Gas-Phase Watson–Crick and Hoogsteen Isomers of the Nucleobase Mimic 9-Methyladenine⋅2-Pyridone
✍ Scribed by Jann A. Frey; Roman Leist; Andreas Müller; Samuel Leutwyler
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 182 KB
- Volume
- 7
- Category
- Article
- ISSN
- 1439-4235
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✦ Synopsis
Abstract
2‐Pyridone (pyridin‐2‐one) is a mimic of the uracil and thymine nucleobases, with only one NH and CO group. It provides a single H‐bonding site, compared to three for the canonical pyrimidine nucleobases. Employing the supersonically cooled 9‐methyladenine⋅2‐pyridone (9MAd⋅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 Δ~f~G^0^ of the Watson–Crick and Hoogsteen isomers agree qualitatively with the experimental isomer concentration ratio of 3:1.