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ABSTRACT: In this work, we employ cyanobacteria, Spirulina platensis, and separate their photosynthetic apparatus, phycobilisome (PBS), thylakoid membrane and phycobilisome-thylakoid membrane complex. The steady state absorption spectra, fluorescence spectra and corresponding deconvoluted spectra and picosecond time-resolved spectra are used to investigate the energy transfer process in phycobilisome-thylakoid membrane complex. The results on steady state spectra show chlorophylls of the photosystem II are able to transfer excitation energy to phycobilisome with Chla molecules selectively excited. The decomposition of the steady state spectra further suggest the uphill energy transfer originate from chlorophylls of photosystem II to cores of phycobilisome, while rods and cores of phycobilisome cannot receive energy from the chlorophylls of photosystem I. The time constant for the back energy transfer process is 18 ps.
Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 07/2004; 60(7):1543-7. · 2.10 Impact Factor
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ABSTRACT: The [2+2] cycloreversion reaction of formylcyclobutane radical anion (c-C4H7-CHO•-) has been investigated at the UB3LYP level with the augmented Dunning's correlation-consistent polarized valence double-ζ basis set supplied with four even-tempered sp shells. Very diffuse p−π*-like singly occupied orbitals (SOMO) are found for the c-C4H7-CHO•- and product CH2CHCHO•- radical anions, necessitating the use of a rather diffuse basis set for mechanistic study. The respective electron affinities of c-C4H7-CHO and CH2CHCHO are calculated to be 5.4 and 16.1 kcal/mol, showing the ability to bind an extra electron. The intermediate structure •(CH2)3CHCHO- is found to be a valence-bound distonic anion apt to the elimination of C2H4. The present two-step “rotating” cycloreversion mechanism for c-C4H7-CHO•- is formally similar to the biradical one for neutral cyclobutane structures, but with evidently lower potential barrier. For efficient electron-attachment catalysis, the extra electron should be trapped by suitable functional groups in some orbitals with substantial overlap with the σ*-orbitals of the cyclobutane structure.
05/2004;
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ABSTRACT: We have studied, by means of sub-microsecond time-resolved absorption spectroscopy, the triplet-excited state dynamics of carotenoids (Cars) in the intermediate-light adapted LH2 complex (ML-LH2) from Rhodopseudomonas palustris containing Cars with different numbers of conjugated double bonds. Following pulsed photo-excitation at 590 nm at room temperature, rapid spectral equilibration was observed either as a red shift of the isosbestic wavelength on a time scale of 0.6-1.0 mus, or as a fast decay in the shorter-wavelength side of the T(n)<--T(1) absorption of Cars with a time constant of 0.5-0.8 mus. Two major spectral components assignable to Cars with 11 and 12 conjugated double bonds were identified. The equilibration was not observed in the ML-LH2 at 77 K, or in the LH2 complex from Rhodobacter sphaeroides G1C containing a single type of Car. The unique spectral equilibration was ascribed to temperature-dependent triplet excitation transfer among different Car compositions. The results suggest that Cars of 11 and 12 conjugated bonds, both in close proximity of BChls, may coexist in an alpha,beta-subunit of the ML-LH2 complex.
Photosynthesis Research 02/2004; 82(1):83-94. · 3.24 Impact Factor
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ABSTRACT: The reactions of BH2+ with propylene (CH2=CHCH3) to form both the adducts BC3H8+ and the H2-elimination products BC3H6+ + H2 have been investigated at the density functional B3LYP/6-311G(d,p) level of theory. It is shown that the electrophilic attacks of BH2+ towards two olefinic carbons of H2C=CHCH3 and two subsequent 1,3-H-shifts may form four low-lying BC3H8+ isomers (with the relative energies in parentheses in kcal/mol): 1 BH2+.CH2CHCH3 (0.0), 1' BH2+.CH3CHCH2 (6.3), 3 BHCH2CH2CH3+ (4.3), and 4 BHCH(CH3)2+ (5.0), respectively. On the other hand, further H2-eliminations may also occur easily between B-C bonds of isomers 1 and 1' and between C-C bonds of isomers 3 and 4 to form two dissociation products (P1) HBCHCHCH3+ + H2 and (P2) HBC(CH3)CH2+ + H2, with H2-elimination from isomer 1 to be energetically most favorable. According to our calculated mechanism, the collisional stabilization processes of low-lying isomers 1, 1', 3, and 4 may compete extensively with their H2-eliminations processes for the title reaction, leading mainly to some linear carborane cations. This study may be helpful for understanding the stereochemical aspects of borohydride cations towards alkylenes.
Journal of Computational Chemistry 02/2004; 25(2):258-64. · 4.58 Impact Factor
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ABSTRACT: The mechanism of cycloreversion of cyclobutane radical anion (c-C(4)H(8) (-)) has been investigated at the UB3LYP/6-31++G(d,p) level, and compared with those of neutral c-C(4)H(8) and c-C(4)H(8) (+) radical cation. Although both c-C(4)H(8) (-) and C(2)H(4) are shown to be Rydberg states unstable with respect to electron ejection, the activation barrier for the "rotating" cycloreversion of c-C(4)H(8) (-) (37.3 kcal/mol) is lower by about 25.2 kcal/mol than that of c-C(4)H(8), and even the intervention of tetramethylene radical anion intermediate may reduce the activation barrier for the cycloreversion of c-C(4)H(8) by about 8.4 kcal/mol, mainly due to stronger electron-deficiency of intermediate biradical species than close-shell cyclobutanes. For the cycloreversion for c-C(4)H(8) (-), side isomerization reaction may be efficiently prevented by the low kinetic stability of tetramethylene radical anion intermediate towards dissociation, just different from the radical cation case. Our theoretical results have suggested the possibility of electron-attachment catalysis of the cycloreversion of some electron-deficient substituted cyclobutanes.
