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6.3: Energy transfer from carotenoids to bacteriochlorophylls in the photosynthetic light-harvesting complexes

In this section, we once again look into the excited state dynamics of carotenoids. This time, however, we will look at these molecules embedded in photosynthetic pigment-protein complexes, rather than freely floating in solution. The protein in question is the light harvesting antenna complex LH2 of bacterium Rhodobacter sphaeroides. Its structure and energy transfer functions were introduced in section 4.1.

After the excitation of carotenoid molecule (LH2 of Rhodobacter spheroids contains carotenoid called spheroidene), we may expect energy transfer from S2 state to bacteriochlorophyll (the latter absorbs at 800 nm, or 850 nm, depending on which ring of bacteriochlorophylls are embedded in). Additionally, carotenoid can relax to S1 state and then transfer energy to bacteriochlorophylls. The energy can go both to B800 and B850 bacteriochlorophylls, both of which have Qx and Qy electronic states. This makes the overall energy transfer scheme rather complicated (Fig. 33, [28]).

Pump-probe experiments on LH2 in broad probe spectral range have provided detailed understanding of how carotenoids help bacteria to collect solar light. The first such study, where energy transfer from carotenoids to bacteriochlorophylls was observed, is due to A.P Shreve and coworkers [29]. In the further, we will analyze their results shown in Fig. 34.

In the first experiment, B800 bacteriochlorophylls were excited and their dynamics was probed at the same wavelength, namely 800 nm. The results of this experiment are shown in Fig. 34A. Difference absorption kinetics shows that upon the excitation of B800 bacteriochlorophylls, initially we observe decrease in absorption that lasts for roughly 700 fs (see raw data shown by triangles in Fig. 34A). Within 700 fs, the bleaching is replaced by induced absorption because the excitation energy is transferred to B850 bacteriocholorophylls that feature positive signal at 800 nm.[1] Therefore, we conclude that the excitation energy is transferred from B800 to B850 bacteirochlorophylls within 700 fs. The obtained transfer rate beautifully matches the number yielded by fluorescence upconversion experiment described in section 4.1.

            The second experiment was analogous to the pump-probe experiment on b-caroten in solution, described above. Spheroidene was excited using 480 nm light and probed at 498 nm, 510 nm and 540 nm. Similarly to b-carotene, at 498 and 510 nm we observe the S2 stimulated emission – a negative difference absorption signal at early times. After the decay of this signal, S1 excited state absorption appears at 540 nm and, to some extent, at 510 nm. Rise of this signal and the start of its decay is discernible in the data. So far, all the observations are analogous to the dynamics observed in solution: LH2 carotenoids are excited into S2, and subsequently relax to S1 on 100 fs time scale. The lowest excited states decays within approximately 20 ps (the start of decay is clearly visible in the kinetic trace probed at 540 nm in Fig. 34B).

 

Fig. 1) Generalized scheme of energy transfer pathways in LH2. Reproduced from [28].

 

The important question, however, is: do S1 and/or S2 transfer their energy to bacteriochlophylls, and, if so, how fast is this transfer? This question was answered by another PP experiment, the results of which are shown in Fig. 34C. In this experiment, the spheroidenes were excited using 510 nm pulses and the absorption change was monitored at B800 absorption band. The corresponding difference absorption trace shows that B800 band is bleached almost simultaneously with carotenoid excitation. This means that it has to receive energy from S2 during the lifetime of this state (which is of the order of 100 fs, as stated above). If the energy were transferred to B800 bacteriochlorophylls only from S2, their bleach should disappear within 700 fs, as is the case when B800 is excited directly (Fig. 34A). The experiment, however, shows a different behavior: B800 bleach after carotenoid excitation persists significantly longer and disappears only after several picoseconds. The only reasonable explanation for this experimental fact is that S1 state of spheroidene also transfers energy to B800 bacteriochlorophylls during its lifetime. As a result, a dynamic equilibirium between B800 and S1 is established (i.e. for some time B800 is transferring energy to B850, while also receiving it from S1). Effectively, the lifetime of B800 population becomes longer, compared to direct B800 excitation.

A.P. Shreve with co-authors have also performed the modeling of experimental data according to the connectivity scheme shown in Fig. 34D. The fitting allowed estimating the energy transfer rates and pathways between different pigment pools in LH2 complex. The estimated time constants are shown next to the corresponding arrows in Fig. 34D.

 


[1] This signal was observed by exciting B850 bacteriochlorophylls directly.