Guanine ionization potentials

Guanine is a preferential DNA target to several oxidants it shows the lowest ionization potential among the different purine and pyrimidine nucleobases and it is the only nucleic acid component that exhibits significant reactivity toward singlet oxygen ( O2) at neutral pH. ... 

Density of states weighted Franck-Condon factor Deoxyribonucleic acid Barrier height for the adiabatic hole motion Difference in ionization potentials of adenine-thymine and guanine-cytosine base pairs... 

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. 

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-... 

The following papers may bo consulted for the ionization potentials and electron affinities of cytosine and of its complexes with different partners cytosine,171-173-177-178,180.184.186.200,201,203.207,259,260 guanine-cytosine,171-182 18 -106 2-NH2-purine (amine form)-cytosine, 2-NH2-purine(imine form)-cytosine and 2-NH2-purine(amino form)-cytosine (form 6),190 cytosine-cytosine and cytosine-cytosine-cytosine.171... 

Since the suggestion of the sequential QM/MM hybrid method, Canuto, Coutinho and co-authors have applied this method with success in the study of several systems and properties shift of the electronic absorption spectrum of benzene [42], pyrimidine [51] and (3-carotene [47] in several solvents shift of the ortho-betaine in water [52] shift of the electronic absorption and emission spectrum of formaldehyde in water [53] and acetone in water [54] hydrogen interaction energy of pyridine [46] and guanine-cytosine in water [55] differential solvation of phenol and phenoxy radical in different solvents [56,57] hydrated electron [58] dipole polarizability of F in water [59] tautomeric equilibrium of 2-mercaptopyridine in water [60] NMR chemical shifts in liquid water [61] electron affinity and ionization potential of liquid water [62] and liquid ammonia [35] dipole polarizability of atomic liquids [63] etc. 

In the experimental sections there was much discussion about products formed after one-electron loss. For example, ionization of DNA is a random process, yet 90% of the radical cations end up on guanine (which has the lowest gas-phase ionization potential). This brings up several questions that can be answered by theoretical... 

First of all, can one verify that guanine has the lowest gas-phase ionization potential Does guanine have the lowest ionization potential in aqueous solution Are there arrangements of several stacked bases that may be more easily oxidized than guanine Are there situations when the deoxyribose or the phosphate may be oxidized, say in a nucleotide ... 

Gas phase ionization potentials (IPs) of DNA bases, guanine, adenine, thymine and cytosine, have been calculated using a variety of levels of theory [33-40], In Table 20-1, we compare representative theoretical values with available... 

The guanine moiety has the lowest ionization potential of any of the DNA bases or of the sugar-phosphate backbone. As a result, radiation-produced holes are stabilized as dG for hydrated DNA irradiated at 77 K There is an extensive literature describing the role of dG in the radiation chemistry of DNA as studied by pulse radiolysis, flash photolysis, and product analysis. In order to explicate the oxidative reaction sequence in irradiated DNA and to more firmly identify the relevant radical intermediates, ESR spectroscopy was employed to investigate y-irradiated hydrated DNA (T = 12 2). Some experiments were also performed on hydrated (T = 12 2) DNA in which an electron scavenger [thallium(ni) (TP )] was employed to isolate the oxidative path. Oxygen-17 isotopically enriched water was also used to confirm a proposed water addition step to G and the subsequent transformations that follow These experiments were run in oxygen-free samples under conditions for which indirect effects were unimportant. 

Base pairing of the nucleobases in duplex DNA can affect their calculated ionization potentials and electron affinities. Colson et al. [33b] reported that base pairing lowers the IP of guanine, but has little effect on the ionization of adenine in the A T base pair. The effects of base pairing on ionization potentials has been investigated by Hunter and Clark [34] using ab initio and density functional calculations. The isodesmic relations in Eqs. 1 and 2 show that A" " and G+ are stabilized by 10.2 and 17.4 kcal moC , respectively, upon base pairing. 

The identity of the radical ions formed upon steady-state radiation of DNA in low-temperature glasses has been established by means of electron paramagnetic resonance (EPR) spectroscopy [53]. EPR analysis indicates that electrons and holes are localized on a single nucleobase rather than being delocalized over several stacked bases at low temperatures. Radical ion formation is presumed to occur randomly at all four nucleosides. However, EPR studies establish that the electron holes are localized predominately on guanine, which has the lowest gas phase ionization potential and solution oxidation potential (Tables 1 and 3). Yan et al. [54]... 

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