Photoinduced electron transfer in DNA matrix from intercalated ethidium to condensed methylviologen

Fromherz, Peter; Rieger, Bernhard

doi:10.1021/ja00277a060

Cited by 155 publications

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“…In the 1980s and early 90s, a class of experiments on noncovalently bound electron donors (D) and electron acceptors (A) in DNA was reported (7)(8)(9)(10)(11)(12). A major debate focused on whether or not ET through DNA may proceed rapidly and differently from that found in -bonded systems.…”

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confidence: 99%

Femtosecond dynamics of DNA-mediated electron transfer

Wan

Fiebig

Kelley

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

373

362

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Diverse biophysical and biochemical studies have sought to understand electron transfer (ET) in DNA in part because of its importance to DNA damage and its repair. However, the dynamics and mechanisms of the elementary processes of ET in this medium are not fully understood and have been heavily debated. Two fundamental issues are the distance over which charge is transported and the time-scale on which the transport through the -stack of the DNA base pairs may occur. With femtosecond resolution, we report direct observation in DNA of ultrafast ET, initiated by excitation of tethered ethidium (E), the intercalated electron acceptor (A); the electron donor (D) is 7-deazaguanine (Z), a modified base, placed at different, fixed distances from A. The ultrafast ET between these reactants in DNA has been observed with time constants of 5 ps and 75 ps and was found to be essentially independent of the D-A separation (10-17 Å). However, the ET efficiency does depend on the D-A distance. The 5-ps decay corresponds to direct ET observed from 7-deazaguanine but not guanine to E. From measurements of orientation anisotropies, we conclude that the slower 75-ps process requires the reorientation of E before ET, similar to E͞nucleotide complexes in water. These results reveal the nature of ultrafast ET and its mechanism: in DNA, ET cannot be described as in proteins simply by a phenomenological parameter, ␤. Instead, the involvement of the base pairs controls the time scale and the degree of coherent transport.The striking resemblance of the base-pair stack of DNA to conductive one-dimensional aromatic crystals prompted, over 30 years ago, the proposal that long-range charge transport might proceed through DNA (1). In the three decades since, biochemical, biophysical, and theoretical studies have sought to address the possibility and efficiency of the transport (2-29). Such charge migration through DNA is significant, because radical migration is a critical issue to our understanding of carcinogenesis and mutagenesis (5, 6).Photoinduced electron transfer (ET) reactions have provided a useful tool in elucidating parameters governing ET through DNA. In the 1980s and early 90s, a class of experiments on noncovalently bound electron donors (D) and electron acceptors (A) in DNA was reported (7-12). A major debate focused on whether or not ET through DNA may proceed rapidly and differently from that found in -bonded systems. These experiments provided valuable information and raised many questions, but a key issue was the distance between D and A, which was not well defined.With D and A covalently bonded to DNA, studies of ET on more well defined assemblies were made possible, and the effect of distance could be addressed (13)(14)(15)(16)(17)(18)(19)(20). Values of the parameter ␤, which reflects the distance scale of ET through a given medium (30), remarkably, ranged from Յ0.1 Å Ϫ1 to Ͼ1.4 Å Ϫ1; ␤ was estimated by using one system of a fixed distance (13,14) or by varying the distance (15-20). The fact that values of ␤ coul...

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confidence: 99%

Femtosecond dynamics of DNA-mediated electron transfer

Wan

Fiebig

Kelley

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

373

362

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“…E has also been employed as a probe of DNA-mediated electron transfer (ET) reactions (11)(12)(13)(14)(15)(16). Recently, it has been shown, in systems employing E covalently tethered to DNA duplexes, that on photoexcitation this intercalator acts as an electron acceptor with respect to a modified guanine base, 7-deazaguanine (11), or as a donor with respect to a rhodium intercalator (12).…”

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confidence: 99%

Femtosecond dynamics of the DNA intercalator ethidium and electron transfer with mononucleotides in water

