Ritsuko Fujii

Two different pathways of internal conversion in carotenoids depending on the length of the conjugated chain

Abstract Near-infrared, subpicosecond time-resolved absorption spectroscopy of all- trans carotenoids having the number of conjugated double bonds, n =9–13, identified two different pathways of internal conversion in accordance with the energy diagram recently determined by measurements of resonance-Raman excitation profiles (RREPs) [K. Furuichi, T. Sashima, Y. Koyama, Chem. Phys. Lett. 356 (2002) 547]: the 1B u + →1B u − →2A g − internal conversion for neurosporene ( n =9) and spheroidene ( n =10), whereas the 1B u + →3A g − →2A g − internal conversion for lycopene ( n =11), anhydrorhodovibrin ( n =12) and spirilloxanthin ( n =13).

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.

The dependence of the ultrafast relaxation kinetics of the S2 and S1 states in β-carotene homologs and lycopene on conjugation length studied by femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies

The ultrafast relaxation kinetics of all-trans-beta-carotene homologs with varying numbers of conjugated double bonds n(n=7-15) and lycopene (n=11) has been investigated using femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies, both carried out under identical excitation conditions. The nonradiative relaxation rates of the optically allowed S(2)(1(1)B(u) (+)) state were precisely determined by the time-resolved fluorescence. The kinetics of the optically forbidden S(1)(2(1)A(g) (-)) state were observed by the time-resolved absorption measurements. The dependence of the S(1) relaxation rates upon the conjugation length is adequately described by application of the energy gap law. In contrast to this, the nonradiative relaxation rates of S(2) have a minimum at n=9 and show a reverse energy gap law dependence for values of n above 11. This anomalous behavior of the S(2) relaxation rates can be explained by the presence of an intermediate state (here called the S(x) state) located between the S(2) and S(1) states at large values of n (such as n=11). The presence of such an intermediate state would then result in the following sequential relaxation pathway S(2)-->S(x)-->S(1)-->S(0). A model based on conical intersections between the potential energy curves of these excited singlet states can readily explain the measured relationships between the decay rates and the energy gaps.

Cis-Trans Carotenoids in Photosynthesis: Configurations, Excited-State Properties and Physiological Functions

Correlations between the 1H-NMR, electronic-absorption and resonance-Raman spectra and the cis-trans configurations have been identified for β-carotene and other carotenoids: (1) the chemical shifts of the olefinic 1Hs in NMR, (2) the wavelength of the A g − →B u + absorption and the relative intensity of the A g − →A g + vs. the A g − →B u + absorption in electronic absorption, and (3) the C=C stretching frequency, the relative intensity of the C10–C11 vs. the C14–C15 stretching vibration and the appearance of key modes in resonance Raman can be used to identify the all-trans or a mono-cis configuration.

Transient Absorption from the 1Bu+ State of All‐trans‐β‐carotene Newly Identified in the Near‐infrared Region¶

Abstract We have attempted subpicosecond time-resolved absorption spectroscopy of all-trans-β-carotene in organic solvents in the 820–1060 nm region and found novel transient absorption features which lived in subpicosecond time scales. A first component that appeared immediately after excitation showed a lifetime of 190 ± 10 fs in n-hexane in agreement with the 1Bu+ lifetime that had been determined by fluorescence upconversion spectroscopy (195 ± 10 fs). (Kandori et al. [1994] J. Am. Chem. Soc. 116, 2671–2672.) Therefore, this component is assigned to a transient absorption from the 1Bu+ state.

