Michitaka Kuki

Singlet Excited States and the Light-Harvesting Function of Carotenoids in Bacterial Photosynthesis

Abstract— The recent results of stationary‐state and time‐resolved absorption, fluorescence and Raman spectroscopies of some typical carotenoids are summarized. Theoretical analyses of carotenoid singlet states and of carotenoid‐to‐bacteriochlorophyll singlet‐energy transfer are also included. On the bases of the energies, the lifetimes and other properties of singlet excited states of the carotenoids in solution and bound to the light‐harvesting complexes, the energetics and the dynamics of the light‐harvesting function in purple photosynthetic bacteria are discussed with emphasis on the 2Ag and Bu+ states.

A new singlet-excited state of all-trans-spheroidene as detected by resonance-Raman excitation profiles

Abstract Measurements of resonance-Raman excitation profiles of the CC and C–C stretching Raman lines for crystalline all- trans -spheroidene in KBr disc at 77 K identified a new singlet state (17600 cm −1 ) that is located in-between the 2A g − state (14200 cm −1 ) and the 1B u + state (19700 cm −1 ). The particular state is tentatively assigned to the 1B u − state on the basis of the extrapolation of the PPP-MRD-CI calculations for the low-lying singlet states of shorter polyenes [P. Tavan, K. Schulten, J. Chem. Phys. 85 (1986) 6602], and its possible role in mediating the rapid 1B u + to 2A g − internal conversion is discussed.

The 2Ag− energies of all-trans-neurosporene and spheroidene as determined by fluorescence spectroscopy

Abstract The fluorescence spectra of neurosporene and spheroidene were recorded in order to determine their 2Ag− levels: each fluorescence spectrum was deconvoluted into two series of 1Bu+→1Ag− and 2Ag−→1Ag− vibronic transitions, and the energy of the 2Ag− (v=0) state was determined to be 15300 cm−1 in neurosporene and 14200 cm−1 in spheroidene. No temperature dependence of these energies was seen.

Shifts of the 1A−g→1B+u electronic absorption of carotenoids in nonpolar and polar solvents

The 1B+u energy of carotenoids (spheroidene, neurosporene, and β carotene) exhibits a linear dependence on R(n)=(n2−1)/(n2+2) in both nonpolar and polar solvents: The 1B+u energy is more stabilized in polar solvents in the limit of R(n)→0, and the line for polar solvents has a gentler slope. [As a result, the line for polar solvents crosses the line for nonpolar solvents at R(n)≂0.3.] A theory has been developed, which explains the aforementioned observations as follows. In polar solvents, an electric field is generated by fluctuation of the solvent permanent dipoles, and it affects the rodlike, conjugated chain of the carotenoid in a long spheroidal cavity. The electric field stabilizes the 1B+u energy through the polarization effect and it substantially reduces the dispersive interaction. The higher transition multipoles, rather than the transition dipole of carotenoid, play major roles in the aforementioned interactions between the carotenoid and the solvent molecules.

The 2 1Ag− state and two additional low-lying electronic states of spheroidene newly identified by fluorescence and fluorescence-excitation spectroscopy at 170 K

Abstract Fluorescence and fluorescence-excitation spectroscopy of spheroidene at 170 K in n-hexane, CS2, and their mixtures has identified the 21Ag− state (level I) and two additional electronic states, i.e. one below (level II) and the other above (level IV) the optically allowed 1Bu+ state (level III). The energies of levels, I, II, III and IV, determined as absorptive 0←0 transitions, are 14900, 18700, 20200 and 24100 cm−1 in n-hexane, and 14600, 18000, 18700 and 23400 cm−1 in CS2. Fluorescence-excitation spectra showed that internal conversion is efficient from level IV to level I, less efficient from level II to level I, and inefficient from level III to level I.

The structures of S0 spheroidene in the light‐harvesting (LH2) complex and S0 and T1 spheroidene in the reaction center of Rhodobacter sphaeroides 2.4.1 as revealed by Raman spectroscopy

The Raman spectrum of S0 spheroidene in the light-harvesting (LH2) complex (LHC) and those of S0 and T1 spheroidene in the reaction center (RC) of Rhodobacter sphaeroides 2.4.1 were recorded. Comparison of the S0 Raman spectrum of the all-trans isomer bound to the LHC of R. sphaeroides 2.4.1 with that free in n-hexane solution suggests that the LHC-bound carotenoid takes a flat, all-trans configuration with possible distortion in the plane of the conjugated chain. On the other hand comparison of the S0 Raman spectrum of the 15-cis isomer bound to the RC with that free in n-hexane solution suggests that the RC-bound carotenoid takes a 15-cis configuration which is twisted around the C15(double bond)C15′ bond and distorted in the (single bond)C15H(double bond)C15′H(single bond) plane. The T1 Raman spectra of the RC-bound spheroidene indicated substantial twisting and in-plane distortion of the conjugated backbone. Based on the above results, a mechanism of triplet-energy dissipation by the RC-bound carotenoid, which involves an internal rotation around the C15(double bond)C15′ bond, is proposed.