Fiebig

Wan

Kelley

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

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Ethidium (E) is a powerful probe of DNA dynamics and DNA-mediated electron transfer (ET). Molecular dynamical processes, such as solvation and orientation, are important on the time scale of ET. Here, we report studies of the femtosecond and picosecond time-resolved dynamics of E, E with 2deoxyguanosine triphosphate (GTP) in water, and E with 7-deaza-2-deoxyguanosine triphosphate (ZTP) in water; E undergoes ET with ZTP but not GTP. These studies elucidate the critical role of relative orientational motions of the donor-acceptor complex on ET processes in solution. For ET from ZTP to E, such motions are in fact the ratedetermining step. Our results indicate that these complexes reorient before ET. The time scale for the solvation of E in water is 1 ps, and the orientational relaxation time of E is 70 ps. The impact of orientational and solvation effects on ET between E and mononucleotides must be considered in the application of E as a probe of DNA ET.Because of its unique properties, ethidium (E) bromide has been used widely for various biochemical and biophysical applications (e.g., refs. 1-10). The cationic dye E intercalates in helical polynucleotides and can be used as a nucleic acid stain. The intercalated probe has different nanosecond fluorescence properties from those of E in water, and one can distinguish between intercalated and nonintercalated E molecules (1-5). Time-resolved studies of E intercalated in DNA on the picosecond time scale provide a probe of torsional dynamics and DNA flexibility, which are important structural features of DNA (7-10).E has also been employed as a probe of DNA-mediated electron transfer (ET) reactions (11-16). Recently, it has been shown, in systems employing E covalently tethered to DNA duplexes, that on photoexcitation this intercalator acts as an electron acceptor with respect to a modified guanine base, 7-deazaguanine (11), or as a donor with respect to a rhodium intercalator (12). In these studies, measurements of fluorescence quenching suggested fast reaction kinetics (Ն10 10 s Ϫ1) and a sensitivity of quenching to stacking.Despite the general utility of E as a DNA probe, its photophysical properties on the ultrafast time scale have not been examined extensively. Sommer et al. (17) measured the transient fluorescence spectra with picosecond time resolution and observed a time-dependent spectral shift on a subnanosecond time scale in glycerol. This shift was attributed to an intramolecular relaxation process involving phenyl-group rotation. The fact that they could not resolve the dynamics of this process when water was used as a solvent has been attributed to the lower viscosity, which offers no significant hindrance to phenyl-group rotation.Studies of the femtosecond dynamics of E in water and ET processes of E with mononucleotides lay the foundation for studies of ultrafast ET dynamics in DNA and time-resolved studies of biological dynamics. Here, we report our studies of the femtosecond dynamics of E and of ET between E and mononucleotides in water. Expe...

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“…To further characterize the interaction of ClAlTMPyPa with DNA we analyzed the fluorescence pattern in the presence of cthidium, a well known DNA intercalator. A similar approach has been reported to study DNA-binding properties of other ligands (Baguley and Le Bret, 1984;Fromherz and Rieger, 1986), including cationic porphyrins (Pasternack et al, 1991). In the absence of DNA, the ClAlTMPyPa concentration-dependence of the quenching of ethidium fluorescence (excitation wavelength 490 nm ; emission wavelength 604 nm) followed a standard (linear) Stern-Volmer relationship of the form FJF = l+K,[Q], with K, = 8.3 X 10' M-' (Fig.…”

Section: Fluorescence Spectroscopymentioning

confidence: 94%

Interactions of chloroaluminium‐tetramethyl‐tetrapyridino‐porphyrazine with DNA

Gantchev

Ali

Lier

1993

European Journal of Biochemistry

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The interaction of chloroaluminium-tetramethyl-tetrapyridinoporphyrazine (ClAlTMPyPa) with DNA was investigated by absorptiodemission and circular-dichroic spectroscopy. Formation of a DNA-porphyrazine complex was evidenced by quenching of the ClAlTMPyPa fluorescence, a hypochromic red shift of the absorption band in the visible part of the spectrum and induction of a characteristic CD band with high positive ellipticity. Scatchard analysis of the spectral changes in low ionic strength buffer suggested an association constant of 9.7 2 0 . 6 X 10' M-'. Quenching of the fluorescence of a DNA-ethidium complex by added ClAlTMPyPa was shown to result rrom direct DNA-dye interaction rather than the displacement of ethidium from DNA. Our data suggest that porphyrazine binds to the outside of the DNA helix, involving interactions which are not limited to electrostatic forces only. The computer assisted modelling of ClAlTMPyPa-DNA interactions showed that the energy-minimized intermolecular orientation involves face-on binding of the ligand, preferably on the minor DNA groove.The interaction of cationic meso-tetrapyridyl substituted porphyrins with polynucleotides has previously been studied to probe nucleic acid structure and dynamics (Pasternack and Gibbs, 1989; Raner ct al., 1989;Marzilli, 1990). It has been established that the DNA-binding-mode preference of cationic porphyrins (e.g. the external binding versus intercalation) can be modulated not only by changes in the coordinated metal, but also by altcration in their periphery (Carvlin et al., 1983;Pasternack et al., 1983). Phthalocyanines closely resemble porphyrins in many of their phy sico-chemical, photophysical and photosensitizing properties. Current interest in the biological activities of phthalocyanines relates to their potential use as tumor-localizing photosensitizers for the photodynamic therapy of cancer (Rosenthal, 1991 ; Paquette and van Lier, 1992). Among the new photosensitizers for photodynamic therapy and other photobiological applications (e.g. in photochemical DNA footprinting), phthalocyanine analogues, such as tetramethyl-metalloporphyrazincs, are promising candidates. Due to the size criterion and their histochemical staining behaviour, tetramethyl-porphyrazines are believcd to bind intrahelically to DNA (Scott, 1972). However, detailed analysis of the DNA-binding properties of cationic porphyrazines have not been reported. In the present study. we characterize the interactions of chloroaluminiumLctramcthyl-tetrapyridinoporphyrazine (Cl AlTMPyPa, Fig. 1 ) with natural, long-chain DNA using absorption, fluorescence and CD spectroscopy. The optical properties of the DNA-

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Photoinduced electron transfer in DNA matrix from intercalated ethidium to condensed methylviologen

Cited by 155 publications

References 0 publications

Femtosecond dynamics of DNA-mediated electron transfer

Femtosecond dynamics of DNA-mediated electron transfer

Femtosecond dynamics of the DNA intercalator ethidium and electron transfer with mononucleotides in water

Interactions of chloroaluminium‐tetramethyl‐tetrapyridino‐porphyrazine with DNA

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