title: Gas-phase Watson-Crick and Hoogsteen isomers of the nucleobase mimic 9-methyladenine x 2-pyridone.
abstract: 2-Pyridone (pyridin-2-one) is a mimic of the uracil and thymine nucleobases, with only one N--H and C==O group. It provides a single H-bonding site, compared to three for the canonical pyrimidine nucleobases. Employing the supersonically cooled 9-methyladenine2-pyridone (9MAd x 2PY) complex, which is the simplest base pair to mimic adenine-uracil or adenine-thymine, we show that its gas-phase UV spectrum consists of contributions from two isomers. Based on the H-bonding sites of 9-methyladenine, these are the Watson-Crick and Hoogsteen forms. Combining two-color two-photon ionisation (2C-R2PI), UV-UV depletion and laser-induced fluorescence spectroscopies allows separation of the two band systems, revealing characteristic intermolecular in-plane vibrations of the two isomers. The calculated S(0) and S(1) intermolecular frequencies are in good agreement with the experimental ones. Ab initio calculations predict the Watson-Crick isomer to be slightly more stable (D(0)=-16.0 kcal mol(-1)) than the Hoogsteen isomer (D(0)=-15.0 kcal mol(-1)). The calculated free energies Delta(f)G(0) of the Watson-Crick and Hoogsteen isomers agree qualitatively with the experimental isomer concentration ratio of 3:1.
authors.affiliation: Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, 3012 Bern, Switzerland.
authors.forename: Jann A
authors.initials: JA
authors.lastname: Frey
authors.forename: Roman
authors.initials: R
authors.lastname: Leist
authors.forename: Andreas
authors.initials: A
authors.lastname: Müller
authors.forename: Samuel
authors.initials: S
authors.lastname: Leutwyler
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives* / Q000737:chemistry; D001482:Base Composition; D020029:Base Pairing; D019281:Dimerization; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D009690:Nucleic Acid Conformation; D017785:Photons; D011728:Pyridones / Q000737:chemistry*; D012984:Software; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
chemical_list: D011728:Pyridones; C013115:9-methyladenine; D000225:Adenine