The Light Harvesting Process in Purple Bacteria

Abstract

We present and review the results of fluorescence upconversion and photon echo experiments, and ab initio calculations performed in our group within the last few years with respect to the light harvesting process in purple bacteria. Carotenoids transfer energy to bacteriochlorophyll (BChl) mainly via the carotenoid S2 ->BChl Q pathway on a ~ 100 fs timescale.This transfer is reasonably reproduced by considering the Coulombic coupling calculated using the transition density cube method which is valid at all molecular separations. Carotenoids may also serve a role in mediating B800 ->B850 energy transfer in LH2 by perturbing the transition density of the B850 as shown by ab initio calculations on a supermolecule of two B850 BChls, one carotenoid and one B800 BCh1. Further calculations on dimers of B850 BChl estimate the intra-and interpolypeptide coupling to be 315 and 245 cm -", respectively. These interactions are dominated by Coulombic coupling, while the orbital overlap dependent coupling is 20% of the total. Photon echo peak shift experiments (3PEPS) on LH1 and the B820 subunit are quantitatively simulated with identical parameters aside from an energy transfer time of 90 fs in LH1 and oo in B820, suggesting that excitation is delocalized over roughly two pigments in LH1. 3PEPS data taken at room and low temperature (34 K) on the B800-B820 suggest that static disorder is the dominant mechanism localizing excitation in LH1 and LH2. We suggest that the competition between the delocalizing effects of strong electronic coupling and the localizing effects of disorder and nuclear motion results in excitation in the B850 and B875 rings being localized on 2-4 pigments within approximately 60 fs.

Ultrafast light harvesting dynamics in the cryptophyte phycocyanin 645

“…A diverse catalog of light harvesting antenna structures are known. For example, higher plants employ chlorophyll (a and b), predominantly bound in light harvesting complex II (LHCII), purple bacteria employ bacteriochlorophyll-a in remarkably well-ordered antenna systems (LH2 and LH1), [12][13][14][15] while the chlorosome antenna from green sulfur bacteria binds ∼250000 bacteriochlorophyll-c molecules. [16][17][18] Chlorophyll, though common, is not the only molecule type found in antenna systems.…”

Quantum Chemical Evidence for an Intramolecular Charge-Transfer State in the Carotenoid Peridinin of Peridinin−Chlorophyll−Protein

“…The B u + state is primarily involved in the lightharvesting function of carotenoids. [8][9][10][11][12][13] A second optically dark state with B usymmetry is predicted by multireference double excitation configuration interaction calculations using the Pariser-Parr-Pople Hamiltonian, 14 and may facilitate rapid internal conversion from the B u + to the A gstate. [14][15][16] Experimental observation of this state [15][16][17] has proved controversial, and therefore we will retain the traditional convention of labeling A gand B u + as the first and second excited singlet states, respectively.…”

“…For nonpolar light-harvesting carotenoids, the dominant carotenoid- to-chlorophyll energy transfer is generally found to occur directly from the carotenoid S 2 state. [8][9][10][11][12][13] The fluorescence results prompted solvent-dependent transient absorption studies of peridinin. 22 The S 1 lifetime was found to be strongly dependent on solvent polarity: 7 ps in trifluoroethanol and 172 ps in cyclohexane.…”

“…The state is accessible from the ground state via one-photon, but usually not two-photon, spectroscopy. The B u + state is primarily involved in the light-harvesting function of carotenoids. − A second optically dark state with B u - symmetry is predicted by multireference double excitation configuration interaction calculations using the Pariser−Parr−Pople Hamiltonian, and may facilitate rapid internal conversion from the B u + to the A g - state. − Experimental observation of this state − has proved controversial, and therefore we will retain the traditional convention of labeling A g - and B u + as the first and second excited singlet states, respectively. To further complicate the matter, in some carotenoids, van Grondelle and co-workers have detected an additional excited-state called S* by means of femtosecond transient absorption spectroscopy .…”

“…This suggests that the peridinin-to-chlorophyll energy transfer (Per → Chl) occurs predominantly via the S 1 state, even though the S 0 → S 1 transition is not observed in the absorption spectrum. For nonpolar light-harvesting carotenoids, the dominant carotenoid- to-chlorophyll energy transfer is generally found to occur directly from the carotenoid S 2 state. − …”