The nucleobase cytosine and the cytosine dimer investigated by double resonance laser spectroscopy and ab initio calculations

Abstract

The vibronic spectrum of laser desorbed and jet cooled cytosine consists of bands from two major tautomers (keto and enol) as revealed by UV-UV and IR-UV double resonance spectroscopy and methyl blocking experiments. Only one isomer each was observed for the cytosine dimer and for the cytosine -1-methylcytosine mixed dimer. These isomers form C=OÁ Á ÁHNH/NHÁ Á ÁN hydrogen bonds. Cytosine -5-methylcytosine exhibits three isomers: one again with C=OÁ Á ÁHNH/NHÁ Á ÁN connectivity, the second with C=OÁ Á ÁHNH/NHÁ Á ÁN interaction but one cytosine in the enol form and the third with symmetrical C=OÁ Á ÁNH/NHÁ Á ÁO=C bonds. These are also the most stable clusters according to molecular dynamics/quenching and ab initio quantum chemical calculations. The experimental IR spectra of these isomers agree well with the calculated normal mode vibrational spectra. The vibronic spectra of the clusters are blue shifted relative to the monomer spectra by more than 1000 cm À1 indicating a considerable reduction of dimer stability upon electronic excitation.

Photophysics of cytosine tautomers: new insights into the nonradiative decay mechanisms from MS-CASPT2 potential energy calculations and excited-state molecular dynamics simulations

“…22,30 It also agrees quite well with the experimental adiabatic excitation energy of 3.95 eV estimated from the resonance-enhanced multiphoton ionization (REMPI) spectrum. 10,60 It has been proposed previously that the minimum energy path (MEP) approach is appropriate for the determination of the deactivaiton pathway since it provides information about the accessibility of the conical intersection. 29,[61][62][63] There is the possibility that it leads to the seam of CIs before reaching the ( 1 ππ*) min structure, thus providing the most relevant photochemical deactivation pathway.…”

Gas-Phase IR Spectroscopy of Nucleobases

“…While the energy difference between the enol and keto forms is very small, some of their other properties, such as their vertical excitation energies, are very different. Nir et al reported REMPI and hole burning spectra and concluded that the keto and enol tautomers have band origins that differ by a remarkable half electron volt at 314 nm and 278 nm, respectively (42,43). The REMPI spectra of U and T exhibit a very broad structure with an onset in the frequency range of the origin of the enol cytosine first reported by Brady et al (4).. No spectroscopic detail could be extracted from these broad spectra and tautomeric information was limited for a long time to bulk measurements.…”

Ionization of cytosine monomer and dimer studied by VUV photoionization and electronic structure calculations

“…1 They produced cytosine dimers using laser desorption and presented strong evidence for only one isomer -(C1) 2 HB1 -being present in their molecular beam. 13 According to Kabelac and Hobza,54 this is the most populated isomer based on their molecular dynamics/quenching calculations (albeit of a 298K ensemble). The absence of other isomers could be explained 1 by a number of reasons stemming from the detection scheme in a REMPI experiment: (1) poor absorption in the first excited state; (2) ionization energy higher than accessible by two photons; (3) fragmentation of the cation; (4) lifetimes of the excited state being too short for second photon absorption.…”

“…Previous ionization studies of cytosine consist of photoionization mass spectrometry (PIMS) measurements of AIEs, 6,7 photoelectron spectroscopy (PES) at the valence [8][9][10][11] and core level, 12 resonance enhanced multiphoton ionization (REMPI) experiments 13,14 and a number of electronic structure calculations. 9,[15][16][17][18] Trofimov et al 9 measured a valence-shell PES of cytosine.…”