2026-05-07 京都大学
本研究の概要図：紫外光照射による水中の核酸塩基の構造変化と水和反応。Suzuki group all rights reserved.
＜関連情報＞
- https://www.kyoto-u.ac.jp/ja/research-news/2026-05-07-3
- https://pubs.acs.org/doi/10.1021/jacs.6c00684
ピリミジンヌクレオシド・ヌクレオチドの電子緩和に現れるねじれた中間体：C5 メチル化が非断熱遷移と水和損傷に及ぼす影響 A Twist in Electronic Relaxation of Pyrimidine Nucleosides and Nucleotides: Impact of C5 Methylation on Nonadiabatic Transition and Photohydration Damage
Srijon Ghosh,Yuki Obara,Vishal Kumar Jaiswal,Mario Taddei,Artur Nenov,Irene Conti,Marco Garavelli,and Toshinori Suzuk
Journal of the American Chemical Society Published: May 4, 2026
DOI:https://doi.org/10.1021/jacs.6c00684
Abstract
We report a combined experimental and theoretical study of the formation and decay dynamics of a ground-state twisted intermediate (TI) involved in the ultrafast internal conversion of UV-excited pyrimidine nucleosides and nucleotides in aqueous solution. Infrared transient absorption spectroscopy identifies a TI featuring a strongly twisted C5=C6 double bond, and its quantum yield (ΦTI) and lifetime (τTI) are determined for structurally distinct nucleosides and nucleotides. Photohydration rates (khyd) measured under continuous 266 nm irradiation correlate directly with ΦTI × τTI, providing unambiguous evidence that the TI mediates the hydration reaction. Comparison of C5–H and C5–CH3 derivatives reveals pronounced reductions in both ΦTI and khyd upon methylation. Quantum mechanics/molecular mechanics dynamical simulations show that TI formation requires sufficient nuclear momentum along the TI-forming coordinate at the conical intersection, whereas vibrational energy randomization induced by C5 methylation and solvent interactions diminishes this momentum and consequently ΦTI. The TI is neither diradical nor zwitterionic but instead contains an elongated, highly reactive C5=C6 double bond whose polarization and hydration reactivity are attenuated by C5 methylation. Consistently, the normalized reactivity index, khyd/(ΦTI × τTI), is substantially lower for C5-methylated compounds. A high activation barrier limits the photohydration quantum yield (∼0.01), and τTI is primarily governed by isomerization back to the planar ground-state structure.
