Guanine radical

The two overwhelming oxidation products of the purine moiety of dGuo 37 arising from the transformation of guanine radical cations 38 were isolated and identified as 2,2-diamino-4-[(2-deoxy-/l-D-eryfhro-pentofura-nosyl)amino]-5(2H)-oxazolone (41) and its precursor 2-amino-5-[(2-deoxy-... 

Relevant kinetic information on two competitive reactions of guanine radical cations within double stranded DNA, namely hydration and hole transfer to another guanine residue, has been examined [13]. Thus, the pseudoorder rate for hydration of guanine radical cations 38 has been estimated to... 

Scheme 1 Generation of guanine radical cation 4 and sugar radical cation 7...

The guanine radical cations (G +) are detected by their reactions with water, which leads after treatment with piperidine or ammonia to selective strand cleavage [14]. A similar charge detection method was used by J.K. Barton, G.B. Schuster and I. Saito as described in their articles in this volume. The cleavage products were separated and quantified by gel electrophoresis. A typical example is shown in Fig. 7 where the GGG unit acts as a thermodynamic sink for the positive charge, and the efficiency of the charge transfer can be measured by the product ratio Pggg/Pg-... 

Fig. 7 Histogram showing the products PG and PGgg> formed after charge injection into G, water trapping of the guanine radical cations and subsequent strand cleavage...

Fig. 11 Yields of the water trapping products Pq at the single guanines, and reaction profile diagram for the electron transfer as well as water trapping of the guanine radical cations...

Fig. 13 Influence of the (A T)n sequence on the hole transfer between a guanine radical cation G + and a GGG triplet... 

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...

In the perfectly paired double strand 22, the yield of product PGgg> which indicates the amount of charge that has reached the hole trap GGG, is 68%. But if the intermediate G C base pair is exchanged by a G T mismatch, the efficiency of the charge transport drops to 23%. With an abasic site (H) opposite to G the hole transport nearly stops at this mismatched site (Fig. 15). We have explained this influence of a mismatch on the efficiency of the charge transport by a proton transfer from the guanine radical cation (G2 +)... 

Scheme 6 Deprotonation of a guanine radical cation with and without a complementary base...

DNA-mediated charge transport has been a hot topic in chemistry for the last decade. Studies of DNA-mediated guanine radical cation (hole) transport are often focused on the efficiency of the transport between two guanines separated by a large distance, through the /r-stack of DNA base pairs [1-14]. The efficiency of the hole transport could be easily determined if one knew the distance between the hole donor and acceptor, the relative number of holes that reach the acceptor, and the time required for the transport. 

Our studies showed that i) substitution of an exocyclic amino group of dG is effective in modulating the chemical properties of dG toward one-electron oxidation, and ii) decomposition of the guanine radical cation was effectively suppressed near dphG. These results indicate that dphG is a prototype of nucleosides functioning as an intrinsic antioxidant of duplex DNA toward one-electron oxidation. 

All researchers in the field, however, agree that the guanine radical cations react with water or oxygen to give oxidative DNA lesions, which are in vivo... 

Scheme 4 Proposed pathways for the reaction of nitrogen dioxide radical (NOj) with the neutral guanine radical.

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