UV-vis action spectroscopy and structures of hydrogen-rich 2′-deoxycytidine dinucleotide cation radicals. A difficult case

Abstract

Hydrogen-rich cation radicals of 2′-deoxycytidine dinucleotide, (dCC + 2H)+●, were generated by electron transfer dissociation of their doubly charged complexes with crown ethers and found to be stable in the gas phase. The (dCC + 2H)+● ions were calculated to be formed with ca. 280 kJ mol-1 internal energy that was rapidly dissipated below a 130 kJ mol-1 energy threshold for the lowest-energy dissociation by loss of water. UV-vis action spectroscopy was applied to monitor several photodissociation channels of (dCC + 2H)+● and to compose spectra that showed distinct absorption bands at 220, 270, 290, and 350 nm, and a broad featureless band at 500-700 nm. UV-vis action spectra were also obtained for the (dCC ‒ H2O + 2H)+● cation radical that showed bands at 225, 270, 330, 430, and 530 nm. Born-Oppenheimer molecular dynamics and density functional theory (DFT) calculations were employed to generate a complete set of tautomeric (dCC + crown + 2H)2+ precursor dications and identify the lowest-energy structures in the gas phase and aqueous solution that were the cytosine N-3-H tautomers. Calculations also indicated that cytosine cations and radicals in the lowest-energy (dCC + 2H)+● ions were N-3-H tautomers. Absorption spectra of several (dCC + 2H)+● tautomers were obtained by time-dependent DFT calculations that provided a limited match with the action spectrum. In contrast, a satisfactory match was obtained for the action and absorption spectra of (dCC ‒ H2O + 2H)+● fragment ions. (dCC + 2H)+● represents a difficult case because of a multitude of cation and radical tautomers to be analyzed and the propensity for consecutive photofragment dissociation upon photon absorption.