Pyrimidine ionization potential

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. ... 

The values of the ionization potentials of the bases were unknown experimentally until quite recently, and the electron affinities are still unknown. The first experimental determination of the ionization potentials of the pyrimidine bases (uracil, thymine), by Akopyan and Vilesov,253 dates from 1965. Two years later Bergmann et al.25i measured the ionization potentials of some purine and pyrimidine bases including cytosine. The cytosine ionization potential as determined by mass spectrometry254 was found to be 8.90 + 0.2 eV. On the other hand the potential determined from the charge transfer spectra by Fulton and Lyons255 was 7.98 eV. 

The values of the ionization potentials of a few minor pyrimidine bases were estimated from the charge transfer spectra255 (Table XXXIII). These values compare well with the ionization potentials calculated by means of the 77-SCF MO method.156 However, the relative order of the ionization potentials given by the calculations (77-SCF MO and CNDO/2 methods, Tables XXXIII and XXXIV)—thiocytosine... 

Theoretical (77-SCF MO)156 and Experimental (from CT Spectra)255 Ionization Potentials of Rare Pyrimidine Bases... 

There remains the choice of Up values. As a starting point we have adopted Uc = —9.5 eV, which is known to give satisfactory values for the ionization potentials in hydrocarbons86 and determined U0, Ux, UNH> and t/NHl so as to reproduce the ionization potentials of formaldehyde, pyridine, pyrimidine, pyrrole, and aniline. Moreover,... 

The lowest ionization potential of pyrazine and related nitrogen-containing heteroaromatic compounds has been measured. The experimental value of 9.29 eV is very similar to that of benzene (9.25 eV) and pyrimidine (9.35 eV) and is thought to be due to ionization from the highest occupied 77- orbital. Subsequent calculations have supported this assignment.04,65 (For a discussion of the relative ordering of 7r, a, and n orbitals, see Turner et ol.65a)... 

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. 

Electrons are selectively captured by the pyrimidines and the holes formed on the base stack, or transferred to the base stack, are nearly all trapped by Gua, giving Gua +. This species is in a reversible equilibrium with its deprotonated form Gua(NI-H) (Fig. 2). While initial distributions between the pyrimidines are nearly equal upon annealing, or irradiation to high dose, the excess electron transfers to cytosine as the most stable site as a result of protonation of Cyt at N3 forming Cyt(N3+H) [10]. Molecular orbital calculations show that the major factor that affects the relative yields of DNA base ion radicals is the difference in both the ionization potential and electron affinity ofthe DNA bases [11]. In addition, experimental and theoretical work shows that variations in relative radical yields are also affected by the... 

The photoelectron spectrum of pyridazine is similar to those of pyrazine, pyrimidine, and triazine i.e., the lowest ionization potential corresponds to ionization of a lone-pair electron. The ionization potential is in agreement with the calculated value. These spectra were recorded also of pyridazine 1-oxide and 1,2-dioxide, and it was found that the perturbation of the t-system by the N—O group results in the separation of the lower excited states of the AT-oxide ions. ... 

The fundamental physico-chemical characteristics which enter into the evaluation of these component forces are the dipole moments, fi, the ionization potentials, /, and the polarizabilities, a, of the base pairs. The polarizabilities may be obtained relatively easily by the use of the usual additivity rules . The problems of the ionization potentials and of the dipole moments are however much more difficult. As concerns the ionization potentials they are completely unknown experimentally for the biological purines and pyrimidines (as they are, in fact, for the great majority of biomolecules). As concerns the dipole moments only those of some simple derivatives of purine, adenine and uracil are known no information exists about the moments of guanine or cytosine. 

It has been reported that the direct substitution of the benzene ring of ben-zothiophene in such compounds can be used as a tool for modification of electronic and photophysical properties of Tt-conjugated materials. In particular, the substituent effect on HOMO/LUMO energies, ionization potentials, electron affinities, and reorganization energies have been described for 4,6-di(thiophen-2-yl)pyrimidine derivatives (Scheme 36(b) 2014CTC(1031)76). 

The total energies of the mixed pyrimidine dimers and their radical cations are listed in Table 15. Each of the adiabatic ionization potential, which is the difference between the total energies of the optimized pyrimidine dimer and its radical cation, is 8.25, 7.64, 7.93 and 7.90 eV, for T<>T, CoC, T<>C and Cot, respectively. It is predicted that the adiabatic ionization potential is largest in ToT, and smallest in CoC, and those of the mixed pyrimidine dimers are in-between. No experimental data are available for the ionization potentials of the pyrimidine dimers. 

Figure 11-11. Proposed potential energy surfaces and processes for the pyrimidine bases. Ionization from the S2 state and the dark state S samples a different Franck-Condon region of the ionic state (D0(tt 1)) or reaches a different ionic state (D n-1)), resulting in different ionization energies for these two states...

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