Pairing of Isolated Nucleobases: Double Resonance Laser Spectroscopy of Adenine-Thymine

• Published in: Chemphyschem 2003-08, Vol.4 (8), p.838-842
• Main Authors: Plützer, Christian, Hünig, Isabel, Kleinermanns, Karl, Nir, Eyal, de Vries, Mattanjah S.
• Format: Article
• Language: English
• Subjects: ab initio calculations; Adenine - chemistry; Biological and medical sciences; Fundamental and applied biological sciences. Psychology; Hydrogen Bonding; Interactions. Associations; Intermolecular phenomena; IR spectroscopy; laser spectroscopy; Methylation; Models, Chemical; Molecular biophysics; nucleobases; Photons; Spectrophotometry - methods; Spectrophotometry, Infrared - methods; Thymine - chemistry; Ultraviolet Rays; UV/Vis spectroscopy
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200300648

The vibronic spectrum of the adenine–thymine (A–T) base pair was obtained by one‐color resonant two‐photon ionization (R2PI) spectroscopy in a free jet of thermally evaporated A and T under conditions favorable for formation of small clusters. The onset of the spectrum at 35 064 cm−1 exhibits a large red shift relative to the π–π* origin of 9H‐adenine at 36 105 cm−1. The IR–UV spectrum was assigned to cluster structures with HNH⋅⋅⋅OC/N⋅⋅⋅HN hydrogen bonding by comparison with the IR spectra of A and T monomers and with ab initio calculated vibrational spectra of the most stable A–T isomers. The Watson–Crick A–T base pair is not the most stable base‐pair structure at different levels of ab initio theory, and its vibrational spectrum is not in agreement with the observed experimental spectrum. Experiments with methylated A and T were performed to further support the structural assignment. More stable than the Watson‐Crick pair is an isomer of adenine–thymine (A–T) that was determined by analyzing the vibronic spectrum of the A–T base pair, recorded by one‐color resonant two‐photon ionization (R2PI) spectroscopy in a free jet of thermally evaporated A and T under conditions favorable for formation of small clusters. The IR‐UV spectrum was assigned to structures (see picture) having HNH⋅⋅⋅OC/N⋅⋅⋅HN hydrogen bonding, which excludes the Watson–Crick A–T base pair, on the basis of comparisons with the IR spectra of A and T monomers and with ab initio calculated vibrational spectra of the most stable A–T isomers. Experiments with methylated A and T further support the structural assignment.