title: Ultrafast excited-state dynamics of isocytosine.
abstract: 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 τketo = 182 fs and τenol = 533 fs. The dominant photorelaxation pathways correspond to ring-puckering (ππ* surface) and C[double bond, length as m-dash]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 S1 [radiolysis arrow - arrow with voltage kink] T1 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.
authors.affiliation: Institute of Biophysics, Academy of Sciences of the Czech Republic, Královopolská 135, 61265, Brno, Czech Republic. rafal.szabla@gmail.com.
authors.forename: Rafał
authors.initials: R
authors.lastname: Szabla
authors.affiliation: Department of Physical and Quantum Chemistry, Faculty of Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland. robert.gora@pwr.edu.pl.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Góra
authors.affiliation: Institute of Biophysics, Academy of Sciences of the Czech Republic, Královopolská 135, 61265, Brno, Czech Republic. rafal.szabla@gmail.com and CEITEC-Central European Institute of Technology, Masaryk University, Campus Bohunice, Kamenice 5, CZ-62500 Brno, Czech Republic.
authors.forename: Jiří
authors.initials: J
authors.lastname: Šponer