On the Apparently Anomalous Distance Dependence of Charge-Transfer Rates in 9-Amino-6-chloro-2-methoxyacridine-Modified DNA

From previous thermal and photoinduced charge-transfer reactions in duplex DNA there is accumulative evidence for an attenuation parameter β of the distance dependence in the range 0.6−0.8 Å-1, with the exception of one specific system exhibiting β = 1.5 Å-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 β value of >2.0 Å-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 To test the hypothesis of distance-dependent activation energy, guanine has been replaced by 7-deazaguanine, its easier-to-oxidize purine analogue.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