Journal of Computational Chemistry 03/2003; 24(3):340-4. · 4.58 Impact Factor
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ABSTRACT: The mechanism of the primary electron transfer (ET) process in the photosynthetic reaction center (PRC) of Rhodobacter sphaeroides has been studied with quantum chemistry method of ab initio density functional theory (DFT) (B3LYP/6-31G) based on the optimized X-ray crystallographic structure. The calculation was carried out on different structural levels. The electronic structure of pigment molecules was first studied, and then the influence of the neighboring protein was taken into account at three approximation levels: (a) the surrounding proteins were treated as a homogeneous medium with a uniform dielectric constant (SCRF); (b) both the influence of axial coordination of His to the special pair P and ABChl as, and the hydrogen bonds between related residues and P and also BPhas were included; and (c) the influence of the electronic structure of the protein subunit chains as a whole was studied. The results suggest that: (1) according to the composition of the HOMO and LUMO of P, there might be a charge-separated state of (BChl(L) (+)BChl(M) (-)) for the excited state of P; (2) to treat the protein surroundings as a homogeneous medium is not sufficient. Different interactions between pigment molecules and related residues play different roles in the ET process; (3) the axial coordination of His to P raises the E (LUMO) of P greatly, and it is very important for the ET process to occur in the PRC of wild-type bacterium; the axial coordination of His to ABChl as also raises their E (LUMO) significantly; (4) the hydrogen-bonds between amino acid residues and P and also BPh as depress the E (LUMO) of the pigment molecules to some extent, which makes the E (LUMO) of P lower than those of ABChlas, and the E (LUMO) of BPh a (L) lower than that of BPh a (M). Consequently, the ET process from P to BPh a (L) does not, according to our calculation model, occur via ABChl a (L). The possibility of the ET pathway from P to BPh a (L) via ABChl a (L) was discussed; (5) the frontier orbitals of protein subunit chains L and M are localized at the random coil area and the alpha-helix areas, respectively. Results mentioned above support the fact that the ET process proceeds in favourable circumstances along the branch L.
Photosynthesis Research 02/2002; 74(1):11-36. · 3.24 Impact Factor
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ABSTRACT: In this paper, the micro-solvated effects on the lowest-energy vertical transition state and adiabatic excited states of 2-aminopurine (2Ap) were studied by Supramolecular method (B3LYP/6-31++G(d)) and ONIOM (B3LYP/6-31++G(d):PM3) method. The results are as follows: (1) In 2Ap molecule surrounded by six water molecules the pyramidalization of amino group in 2Ap almost disappears, and the hex-atomic ring is obviously buckled. The adiabatic lowest-energy valence excitation of gaseous 2Ap also causes the disappearance of amino pyramidalization. (2) The energy for lowest-energy singlet π→π∗ vertical transition in water is predicted as 3.99 and 4.29 eV by Supramolecular and ONIOM method, respectively. Both values are in good agreement with the reported experimental result, 4.11 eV. The energy for the second lowest-energy n→π∗ transition, 4.72 eV, by the Supramolecular method is obviously deviated from the reported experimental value 4.46 eV. The corresponding value given by the two-layer ONIOM method, 4.43 eV, is in good agreement with the experimental value. (3) The optimized energy of the fluorescent emission state (S1 state) are 3.61 and 3.87 eV by Supramolecular and ONIOM methods, respectively. The calculated oscillator strengths, in both gas and water clusters, were compared with reported experimental and theoretical results. These results indicated that both Supramolecular and ONIOM methods, combined with TD DFT B3LYP/6311++G(d), can provide good results of calculating excited state and spectra properties of 2Ap in condensed phase. This fact encouraged us to extend our study to 2Ap-T base pair and its solvated model so as to obtain the spectra properties of 2Ap in real DNA environment.
Journal of Molecular Structure THEOCHEM 680:21-27. · 1.44 Impact Factor
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ABSTRACT: The cycloreversion of cyclobutane radical cation (1 c-C4H8+) has been investigated in detail at the UB3LYP/6-31++G(d,p) level of theory. The `rotating' mechanism of cycloreversion via the `tightly bound' intermediate of tetramethylene radical cation (2 CH2CH2CH2CH2+) has been revealed for the first time. It is shown that both the present `rotating' mechanism and the previous `shifting' one via the `loosely bound' complex (3 C2H4·C2H4+) may compete intensively with each other for this reaction, leading to the low-lying isomers (4c-CH2CH2CH–CH3+ and 5 CH3CH2CHCH2+) in low temperature but to the dissociation products 6 C2H4+C2H4+ in high temperature above 600 K.
Chemical Physics Letters 360:283-288. · 2.34 Impact Factor
