Guanine, hole transfer

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... 

In order to determine the influence of the bridge length on the efficiency of the hole transfer between guanines, we studied double strands where the number of (A T)n base pairs between the guanines were varied (Fig. 9). In these experiments the GGG unit acts as a thermodynamic sink for the charge [17]. 

In extreme cases where all electron transfer steps are reversible and the water trapping reactions are very slow, the charge is distributed over the guanines according to the thermodynamic stabilization. From these experiments one cannot deduce the influence of the sequence on the hole transfer rate. Therefore, using a chemical assay of this type leads to results that have to be discussed with great care. 

These results demonstrate that hole transfer between guanines over short and long (A T)n sequences follow different distance dependencies and there-... 

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 conclusion, hole transfer between guanines that are separated by long (A T)n sequences is possible because also the adenines become involved as charge carriers. Such a change of the reaction mechanism can also be explained by calculations [21], and has been described by J. Jortner as well as D. Beratan and M.A. Ratner in their articles of this volume. 

Experiments with methyl guanine (27), in which the acidic proton of the radical cation is exchanged by a methyl group, support this explanation [22]. With this base in a mismatch situation (strand 25) the hole transfer becomes efficient again because a deprotonation cannot occur (Fig. 15). 

The G doublet and triplet effectively functions as a thermodynamic sink in DNA-mediated hole transport. However, the rate determining step of hole trapping at guanine clusters and the rate of hole trapping are not well understood. Furthermore, hole transfer between the donor and acceptor should compete with the hole trapping reaction the relative rate of hole trapping versus the hole transfer rate determines the overall efficiency for the hole transfer. 

Very Shallow Traps. It has been proposed that the neutral Gua(Nl—H) radical, formed by proton transfer from the Gua radical by proton transfer from N1 of Gua to N3 of Cyt, is a shallow trap [143,144]. This proposal is based on projections from made on monomers in dilute aqueous solution, which predict that proton transfer is favored by 2.3 kJ/mol [22,145]. Ab initio calculations are in excellent agreement with this value [146,147]. So one expects that an energy of at least 0.025 eV is needed to activate the return of the proton to N1 Gua, reforming Gua . Once Gua is reformed, tunneling to nearby guanines is reestablished as a competitive pathway. Proton transfer therefore is a gate for hole transfer. Proton-coupled hole transfer describes the thermally driven transfer of holes from one Gua Cyt base pair to another. 

Electron and Hole Transfer to Oxidized Guanine Sites. 124... 

ESR results show that hole transfer in DNA at low temperatures is initially quenched after hole localization on guanine [9] which has the lowest ionization potential of the DNA bases [15] (Scheme 2). 

Hole transfer from guanine to sites of lower ionization potential such as intercalators occurs and is discussed later. ESR results also show that sugar and phosphate radicals are produced in smaller abundance [3, 9, 20-23] than expected from the 50% fraction of ionization that occurs on the sugar-phosphate backbone. This is evidence for hole transfer from the sugar ion-... 

Analysis of the transient absorption spectra recorded at 100 ns allows for the determination of the prompt relative yields of the G /G(-H) radicals, I g (<100 ns). This yield is probably due, in part, to the oxidation of the guanines by the 2AP + radical cations [11]. Assuming that the 2AP radical cations decay only via the deprotonation of 2AP (rate constant ku) and hole transfer from 2AP + to guanine (fct), the prompt yield Og may be expressed as follows ... 

---