Guanine ionization energies

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. 

Nucleic acids absorb strongly in the near-UV region because of the purine and pyrimidine bases they contain. However, the energies of the strong transitions for the five most common bases are virtually the same, which results in very close absorbance maxima (260, 246, 259, 265, and 267nm for adenine, guanine, uracil, thymine and cytosine, respectively), even though their exact position is pH dependent. This makes it rather difficult to assess the contribution of each band to the spectra of DNA or RNA, which exhibit a broad absorbance band between 240 and 280 nm due to the bases. The spectrophotometric quantification of nucleic acids is also hindered by marked variations in the spectra with pH as a result of the ionization of their constituent bases. 

Figure 9. Free energies of ionization in solution (AG, j.(solution)) for 5 -dGMP and 5 -dAMP. Orbital descriptions are the same as those given in the text and in Figures 5, 6 and 7. As in Figures 6 and 1, the hatched areas corresponds to energy regions containing orbitals in 1,9.dimethyl-guanine (l,9-Me2G) and 9-methyladenine (9-MeA) that rise to unresolved PE bands.

Excitation energies and ionization potentials are also tautomer-dependent properties [51]. This property can be seen in the observation of different photoionization curves for guanine resulting from different types of molecular beams, presumably reflecting different tautomer distributions [52]. 

---