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A. G. Gagliano † N. E. Geacintov ‡ J. Breton S. Acker R. Remy § 《Photochemistry and photobiology》1979,29(2):415-418
Abstract— The P700 chlorophyll a -protein complex (CPI) isolated from green plants was oriented in aqueous solutions using pulsed electric fields of up to 6700 V cm-1 . The electric linear dichroism spectrum is reported in the range of 400–720nm. Positive peaks in the linear dichroism Δ A = A I - A 1 (where AI and A1 are the absorbance components in which the polarizer orientation is parallel and perpendicular with respect to the electric field. respectively) are observed at 443 and 686 nm. The ΔA signal at 686 nm is discussed in terms of either a specialized chlorophyll form absorbing at 686 nm. or due to an exciton component absorbing at the same wavelength. 相似文献
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Abstract— The mechanisms of orientation in pulsed and alternating electric fields of thylakoids (derived from the sonication of spinach chloroplasts) and of light-harvesting chlorophyll a/b-protein complexes (CPII) were investigated by utilizing linear dichroism techniques. Comparisons of the linear dichroism spectra of thylakoids and CPII particles suggest that the latter are oriented with their directions of largest electronic polarizabilities (and thus probably their largest dimensions) within the thylakoid membrane planes. At low electric field strengths (< 12 V cm?1), and at low frequencies of alternating electric fields (< 0.25 Hz), thylakoid membranes tend to align with their normals parallel to the direction of the applied electric field; the mechanism of orientation involves a permanent dipole moment of the thylakoids which is oriented perpendicular to the planes of the membranes. However, at high field strengths and high frequencies of the applied alternating electric fields, the thylakoids tend to orient with their planes parallel to the applied field, thus exhibiting an inversion of the sign of the linear dichroism as the electric field strength is increased. At the higher frequencies and at higher field strengths, the orientation mechanisms of the thylakoids involve induced dipole moments related to anisotropies in the electronic polarizabilities. The polarizability is higher within the plane than along a normal to the plane, thus accounting for the inversion of the dichroism as the electric field strength is increased. The CPII particles align with their largest dimension parallel to the applied field at all field strength, indicating that the induced dipole moment dominates the orientation mechanisms in pulsed electric fields. The magnitude of the absolute linear dichroism of CPII suspensions increases with increasing dilution, indicating that aggregates of lower symmetry are formed at higher concentrations of the CPII complexes. 相似文献
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Shafirovich V Dourandin A Luneva NP Singh C Kirigin F Geacintov NE 《Photochemistry and photobiology》1999,69(3):265-274
The excitation of pBr322 supercoiled plasmid DNA with intense near-IR 810 nm fs laser pulses by a simultaneous multiphoton absorption mechanism results in single-strand breaks after treatment of the irradiated samples with Micrococcus luteus UV endonuclease. This enzyme cleaves DNA strands at sites of cyclobutane dimers that are formed by the simultaneous absorption of three (or more) 810 nm IR photons (pulse width approximately 140 fs, 76 MHz pulse repetition, average power output focused through 10x microscope objective is approximately 1.2 MW/cm2). Direct single-strand breaks (without treatment with M. luteus) were not observed under these conditions. However, in the presence of 6 microM of the intercalator proflavine (PF), both direct single- and double-strand breaks are observed under conditions where substantial fractions of undamaged supercoiled DNA molecules are still present. The fraction of direct double-strand breaks is 30 +/- 5% of all measurable strand cleavage events, is independent of dosage (up to 6.4 GJ/cm2) and is proportional to In, where I is the average power/area of the 810 nm fs laser pulses, and n = 3 +/- 1. The nicking of two DNA strands in the immediate vicinity of the excited PF molecules gives rise to this double-strand cleavage. In contrast, excitation of the same samples under low-power, single-photon absorption conditions (approximately 400-500 nm) gives rise predominantly to single-strand breaks, but some double-strand breaks are observed at the higher dosages. Thus, single-photon excitation with 400-500 nm light and multiphoton activation of PF by near-IR fs laser pulses produces different distributions of single- and double-strand breaks. These results suggest that DNA strand cleavage originates from unrelaxed, higher excited states when PF is excited by simultaneous IR multiphoton absorption processes. 相似文献
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O. V. Lukashevich N. A. Cherepanova A. Kolbanovsky N. E. Geacintov E. S. Gromova 《Moscow University Chemistry Bulletin》2014,69(2):92-96
We have studied the impact of DNA damage by one of the most common carcinogens, benzo[a]pyrene, on the functioning of the truncated isoform of DNA methyltransferase Dnmt3a (Dnmt3a2). It is revealed with 30-mer model DNA substrates that DNA methylation rates are drastically reduced when the lesions disturb the structure of the Dnmt3a recognition site or hinder the interaction of the enzyme catalytic loop with the DNA minor groove. Under the chosen conditions, the PWWP domain of Dnmt3a possessing lower affinity to DNA in comparison with the catalytic domain does not influence catalysis. 相似文献
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Byeong Hwa Yun Peter C. Dedon Nicholas E. Geacintov Vladimir Shafirovich 《Photochemistry and photobiology》2010,86(3):563-570
The photosensitized hole injection and guanine base damage phenomena have been investigated in the DNA sequence, 5′-d(CAT G 1 Py C G 2 TCCTAC) with a site-specifically positioned pyrene-like (Py) benzo[ a ]pyrene 7,8-diol 9,10-epoxide-derived N2 -guanine adduct (G1 Py ). Generation of the Py radical cation and subsequent hole injection into the DNA strand by a 355 nm nanosecond laser pulses (∼4 mJ cm−2) results in the transformation of G1 Py to the imidazolone derivative Iz1 Py and a novel G1 Py * photoproduct that has a mass larger by 16 Da (M+16) than the mass (M) of G1 Py . In addition, hole transfer and the irreversible oxidation of G2, followed by the formation of Iz2 was observed (Yun et al. [2007], J. Am. Chem. Soc., 129, 9321). Oxygen-18 and deuterium isotope labeling methods, in combination with an extensive analysis of the MS/MS fragmentation patterns of the individual dG Py * nucleoside adduct and other data show that dG Py * has an unusual structure with a ruptured cyclohexenyl ring with a carbonyl group at the rupture site and intact guanine and pyrenyl residues. The formation of this product competes with hole injection and thus diminishes the efficiency of oxidation of guanines within the oligonucleotide strand by at least 15% in comparison with that in the dG Py nucleoside adduct. 相似文献
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A computer solution of the Pauli Master Equation is used to study the fluorescence quantum yield of a two-dimensional random host-trap-anti-trap system. Calculations predict that both the quantum yield and prompt fluorescence lifetime from photosynthetic units increase as pigment heterogeneity increases. From the temperature dependent action spectrum of fluorescence the pairwise Forster transfer rate is predicted to decrease with decreasing temperature. 相似文献
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Photogeneration of charge carriers in tetracene 总被引:1,自引:0,他引:1