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The Natural Selection of Configurations and Conjugation Lengths in Carotenoids Bound to the Antenna and Reaction Center Complexes

Professor Yasushi Koyama

Faculty of Science and Technology, Kwansei Gakuin University, Japan.

Wednesday, 7th March 2007, 3.30PM, Lecture Theatre EN101 - Ground Floor of EN (Engineering) Building, Hawthorn.

The natural selection of carotenoid (Car) configurations, i.e., the all-trans configuration by antenna LH1 and LH2 complexes, whereas the 15-cis configuration, by reaction center (RC), has been found. The selection is due to the major-functions of Cars, i.e., the light-harvesting function in antenna and the photo-protective function in RC. We have revealed the reasons for this natural selection:

(1) The all-trans configuration by antenna for the light-harvesting function. The all-trans conjugated chain has approximately C2h symmetry, which gives rise to low-lying singlet states, including the optically-allowed 1Bu+ state and the optically-forbidden (‘dark’) 3Ag-, 1Bu- and 2Ag- states (we discovered the 3Ag- and 1Bu- states in addition to the well-documented 1Bu+ and 2Ag- states). After excitation to the 1Bu+ state, by absorption of a photon, Car relaxes (internal-converts) to the lower 3Ag-, 1Bu- and 2Ag- states; during these internal-conversion processes, Car transfers its singlet energy to the Qx and Qy states of bacteriochlorophyll (BChl). Thus, all-trans Car can have plural channels for efficient singlet-energy transfer.

(2) 15-cis configuration by reaction center for the photo-protective function. The major role of 15-cis Cars in RC is efficient dissipation of triplet energy. It was found that 15-cis Car has a potential to rapidly isomerize into all-trans. We have proposed and evidenced the following mechanism of triplet-energy dissipation: ‘The rotational motion around the 15-cis double bond generates a change in orbital angular momentum, which causes a change in spin angular momentum (through spin-orbit coupling), resulting in the T1 ? S0 intersystem crossing’. Thus, 15-cis Cars can efficiently dissipate the triplet energy.

The natural selection of the Car conjugation length, i.e., a shorter-chain Car by antenna, whereas a longer-chain Car by RC, also, has been found. For example, Cars having the number of conjugated double bonds (n) = 9-10 are selectively bound to LH2, whereas a Car having n = 13, to RC, when all these Cars are available.

(3) The shorter conjugated chain by antenna for the light-harvesting function. We have obtained an energy diagram of the 1Bu+, 3Ag-, 1Bu- and 2Ag- states for Cars with n = 9-13. All the singlet energies decrease with n, as linear functions of 1 / (2n + 1): The slopes of the linear relation have turned out to be 2Ag- : 1Bu- : 3Ag- ≈ 2: 3: 4. The relative singlet energies of Car vs. BChl (Qx and Qy) showed that three singlet energy-transfer channels, 1Bu+, 1Bu- and 2Ag-, can be open for n = 9 and 10, whereas only one channel, 1Bu+, can be open for n = 11-13. A sudden decrease in the Car-to-BChl singlet-energy transfer, on going from n = 10 to 11, was shown by fs absorption and stationary-state fluorescence spectroscopy.

(4) The longer conjugated chain by RC for the photo-protective function. We have determined the triplet lifetimes (τ) of the RC-bound Cars, and compared to those of the LH1- and LH2-bound Cars. The value of ln τ decreases with n, as functions of 1 / (2n + 1), following the Englman-Jortner energy-gap law, and the linear relation of RC shifted to the negative side than that of LH2. Thus, (i) triplet-energy dissipation is more efficient in the longer conjugated chain than in the shorter conjugated chain for both the RC-bound 15-cis Car and the LH-bound all-trans Car, and (ii) when these two configurations are compared, triplet-energy dissipation is more efficient in the RC-bound 15-cis Car than in the LH-bound all-trans Car.

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