Ultrafast excited-state dynamics of isocytosine

• Published in: Physical chemistry chemical physics : PCCP 2016-07, Vol.18 (3), p.228-2218
• Main Authors: Szabla, Rafa, Góra, Robert W, Šponer, Ji í
• Format: Article
• Language: English
• Subjects: Dynamics; Electronics; Gas phases; Mathematical analysis; Molecular dynamics; Quantum chemistry; Simulation; Tautomers
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp01391k

The alternative nucleobase isocytosine has long been considered as a plausible component of hypothetical primordial informational polymers. To examine this hypothesis we investigated the excited-state dynamics of the two most abundant forms of isocytosine in the gas phase (keto and enol). Our surface-hopping nonadiabatic molecular dynamics simulations employing the algebraic diagrammatic construction to the second order [ADC(2)] method for the electronic structure calculations suggest that both tautomers undergo efficient radiationless deactivation to the electronic ground state with time constants which amount to τ keto = 182 fs and τ enol = 533 fs. The dominant photorelaxation pathways correspond to ring-puckering (ππ* surface) and C&z.dbd;O stretching/N-H tilting (nπ* surface) for the enol and keto forms respectively. Based on these findings, we infer that isocytosine is a relatively photostable compound in the gas phase and in these terms resembles biologically relevant nucleobases. The estimated S 1 &z.radarr; T 1 intersystem crossing rate constant of 8.02 × 10 10 s −1 suggests that triplet states might also play an important role in the overall excited-state dynamics of the keto tautomer. The reliability of ADC(2)-based surface-hopping molecular dynamics simulations was tested against multireference quantum-chemical calculations and the potential limitations of the employed ADC(2) approach are briefly discussed. Nonadiabatic molecular dynamics simulations elucidate the ultrafast photodeactivation mechanisms of alternative nucleobase isocytosine.