The dependence of excitation energy transfer pathways on conjugation length of carotenoids in purple bacterial photosynthetic antennae

Abstract

Phone: þ81 6 6605 3627, Fax: þ81 6 6605 3619Ultrafast excited state dynamics of carotenoids in solution and bound to pigment-protein complexes have been investigated by femtosecond pump-probe spectroscopic measurements. Possible excitation energy transfer (EET) pathways between carotenoids and bacteriochlorophylls and their efficiency depend strongly on the conjugation length of carotenoids. In the case of Rhodobacter sphaeroides 2.4.1, dual EET channels from carotenoid to bacteriochlorophyll (S 2 ! Q x and S 1 ! Q y ) upon excitation of carotenoid were observed. In the case of Rhodospirillum rubrum S1, on the other hand, the EET pathway of S 1 ! Q y upon excitation of carotenoid was closed, whereas reverse energy transfer from bacteriochlorophyll to carotenoid was clearly observed upon excitation of bacteriochlorophyll. The role of carotenoids in the light-harvesting complexes is discussed in terms of their conjugation length.

Energy Transfer Pathways in Light-Harvesting Complexes of Purple Bacteria as Revealed by Global Kinetic Analysis of Two-Dimensional Transient Spectra

“…In the spectra, taken at various t 2 delays, shown in Figures and , a substantial decrease of the amplitude of GSB and SE signals from S 2 , X and Q x can be observed as a function of t 2 , while the amplitude of the negative ESA signal, attributed to S 1 state, increases as t 2 increases. This observation is in agreement with the literature, since it is known that the higher lying states (S 2 , X, Q x ) decay on the ∼100 fs time-scale populating the lower energy S 1 state, ,− which has lifetimes of 1.2 and 3.3 ps in LH2 complexes of Rba. sphaeroides and Rps.…”

The Signaling State of Orange Carotenoid Protein

“…5 F) could explain the changes in the 983 cm À1 band intensity, in good agreement with the data presented in Liu et al (71). However, such a decrease in the conjugation length cannot explain the red shift of OCP R hECN S 0 /S 2 absorption, as it occurs in other carotenoid-binding proteins (74,75). Changes in S 0 /S 2 absorption probably are induced by other factors, such as carotenoid environment and bending of the polyene chain.…”

Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms

“…There are cases in which Car-to-BChl energy transfer occurs only from the S 2 state of the carotenoids, which limits the overall efficiency of transfer to <60%, whereas in other systems, EET takes place with increased efficiency from both the S 2 and S 1 excited states. ,, Because of strong light absorption by the Car S 2 state, the efficiency of EET through S 2 can be determined in accordance with Förster theory. However, the full Coulombic interaction between Cars and BChls must be calculated, because the dipole–dipole approximation is invalidated by the close proximity of the interacting pigments. ,− In contrast, the Car S 1 state does not absorb light (it is called a “dark” state), and its participation in the energy transfer can be calculated only if the borrowing of intensity from the S 2 state is taken into account. − The presence of additional low-lying Car excited states between S 2 and S 1 has complicated the picture further. − Evidence for the participation of these additional excited singlet states has been experimentally demonstrated. ,,− …”

“…There are cases in which Car-to-BChl energy transfer occurs only from the S 2 state of the carotenoids, which limits the overall efficiency of transfer to <60%, whereas in other systems, EET takes place with increased efficiency from both the S 2 and S 1 excited states. 88,342,343 Because of strong light absorption by the Car S 2 state, the efficiency of EET through S 2 can be determined in accordance with Forster theory. However, the full Coulombic interaction between Cars and BChls must be calculated, because the dipole−dipole approximation is invalidated by the close proximity of the interacting pigments.…”