title: Gas-Phase Cytosine and Cytosine-N1-Derivatives Have 0.1-1 ns Lifetimes Near the S1 State Minimum.
abstract: Ultraviolet radiative damage to DNA is inefficient because of the ultrafast S1 ⇝ S0 internal conversion of its nucleobases. Using picosecond pump-ionization delay measurements, we find that the S1((1)ππ*) state vibrationless lifetime of gas-phase keto-amino cytosine (Cyt) is τ = 730 ps or ∼ 700 times longer than that measured by femtosecond pump-probe ionization at higher vibrational excess energy, Eexc. N1-Alkylation increases the S1 lifetime up to τ = 1030 ps for N1-ethyl-Cyt but decreases it to 100 ps for N1-isopropyl-Cyt. Increasing the vibrational energy to Eexc = 300-550 cm(-1) decreases the lifetimes to 20-30 ps. The nonradiative dynamics of S1 cytosine is not solely a property of the amino-pyrimidinone chromophore but is strongly influenced by the N1-substituent. Correlated excited-state calculations predict that the gap between the S2((1)nOπ*) and S1((1)ππ*) states decreases along the series of N1-derivatives, thereby influencing the S1 state lifetime.
authors: Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Susan
authors: S
authors: Blaser
authors: Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Maria A
authors: MA
authors: Trachsel
authors: Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Simon
authors: S
authors: Lobsiger
authors: Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Timo
authors: T
authors: Wiedmer
authors: Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Hans-Martin
authors: HM
authors: Frey
authors: Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Samuel
authors: S
authors: Leutwyler
mesh_terms: D003596:Cytosine / Q000737:chemistry*; D004249:DNA Damage; D015394:Molecular Structure
chemical_list: D003596:Cytosine