Search

Cis-to-trans isomerization upon excitation of the all-trans, 9‘-cis, 13‘-cis, 9-cis, 13-cis, and 15-cis isomers of spheroidene to the triplet state (by the use of anthracene as a sensitizer) was traced by submicrosecond time-resolved absorption spectroscopy. The time-resolved spectra were analyzed by means of singular-value decomposition and subsequent global fitting using a scheme having potential minima at the cis and trans positions in both the T1 and S0 states. The quantum yields of triplet-sensitized isomerization were also determined. The process of cis-to-trans isomerization in the T1 state was clearly seen for the 13‘-cis and 9-cis isomers. The quantum yield and the rate of cis-to-trans isomerization increased systematically in the order 13‘-cis, 9-cis, 13-cis, and 15-cis T1 species, whereas their rates of T1 → S0 intersystem crossing were similar to one another and 6 times larger than that of the all-trans T1 species. The Tn ← T1 absorption maximum shifted to the blue in the above order, and reached to that of the all-trans T1 species. Those observations were explained in terms of “the triplet-excited region” localized in the central part of the conjugated chain. A possible reason for the selection of the 15-cis configuration by a bacterial photoreaction center containing this carotenoid was discussed.

Changes in the carotenoid (Car) 1Bu⁺ absorption upon excitation (using a photon density of 10¹⁵ photons cm^-2) of bacteriochlorophyll (BChl) in the LH2 complex from Rhodospirillum molischianum were traced with subpicosecond time resolution. The absorption changes could be empirically fit by two components showing a blue shift and a red shift in a ratio of 1:3.6. The shifts of Car absorption were explained in terms of interaction between change in dipole moment upon the 1Bu⁺ ← ground transition of Car and that upon the Qy ← ground transition of BChl. Calculations of the shifts for three different types of excitation in the B850 ring of the LH2 complex, i.e., (i) localized excitation on B850a or B850b, (ii) partially delocalized excitation over four B850s, and (iii) completely delocalized excitation over the B850 ring predicted the Car shifts with direction (relative magnitude), (i) a blue shift (1) or a red shift (3.3), (ii) a red shift (0.8), and (iii) a small red shift (0.2), respectively. Thus, the blue shift and the red shift components were attributed to localized excitations on B850a and B850b, respectively. The B850a excitation decayed rapidly (~ 0.1 ps), whereas the B850b excitation decayed more slowly (~ 0.4 ps) and then stayed; the former decay was ascribed to singlet−singlet annihilation between B850a and B850b, whereas the latter decay to singlet−singlet annihilation between B850b's. The unique feature of excitation with high photon density, which preferentially generates the localized excitations through multiple excitation, and enhances subsequent singlet−singlet annihilation reactions, is discussed.

Fluorescence spectra of all-trans-anhydrorhodovibrin and spirilloxanthin having the number of conjugated double bonds n = 12 and 13 were recorded in n-hexane solution. The optically forbidden 2Ag^- → 1Ag^- fluorescence was observed for carotenoids having such a long conjugated chain, and the optically forbidden 1Bu^- → 1Ag^- fluorescence was identified for the first time. The proposed energies of the 2Ag^-, 1Bu^-, and 1Bu⁺ fluorescence origins are 12 500, 14 900, and 19 200 cm^-1 for anhydrorhodovibrin and 11 900, 13 600, and 18 900 cm^-1 for spirilloxanthin (at 295 K). The estimated transition dipole moments for fluorescence from the 2Ag^-, 1Bu^-, and 1Bu⁺ states to the ground 1Ag^- state are 1.24, 4.76, and 17.4 D for the former and 1.37, 3.72, and 16.8 D for the latter.

The major role of vibronic coupling through the in-phase, C&dbd;C stretching (ν1) mode in the 2Ag^- to 1Ag^- internal conversion of all-trans-β-carotene has been shown by the use of isotopic effects on the rate of internal conversion and on the strength of vibronic coupling as follows:  (1) The rates of internal conversion for all-trans-β-carotene having natural abundance isotope composition [NA], along with ²H-labeled [²H], ¹³C-labeled [¹³C], and ²H,¹³C-doubly labeled [²H,¹³C] all-trans-β-carotenes, were determined, by subpicosecond time-resolved absorption spectroscopy, to be in the ratio [NA]/[²H]/[¹³C]/[²H,¹³C] = 1:0.92:0.70:0.64. (2) The strength of vibronic coupling was estimated for each isotope species by using the frequency difference between the 2Ag^- and 1Ag^- states, which was determined, by picosecond Raman spectroscopy, to be in the ratio, [NA]/[²H]/[¹³C]/[²H,¹³C] = 1:1.21:0.89:1.07. On the other hand, a theory was presented to show that the nonadiabatic vibronic-coupling constant that determines the rate of internal conversion is proportional to the adiabatic vibronic-coupling constant that determines the frequency difference. The application of the observed relative strength of vibronic coupling to the Englman−Jortner equation, for a single mode ν1, predicted the relative rates of internal conversion to be 1:0.80:0.72:0.60, which are in good agreement with those observed above. (3) A theory showing that the adiabatic vibronic-coupling constant is proportional to the product of the transition bond-order matrix and the L matrix was also presented. In a polyene model, the relative rates of internal conversion were predicted to be 1:0.94:0.68:0.64, which are in excellent agreement with the above observed values.

