Comprehensive Core-Level Study of the Effects of Isomerism, Halogenation, and Methylation on the Tautomeric Equilibrium of Cytosine

Abstract

Core-level X-ray photoemission and near-edge X-ray absorption fine structure spectra of 5-methylcytosine, 5-fluorocytosine, and isocytosine are presented and discussed with the aid of high-level ab initio calculations. The effects of the methylation, halogenation, and isomerization on the relative stabilities of cytosine tautomers are clearly identified spectroscopically. The hydroxy-oxo tautomeric forms of these molecules have been identified, and their quantitative populations at the experimental temperature are calculated and compared with the experimental results and with previous calculations. The calculated values of Gibbs free energy and Boltzmann population ratios are in good agreement with the experimental results characterizing tautomer equilibrium.

Watson–Crick and Sugar-Edge Base Pairing of Cytosine in the Gas Phase: UV and Infrared Spectra of Cytosine·2-Pyridone

“…[23][24][25] Brutschy and co-workers have investigated the IR spectra of the jet-cooled dimers 4-aminopyrimidine-1-methylthymine and 4-aminopyrimidine-6-methyl-4-pyrimidinone as mimics of the adenine-thymine (A-T) Watson-Crick (WC) base pair. 26 Since only the five cytosine tautomers shown in Figure 1 have been observed in the gas phase [4][5][6][7][8]27 we will restrict the discussion to these forms. Each conformer offers two to three double hydrogen binding sites, totalling 11 binding sites.…”

Ultrafast excited‐state dynamics of gas‐phase 5‐methylcytosine tautomers

“…The relative energies and populations of 5mCy major tautomers have been studied theoretically and experimentally by several groups. [8,40,[49][50][51] Table 1 lists the relative energies of major 5mCy tautomers calculated at various levels of theory, [8,49,50] including the G4MP2 energies obtained in this work. All results consistently predict that the enol-2b structure (see Figure 1) is the most stable form.…”

“…Table 2 lists the mole fractions of the three major 5mCy tautomers with their rotamer contributions summed together as derived from Boltzmann populations calculated with ΔG obtained from CCSD//MP2 [49] and present G4MP2 calculations. Two sets of experimental data [48,49] are also listed in Table 2 for comparison.…”

“…Similar to Cy, 5mCy also exists in several tautomeric forms in the vapor phase. [48][49][50][51] Early infrared (IR) studies of 5mCy in cryogenic Ar matrices have confirmed the coexistence of three major tautomers in the vapor (470 K) prior to deposition. [48] They are the aminoketo, amino-enol, and imino-keto forms, hereafter referred to as the keto, enol, and imino forms (see Figure 1 for chemical structures), respectively, for conciseness.…”

“…Note that each major tautomer can exist in at least two rotamers or structural isomers. Feyer et al [49] measured core-level X-ray photoemission and near-edge X-ray absorption spectra of 5mCy and determined the mole fractions of the three tautomers to be keto:enol: imino = 0.13:0.68:0.19 at 445 K, indicating the predominance of the enol form in the vapor phase.…”