On the Apparently Anomalous Distance Dependence of Charge-Transfer Rates in 9-Amino-6-chloro-2-methoxyacridine-Modified DNA
| Autor: | Maria-Elisabeth Michel-Beyerle, Stephan Hess, Mirco Götz, William B. Davis |
|---|---|
| Rok vydání: | 2001 |
| Předmět: |
Aminoacridines
Guanine Base pair DNA General Chemistry Nanosecond Chromophore Biochemistry Intercalating Agents Catalysis Nucleobase Kinetics chemistry.chemical_compound Crystallography Spectrometry Fluorescence Colloid and Surface Chemistry chemistry Acridine Thermodynamics AP site Fluorescent Dyes |
| Zdroj: | Journal of the American Chemical Society. 123:10046-10055 |
| ISSN: | 1520-5126 0002-7863 |
| DOI: | 10.1021/ja010976r |
| Popis: | From previous thermal and photoinduced charge-transfer reactions in duplex DNA there is accumulative evidence for an attenuation parameter beta of the distance dependence in the range 0.6-0.8 A(-1), with the exception of one specific system exhibiting beta = 1.5 A(-1) which is reinvestigated in this paper. Femtosecond to nanosecond time-resolved pump-probe spectroscopy has been used to follow photoinduced charge-shift dynamics in DNA duplexes containing a covalently appended, protonated 9-alkylamino-6-chloro-2-methoxyacridine chromophore. This acridine derivative (X+) resides in the DNA duplex at a specific abasic site, which is highly defined as reflected in the monoexponentiality of the kinetics. In the presence of only neighboring A:T base pairs, no charge transfer occurs within the excited-state lifetime (18 ns) of the chromophore. However, the presence of a guanine nucleobase as either a nearest neighbor or with one interspersed A:T base pair does result in fluorescence quenching. In the case of nearest neighbors, the intermediate radical state X* is formed within 4 ps and decays on the 30 ps time scale. Placing one A:T base pair between the X+ and guanine slows down the forward transfer rate by 3 orders of magnitude, corresponding to an apparent beta value of2.0 A(-1). This dramatic decrease in the rate is due to a change in charge-transfer mechanism from a (nearly) activationless to a thermally activated regime in which the forward transfer is slower than the back transfer and the X* state is no longer observed. These observations indicate that the distance dependence of charge injection in the X+-labeled DNA duplex is not solely caused by a decrease in electronic couplings but also by a concomitant increase of the activation energy with increasing distance. This increase in activation energy may result from the loss of driving force due to excited-state relaxation competing with charge transfer, or reflect distance-dependent changes in the energetics, predominantly of the low-frequency reorganization energy in this charge-shift reaction, on purely electrostatic grounds. To test the hypothesis of distance-dependent activation energy, guanine has been replaced by 7-deazaguanine, its easier-to-oxidize purine analogue. In these duplexes, a similar change of charge-transfer mechanism is found. However, consistent with an a priori larger driving force this change occurs at a larger donor-acceptor separation than in the X+-guanine systems. Independent of the detailed contributions to the distance-dependent activation energy, this phenomenon illustrates the complex nature of experimental beta values. |
| Databáze: | OpenAIRE |
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