Tautomer‐Specific Resonant Photoelectron Imaging of Deprotonated Cytosine Anions

Abstract

Tautomers of the nucleobases play fundamental roles in spontaneous mutations of DNA. Tautomers of neutral cytosine have been studied in the gas phase, but much less is known about charged species. Here, we report the observation and characterization of three tautomers of deprotonated cytosine anions, [trans‐keto‐amino‐N3H‐H8b] (tKAN3H8b−), [cis‐keto‐amino‐N3H‐H8a] (cKAN3H8a−) and [keto‐amino‐H] (KAN1−), produced by electrospray ionization. Excited dipole‐bound states (DBSs) are uncovered for the three anions by photodetachment spectroscopy. Excitations to selected DBS vibrational levels of cKAN3H8a− and tKAN3H8b− yield tautomer‐specific resonant photoelectron spectra. The current study provides further insight into tautomerism of cytosine and suggests a new method to study the tautomers of nucleobases using electrospray ionization and anion spectroscopy.

High-resolution photoelectron imaging and resonant photoelectron spectroscopy via noncovalently bound excited states of cryogenically cooled anions

“…The tKAN3H8b À anion was calculated recently to be more stable by 1.93 kcal mol À1 . 52 In Fig. 12f, the nonresonant PE spectrum of [Cy-H] À at 392.11 nm reveals three major peaks, labeled C 0, T 0 and C ( T 21 1 ).…”

“…The narrowest line width achieved was 1.2 cm À1 for 5.2 cm À1 electrons. 69 The third-generation ESI-PES apparatus has allowed the study of weakly bound non-covalent excited states of anions, including both dipole-bound 5,41,[44][45][46][47][48][49][50][51][52][53] and quadrupole-bound excited states, 54 and the development of rPES and PDS for cold anions. In a typical investigation, we rst measure nonresonant PE spectra to obtain the detachment threshold of an anion.…”

“…However, enhancement of vibrational features for a specic conformer or conformer-selective rPES can be achieved when the detachment laser is tuned to the DBS vibrational levels of a specic conformer. 48,49,52 5.1 Conformer-selective rPES via the DBS of m-HO(C 6 H 4 )O À…”

“…12f, the nonresonant PE spectrum of [Cy-H] À at 392.11 nm reveals three major peaks, labeled C 0, T 0 and C ( T 21 1 ). 52 Note that the superscripts "C" and "T" designate the tautomers of cKAN3H8a À and tKAN3H8b À . Peaks C 0 and T 0 represent the 0-0 detachment transitions and yield the EAs of cKAN3H8a and tKAN3H8b to be 3.047 eV and 3.087 eV, respectively, which are in excellent agreement with the calculated EAs.…”

“…42,43 Subsequently, more deprotonated organic molecular anions (Fig. 1) were found to support excited DBSs [44][45][46][47][48][49][50][51][52][53] or quadrupole-bound states (QBSs) 54 below the anion photodetachment threshold. As shown in Fig.…”