Nucleobase pair analogues 2-pyridone-uracil, 2-pyridone-thymine, and 2-pyridone-5-fluorouracil: Hydrogen-bond strengths and intermolecular vibrations

作者：Muller A; Leutwyler S^*

来源：Journal of Physical Chemistry A, 2004, 108(29): 6156-6164.

DOI：10.1021/jp049033h

Due to the ultrafast nonradiative decay of the uracil (U), thymine (T), and 5-fluorouracil (5-FU) nucleobases, resolved vibronic spectra of nucleobase dimers with U, T, or 5-FU chromophores have not yet been observed in the gas phase. 2-Pyridone (2PY) is a fluorescent hydrogen-bonding analogue of U with a single N-H donor and C = O acceptor group. We report the mass-selected S-1 <-- S-0 vibronic spectra of 2PY(.)U, 2PY(.)T, and 2PY(.)5-FU, revealing properties of their H-bonds free of stacking interactions and backbone constraints. The in-plane H-bond vibrations, omega', chi', sigma', are observed for all three dimers; the out-of-plane vibrations beta', tau' and the overtone of the stagger mode 2delta' are measured for 2PY(.)T and 2PY(.)U. The experimental frequencies are in good agreement with the ab initio calculated frequencies of the most stable H-bonded isomer, which involves the N1-H and C2 = 0 groups of U, T, or 5-FU as H donor/acceptor. The H-bond dissociation energies follow the sequence T approximate to U < 5-FU. However, the experimental S-1 state H-bond stretching force constants increase in the sequence U < T < 5-FU. The absence of fluorescence from these dimers implies that the S, lifetimes of 2PY in these complexes are > 20 times shorter than those in the other 2PY(.)X complexes measured so far. Energy transfer from 2PY to U (T, 5-FU) is suggested as the cause, which implies that the npi*S-1 states of U, T, and 5-FU, which have never been directly observed, lie below the S-1 pipi* state of 2PY.

• 出版日期2004-7-22