This investigation was motivated by a desire to get a deeper insight into the mechanism of carotenoiod-to-bacteriochlorophyll (Car-to-BChl) energy transfer proceeding via the carotenoid S1 state. (Here, we call the 2Ag^- and 1Bu⁺ states “the S1 and S2 states” according to the notation presently accepted.) To systematically examine the effect of the conjugation length of carotenoid on the rate and efficiency of the Car(S1)-to-BChl(Qy) energy transfer, we performed the following experiments. (1) Subpicosecond time-resolved absorption spectroscopy was employed to measure the S1-state lifetimes of lycopene (number of conjugated C&dbd;C bonds, n = 11), spheroidene (n = 10), and neurosporene (n = 9), both free in n-hexane and bound to the LH2 complexes from Rhodospirillum molischianum (Rs. molischianum), Rhodobactor sphaeroides (Rb. sphaeroides) 2.4.1, and Rb. sphaeroides G1C, respectively. The lifetime of each free (bound) carotenoid was determined to be 4.7(3.4) ps for lycopene, 9.3(1.7) ps for spheroidene, and 21.2(1.3) ps for neurosporene. It was found that the rate and the efficiency of the Car(S1)-to-BChl(Qy) energy transfer increase systematically when the number of conjugated C&dbd;C bonds decreases. (2) High-sensitivity steady-state fluorescence spectroscopy was used to measure the spectra of dual emission from the S2 and S1 states for the above carotenoids dissolved in n-hexane. The fluorescence data, combined with the above kinetic data, allowed us to evaluate the magnitudes of the transition-dipole moments associated with the Car(S1) emission. It was found that the S1 emissions of the above carotenoids carry noticeably large oscillator strengths (transition-dipole moments). In the case of the LH2 complex from Rs. molischianum, whose structural information is now available, the time constant of the Car(S1)-to-BChl(Qy) energy transfer (18.6 ps), which was predicted on the basis of a Car(S2)-to-BChl(Qy) full Coulombic coupling scaled by the ratio of the S1 vs S2 transition dipole moments, was in good agreement with the one spectroscopically determined (12.3 ps). The oscillator strength associated with the Car(S1) emission was discussed in terms of the state mixing between the carotenoid S2 and S1 states.

Vibrational relaxation in the 2Ag^- (S1) state of all-trans-lycopene, which takes place in a time scale slower than that of internal conversion, has been found by picosecond time-resolved absorption spectroscopy:  The time-resolved Sn (2Bu⁺) ← S1 (2Ag^-) absorption spectra of this carotenoid in quinoline and CS2 solutions can be explained in terms of vibrational structures originating from the C&dbd;C (ν1) and C−C (ν2) stretching modes, and the time scales of vibrational relaxation (kν1 and kν2 defined for the lowest vibrational transition) relative to that of internal conversion (k) are estimated by spectral simulation; the values of kν1/k and kν2/k are around 0.6 and 0.3 in the quinoline solution and 0.2 and 1.6 in the CS2 solution. The slower vibrational relaxations are discussed in light of the light-harvesting function of carotenoids in photosynthesis.

The protective effect of free radical scavengers against phototoxicity was investigated with tin-protoporphyrin (SnPP)-treated suckling rats. Six kinds of scavengers (L-ascorbic acid, reduced glutathione, α-tocopherol, retinol, uric acid and cystine) were intraperitoneally injected to rats treated with SnPP plus photoirradiation. Among them, L-ascorbic acid was found to be most effective in protecting SnPP-treated rats against phototoxicity. The survival period was markedly prolonged, and the frequency of abnormal behaviors was reduced with the treatment. Lipid peroxidation in vitro with the brain membrane fraction was also suppressed. The other five substances gave only a little antioxidant effect both in vivo and in vitro. The present study shows that L-ascorbic acid may be a promising chemical to prevent the phototoxicity of SnPP.

Lipid peroxidation induced by Sn-protoporphyrin (SnPP) plus photoirradiation was investigated in vivo and in vitro using nonjaundiced Gunn rats. Membrane lipids from young adult rat brain were peroxidized by SnPP plus photoirradiation depending on the SnPP concentration and photoirradiance. Similarly, coadministration of SnPP and photoirradiation to suckling rats increased lipid peroxides in the whole blood and was found lethal. The influence of the wavelength distribution of light sources was also examined by using blue-white and green fluorescent lights. The photodynamic effect by green light irradiation whose energy distribution had no overlap with the Soret band of SnPP was about half of that produced by blue-white light with regard to the membrane peroxidation and the lethal effect on neonatal rats. We therefore conclude that the combination of SnPP and photoirradiation is a potentially hazardous treatment of neonatal jaundice.