The 2 1Ag− state of a carotenoid bound to the chromatophore membrane or Rhodobacter sphaeroides 2.4.1 as revealed by transient resonance raman spectroscopy

Abstract The transient Raman spectrum of the S 1 and T 1 states of carotenoids bound to the chromatophore membranes of Rhodobacter sphaeroides 2.4.1 (recorded by using 532 nm, ≈ 100 ps, ML-QS pulse trains as a difference spectrum of high power minus low power) has been compared with the S 1 and T 1 Raman spectra of sphaeroidene and the T 1 Raman spectrum of sphaeroidenone. The vibronically coupled CC stretching Raman line of bound sphaeroidene at 1766 cm −1 identifies the 2 1 A g − state as the S 1 state, and its frequency, which is lowered by 51 cm −1 from that of free sphaeroidene, indicates weakened vibronic coupling for the bound carotenoid.

RESONANCE RAMAN EVIDENCE FOR 15-cis TO MA,-trans PHOTOISOMERIZATION OF SPIRILLOXANTHIN BOUND TO A REDUCED FORM OF THE REACTION CENTER OF Rhodospirillum rubrum SI

The reaction center (RC) of Rhodospirillum rubrum SI, which was prepared by ultrafiltration, showed one peak in molecular‐sieve HPLC, but it showed two peaks in diethylaminoethyl (DEAE) ion‐exchange HPLC; they were named as RC‐α and RC‐β in the order of elution, Nonequilibrated isoelectric electrophoresis, together with DEAE ion‐exchange HPLC, showed that RC‐β is electronically more negative than RC‐α. Oxidation of RC‐β by addition of ferricyanide caused its transformation into RC‐α, while reduction of RC‐α by adding ascorbate and subsequent illumination caused its transformation into RC‐β. Resonance Raman spectroscopy of the RC at liquid nitrogen temperature detected the all‐trans and the 15‐cis isomers in a ratio of 1:1, but HPLC analyses of the carotenoid extracted from the RC before and after the Raman measurements detected the pair of isomers in a ratio of 1:6. Thus, the 15‐cis to all‐trans isomerization takes place during irradiation at liquid nitrogen temperature, while the reverse isomerization takes place in the dark. The isolated RC‐α and RC‐β exhibited the bleaching of the 868 nm band, and contained the H, M and L subunits and 1.2‐1.4 molecules of ubiquinone‐10 per RC. Each RC slowly equilibrated in the dark toward a mixture of RC‐α and RC‐β. Generation of the all‐trans isomer in the light was found not in RC‐α but in RC‐β.

Triplet excitation of precursors of spirilloxanthin bound to the chromatophores of Rhodospirillum rubrum as detected by transient Raman spectroscopy

Abstract Analysis by HPLC, and by subsequent mass spectrometry, of the carotenoid extract of the chromatophores prepared from the cells of a 3-day culture of Rhodospirillum rubrum S1 shows the presence of precursors of spirilloxanthin (rhodovibrin, anhydrorhodovibrin, and 3,4-dihydrospirilloxanthin) as minor components in addition to the major component, spirilloxanthin. Only spirolloxanthin is found in the extract of the chromatophores from the cells of a 6-day culture. Transient Raman spectra of the carotenoids bound to the above two different kinds of chromatophore have been recorded by using 532 nm, ≈ 100 ps mode-locked (76 MHz) and Q -switched (800 Hz) pulses; a T 1 Raman spectrum has been obtained by a one-color, pump-and-probe technique. The T 1 Raman spectra of rhodovibrin and spirilloxanthin free in tetrahydrofuran solution have also been recorded by using 337 nm pump and 532 nm probe ≈ 10 ns pulses (10 Hz); a T 1 Raman spectrum has been obtained by a two-color, pump and probe technique (delay 1.8 μs) using anthracene as a sensitizer. Spectral comparison indicates that the precursors as well as spirilloxanthin bound to the chromatophores can be excited to the T 1 state. The result strongly suggests that the precursors are tightly bound to the light-harvesting complex and are involved in energy transfer.

Solvent Effects and Vibronic Couplings of ag-Type Stretching Vibrations in T1-Bacteriochlorophyll a and S0- and S1-Spheroidene: Environment of these Pigments in the LH2 Complex of Rhodobacter sphaeroides

Theories of vibronic coupling predict that its strength is proportional to the square of coupling constants, and is inversely proportional to the energy gap of the electronic states involved; a coupling constant is proportional to the Lx matrix of the normal mode and the bond transition-density matrix between the pair of electronic states [1, 2, 3, 4]. Thus, Raman frequencies can reflect changes in the electronic energies through vibronic couplings.