Cytosine, calculated spectra

It is also worth mentioning that in the case of glutamate mutation TD-DFT calculated spectrum presents also a second quite intense band at about 500 nm, as well as the band listed in Table 2. Note also that glutamate could show the same dependence with pH already evidenced for the cytosine mutant. Unfortunately, experimental data reported from Clark et al. [13] are not totally sufficient to entirely clarify this feature, as well as to clarify the pH dependence of the UVAHS spectrum of such mutant... 

Fig. 23-16 Infrared spectrum of cytosine in the -OH, NH, and C=0 stretching region in an argon matrix in 15 K. (A) Initial spectrum of a sample containing both tautomers. (B) Spectrum of the amino-oxo tautomer calculated by substrac-tion of the spectrum of the amino-hydroxy tautomer. (C) Spectrum of the aminohydroxy tautomer obtained by irradiating the initial sample in order to photochemically convert it into the aminohydroxy tautomer. Spectra (B) and (C) were also corrected for residual amounts of the second tautomer. From Szczesniak et al.166 Courtesy of W. B. Person.

The data shown in Table 19 suggest that the observed spectral transitions of cytosine in water can be explained in terms of the scaled transition energies of the hydrated keto-NlH tautomer within the accuracy of about 0.2 eV except the second transition. However, the error becomes smaller if a comparison is made between the computed transitions of the hydrated keto-NlH tautomer and the observed transitions of the cytosine sublimed film. ° Such a difference between the computed and experimental data is not unexpected since the general features of the cytosine spectrum are found to be solvent dependent. It appears that the absorption peak of cytosine in water near 212 nm (5.85 eV) is shifted to 6.07 eV in the sublimed film experiment. The suggested splitting of the 197 nm (6.29 eV) transition in the form of 202 and 189 nm (6.14 and 6.56 eV, respectively) bands found in CD experiments is neither revealed in the CIS calculations nor in the... 

An illustration of the importance of the tautomeric form for base-pairing properties can be found in the case of GC base-pairing. There are about 50 ways to form GC base pairs by hydrogen bonding. The one structure that is prevalent in DNA (the Watson-Crick structure) has a subpicosecond excited-state lifetime, evidenced by a broad UV spectrum, while other structural arrangements of the same base-pair have sharp spectra, consistent with much longer excited-state lifetimes [75]. Figure 7.9 the shows IR-UV spectra for three structural isomers of GC base pairs. The stick spectra are density functional theory (DFT) calculated vibrational frequencies for the structures shown in the insets. Structure A is the Watson-Crick structure, while structure C is almost the same structure but with the cytosine... 

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