Probing the Effect of the Binding Site on the Electrostatic Behavior of a Series of Carotenoids Reconstituted into the Light-Harvesting 1 Complex from Purple Photosynthetic Bacterium Rhodospirillum rubrum Detected by Stark Spectroscopy

Reconstitutions of the LH1 complexes from the purple photosynthetic bacterium Rhodospirillum rubrum S1 were performed with a range of carotenoid molecules having different numbers of C=C conjugated double bonds. Since, as we showed previously, some of the added carotenoids tended to aggregate and then to remain with the reconstituted LH1 complexes (Nakagawa, K.; Suzuki, S.; Fujii, R.; Gardiner, A.T.; Cogdell, R.J.; Nango, M.; Hashimoto, H. Photosynth. Res. 2008, 95, 339-344), a further purification step using a sucrose density gradient centrifugation was introduced to improve purity of the final reconstituted sample. The measured absorption, fluorescence-excitation, and Stark spectra of the LH1 complex reconstituted with spirilloxanthin were identical with those obtained with the native, spirilloxanthin-containing, LH1 complex of Rs. rubrum S1. This shows that the electrostatic environments surrounding the carotenoid and bacteriochlorophyll a (BChl a) molecules in both of these LH1 complexes were essentially the same. In the LH1 complexes reconstituted with either rhodopin or spheroidene, however, the wavelength maximum at the BChl a Qy absorption band was slightly different to that of the native LH1 complexes. These differences in the transition energy of the BChl a Qy absorption band can be explained using the values of the nonlinear optical parameters of this absorption band, i.e., the polarizability change Tr(Deltaalpha) and the static dipole-moment change |Deltamu| upon photoexcitation, as determined using Stark spectroscopy. The local electric field around the BChl a in the native LH1 complex (ES) was determined to be approximately 3.0x10(6) V/cm. Furthermore, on the basis of the values of the nonlinear optical parameters of the carotenoids in the reconstituted LH1 complexes, it is possible to suggest that the conformations of carotenoids, anhydrorhodovibrin and spheroidene, in the LH1 complex were similar to that of rhodopin glucoside in crystal structure of the LH2 complex from Rhodopseudomonas acidophila 10050.

Ultrafast excited state dynamics of fucoxanthin: excitation energy dependent intramolecular charge transfer dynamics

Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.

The 1Bu-type singlet state of β-carotene as a precursor of the radical cation found in chloroform solution by sub-picosecond time-resolved absorption spectroscopy

Abstract Generation of the radical cation from 1B u -type states (1B u + and/or 1B u − ) of all- trans -β-carotene in chloroform was demonstrated by sub-picosecond time-resolved absorption spectroscopy. In the visible region, ground-state absorption bleaching completely recovered in n -hexane but not in chloroform. In near-infrared, broad transient absorptions from 1B u + and 1B u − states appeared and decayed within 1 ps in both solvents, while weak transient absorption ascribable to the cation appeared and stayed until 40 ps after excitation in chloroform, but not in n -hexane. In chloroform, direct transformation from the 1B u -type singlet state to the radical cation had a time constant 0.14±0.03 ps.

Excitation Energy-Transfer Dynamics of Brown Algal Photosynthetic Antennas.

Fucoxanthin-chlorophyll-a/c protein (FCP) complexes from brown algae Cladosiphon okamuranus TOKIDA (Okinawa Mozuku in Japanese) contain the only species of carbonyl carotenoid, fucoxanthin, which exhibits spectral characteristics attributed to an intramolecular charge-transfer (ICT) property that arises in polar environments due to the presence of the carbonyl group in its polyene backbone. Here, we investigated the role of the ICT property of fucoxanthin in ultrafast energy transfer to chlorophyll-a/c in brown algal photosynthesis using femtosecond pump-probe spectroscopic measurements. The observed excited-state dynamics show that the ICT character of fucoxanthin in FCP extends its absorption band to longer wavelengths and enhances its electronic interaction with chlorophyll-a molecules, leading to efficient energy transfer from fucoxanthin to chlorophyll-a.

Ultrafast Energy‐Transfer Pathway in a Purple‐Bacterial Photosynthetic Core Antenna, as Revealed by Femtosecond Time‐Resolved Spectroscopy

Reaping a good light harvest: Femtosecond spectroscopy uncovered a novel pathway of singlet-singlet excitation-energy transfer (EET) from bacteriochlorophyll (Bchl) in a purple bacterium to the carotenoid spirilloxanthin (Spx) upon the excitation of Bchl into the Qx band (see picture). This pathway was also clearly identified in steady-state fluorescence excitation spectra, but only in the presence of Spx. IC=internal conversion.