Koichiro Mukai

Rapid excitation-energy transfer to optically forbidden states in light-harvesting antennas of photosynthetic bacteria

Abstract The rate of excitation-energy transfer (EET) within the light-harvesting complex LH2 of photosynthetic purple bacteria is calculated, based on a new formula for EET between molecular aggregates with a small mutual distance, by taking into account both the exciton–phonon and exciton–disorder interactions. It is elucidated that EET from B800 to higher and optically forbidden exciton states in B850 is the dominant mechanism of the rapid EET.

Sequential or superexchange mechanism in bridged electron transfer distinguished by dynamics at a bridging molecule

Two kinds of mechanisms are well known for electron transfer (ET) in the system DMA where a donor D and an acceptor A are fixed in a close distance by a bridging molecule M. When the free energy DeltaG(m) of the intermediate state |m of DM(-)A is much higher than the thermal energy k(B)T above the initial state of D(-)MA, the ET occurs unistep from D to A by the superexchange (SX) mechanism, passing |m as a quantum-mechanical virtual state. However, when DeltaG(m) becomes not much higher than k(B)T, the ordinary sequential (OS) ET may manifest itself, where the second ET from |m to the final state of DMA(-) takes place after thermalization of phonons in |m. Recently, much interest has been aroused in how the SX mechanism switches to the OS one as DeltaG(m) is lowered. This subject has often been described conventionally by summation of the rate constant for the SX mechanism and that for the OS one. However, such convention cannot be justified, since these mechanisms are realized in mutually opposite limits concerned with dynamics in mediation of ET by a bridging molecule, hence they cannot both be realized simultaneously in a single system. An observation of such a bridged ET by Paulson, Miller, Gan and Closs (J. Am. Chem. Soc. 2005, 127, 4860) provides a good example of this feature. Describing their observation in a unified framework for the ET, it is shown that the switch occurs at DeltaG(m) congruent with-0.5 eV, which is much lower than 0.3 eV reported by them, where the hot-sequential ET reveals itself, taking place during thermalization of phonons in |m.

Detection of the upper edge of exciton multiplets in the antenna complexes LH1 and LH2 of bacterial photosynthesis, by optical reflection

Abstract The second-lowest state in exciton multiplets on the ring aggregate of bacteriochlorophylls in LH1 and LH2 has a transition dipole parallel to the ring plane, absorbing light strongly. The highest state has that perpendicular to the plane, but has not been detected, since its oscillator strength is much smaller than the former’s. Let us prepare an aggregate of LH1s or LH2s so as for their ring planes to be parallel to a surface. The more slantingly is the surface shone by light polarized within the plane of incidence, the more strongly the latter reflects the light, compared with the former.

Effects of positional disorder on optical absorption spectra of light-harvesting antenna complexes in photosynthetic bacteria

Abstract We study theoretically the influence of both diagonal and off-diagonal disorder on the absorption spectra of the light-harvesting antenna complex LH2 consisting of two circular aggregates, B850 and B800, of bacteriochlorophyll pigments in photosynthetic purple bacteria. Off-diagonal disorder, i.e., randomness in excitonic couplings between molecules, is introduced by a model of disorder in the position of each pigment molecule embedded in proteins. We demonstrate that a large contribution of positional disorder provides a natural explanation for the experimental fact that the excitonic B850 absorption peak is broader than that of monomeric B800 in spite of motional narrowing.

Mechanism of Spin-Triplet-State Formation on the Accessory Chlorophyll in the Reaction Center of Photosystem II

In the reaction center of photosystem II (PSII), the primary charge-separated state P+ H- between the primary electron donor P and the pheophytin H occasionally forms a spin triplet state on the accessory chlorophyll B between P and H by recombination of a hole and an electron in P+ H-. This is one of the specificities of PS II from other photosystems, since the triplet state is formed not on B but on P in the purple-bacterial photosystem. In this formation, we can consider three pathways with different intermediate states: (1) the hole-electron type (P+ BH− → PB+ H- → P3B*H), (2) the electronhole type (P+ BH- → P+ B-H → P3B*H), and (3) the excitation-transfer type (P+ BH- → P+ B-H → 3P*BH → P3B*H). In this study, we shows theoretically that the pathway realized depends on the magnitude of the hole-transfer integral J h relative to the electron one J e. Only when J h << J e, it occurs by (3), otherwise by by (1). We also discuss how the intermediate state mediates the triplet-state formation.

A Model for Temperature-Dependent Peak Shift of the Bacterial Reaction-Center Absorption

Temperature dependence of the reactioncenter absorption of purple bacteria was modeled in the light of quantum chemical calculation. The 860 nm absorption, assigned to the lower excited state of the special pair dimer of bacteriochlorophyll-a (Bchla), shows striking temperature dependence, decreasing markedly with lowering temperature. A model for this temperature-dependent peak shift is given, by constructing potential-energy surfaces for the electronic excited and the ground state of the special pair. The excitedstate potential is sensitive to the intra-pair distance and the mutual angle of the dimer molecules, the potential surface becoming markedly-anharmonic. The anharmonic excited-state potential surface was calculated by the quantum-chemical approach. The other protein vibration modes were assumed to be harmonic and were represented by a single reorganization energy U. Employing these potential surfaces, the absorption band was calculated as a function of temperature. The large peak shift with temperature could successfully be reproduced. This mechanism does not rely on the thermal expansion (Chang et al. 2001) which has been assumed so far to be effective in the peak shift without detailed calculation. The peak shift is due to density-of-vibrational-states difference between the excited and the ground state potential surfaces.

THEORY OF EXCITATION-ENERGY TRANSFER PROCESSES INVOLVING OPTICALLY FORBIDDEN EXCITON STATES IN ANTENNA SYSTEMS OF PHOTOSYNTHESIS

The rate of excitation-energy transfer (EET) within the light-harvesting complex (LH) and from LH to the reaction center (RC) of photosynthetic purple bacteria is calculated, based on a formula for EET between molecular aggregates. We show that optically forbidden exciton states participate in EET processes through mulitpole EET interactions with the help of disorder. In the antenna systems of photosynthesis, high efficiency of energy transfer is implemented by these EET processes involving optically forbidden exciton states.

Nonlocal Optical Response of Mesoscopic Aggregates of Polymers

Linear absorption of nanometer scale aggregates of polymers are studied using the nonlocal response theory. Besides the main peak around the exciton levels, additional peaks appears due to the interaction with radiation field, and their size dependence is discussed.