Guanine hopping

Fig. 14 Reaction profile diagram for the hole transfer from a guanine radical cation (G +) to a distant GGG sequence via the activated hopping mechanism, which also involves adenines (A) as charge carriers...

To our knowledge, there are no other direct measurements of the dynamics of interstrand hole transport. However, the strand cleavage studies of Meggers et al. [17] have clearly demonstrated that long-distance hole transport can occur via a G-hopping sequence in which guanines are located in both strands. Other workers have assumed that hole transport occurs exclusively via intrastrand pathways [45]. 

Two different explanations have been advanced for the difference in trapping rates of GG and GGG. Berlin, Burin, and Ratner [54] base their explanation on the assumptions that (1) both traps are deep, i.e., -0.5 eV for GG and -0.7 eV for GGG, and (2) the ionization potential of guanine is lower than that of the other nucleobases by at least 0.4 eV. The different reactivities of the two traps were ascribed in [54] to different relaxation times of a hole in the trap. The GG units were taken to have a long relaxation time, so that a hole is likely to make a further hop before the trap closes on it, while the relaxation time of the GGG units was supposed to be relatively short, faster than the hopping time. 

An energy level scheme for photoinitiated hole hopping in DNA is shown in Figure 16. Photoinduced electron transfer between the singlet acceptor and the nearest guanine, creates the initial hole on guanine, which can then undergo... 

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