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Ultrafast Proton‐Coupled Electron‐Transfer Dynamics in Pyrene‐Modified Pyrimidine Nucleosides: Model Studies towards an Understanding of Reductive Electron Transport in DNA
Author(s) -
Raytchev Milen,
Mayer Elke,
Amann Nicole,
Wagenknecht HansAchim,
Fiebig Torsten
Publication year - 2004
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200301205
Subject(s) - chemistry , photochemistry , protonation , electron transfer , pyrimidine , radical ion , ultrafast laser spectroscopy , proton coupled electron transfer , marcus theory , intramolecular force , photoinduced electron transfer , pyrene , ion , reaction rate constant , spectroscopy , stereochemistry , kinetics , organic chemistry , physics , quantum mechanics
5‐(Pyren‐1‐yl)‐2′‐deoxyuridine ( PydU ) and 5‐(Pyren‐1‐yl)‐2′‐deoxycytidine ( PydC ) were used as model nucleosides for DNA‐mediated reductive electron transport (ET) in steady‐state fluorescence and femtosecond time‐resolved transient absorption spectroscopy studies. Excitation of the pyrene moiety in PydU and PydC leads to an intramolecular electron transfer that yields the pyrenyl radical cation and the corresponding pyrimidine radical anion (dU . − and dC . − . By comparing the excited state dynamics of PydC and PydU , we derived information about the energy difference between the two pyrimidine radical anion states. To determine the influence of protonation on the rates of photoinduced intramolecular ET, the spectroscopic investigations were performed in acetonitrile, MeCN, and in water at different pH values. The results show a significant difference in the basicity of the generated pyrimidine radical anions and imply an involvement of proton transfer during electron hopping in DNA. Our studies revealed that the radical anion dC . − is being protonated even in basic aqueous solution on a picosecond time scale (or faster). These results suggest that protonation of dC . − may also occur in DNA. In contrast, efficient ET in PydU could only be observed at low pH values (<5). In conclusion, we propose—based on the free energy differences and the different basicities—that only dT . − but not dC . − can participate as an intermediate charge carrier for excess electron migration in DNA.