited).

Spectrally resolved pico- and nanosecond fluorescence kinetics of two types of Photosystem II core complex: D1/D2/cyt b559 reaction centres (RCs) and RCs together with CP47 proximal antenna have been studied at room temperature and at 77 K. The kinetics at room temperature were measured with the RCs being in different functional states. In the photoactive RCs at room temperature a picosecond decay with the lifetime components 13 ± 3 ps and 110 ± 30 ps is followed by the complex nanosecond kinetics. In the case of RC + CP47 complexes picosecond decays are slower: 25 ± 10 ps and 190 ± 30 ps, but nanosecond decay has similar behaviour. The data are analyzed by a simple three-state kinetic model postulating the formation of the primary radical pair in an unrelaxed form and allowing a back-recombination from that state. If this is correct, it is the proof that in the charge-separated state nuclear coordinate relaxation takes place on the picosecond time-scale. The following conclusions considering the nature and temporal characteristics of light excitations have been made: (i) At room temperature excitations are in equilibrium between P680 and the accessory chlorophylls including CP47 antenna and, therefore, only the average trapping time could be observed. This time is equal to 13 ± 3 ps in RCs and 25 ± 10 ps in RC + CP47 complexes. (ii) All other decays at room temperature (except probably part of the 5 ± 0.5 ns component) are of a recombination origin and reflect complex relaxation of the metastable radical pair state. (iii) At low temperatures an energetically directed excitation transfer, qualitatively very similar to the one observed in the core antenna of some purple bacteria takes place. This energy transfer is relatively slow with an apparent transfer time 10–20 ps at 77 K. (iv) Not only P680, but also P+ 680 and Pheo− are very efficient quenchers of excitations.