title: Near-IR-induced, UV-induced, and spontaneous isomerizations in 5-methylcytosine and 5-fluorocytosine.
abstract: Monomeric 5-methylcytosine (5mCyt) and 5-fluorocytosine (5FCyt) were studied using the matrix-isolation method. In 5mCyt and 5FCyt, the most stable form, dominating in low-temperature matrixes, is the amino-hydroxy (AH) tautomer. For both compounds, irradiation of the matrixes with near-IR laser light or with broadband near-IR or mid-IR light induces interconversions between the two rotamers of tautomer AH. In addition, for matrixes kept in darkness, a spontaneous tunneling conversion of the higher-energy hydroxy conformer (with the OH group directed toward the N3 atom) into the lower-energy form (OH directed toward N1) was occurring, with half-life time of 70 min for 5mCyt and 127 min for 5FCyt. These tunneling processes are much faster than that found for unsubstituted cytosine, where the half-life time is more than 30 h. UV irradiation of 5mCyt (at 316 nm) led to phototautomeric conversion of the amino-oxo form into the amino-hydroxy tautomer. Another phototransformation induced by irradiation of 5mCyt at 316 nm was the cleavage of the C-N bond in the amino-oxo form, resulting in generation of the open-ring conjugated isocyanate product. Irradiation of 5mCyt at shorter waves (λ ≤ 310 nm) induced the syn-anti photoisomerization within the imino-oxo forms of the compound. For matrix-isolated 5FCyt, the amount of the amino-oxo form was very small (with respect to the amino-hydroxy tautomer), while the imino-oxo isomers were not detected at all.
authors.affiliation: Institute of Physics, Polish Academy of Sciences , Al. Lotnikow 32/46, 02-668 Warsaw, Poland.
authors.forename: Leszek
authors.initials: L
authors.lastname: Lapinski
authors.forename: Igor
authors.initials: I
authors.lastname: Reva
authors.forename: Hanna
authors.initials: H
authors.lastname: Rostkowska
authors.forename: Rui
authors.initials: R
authors.lastname: Fausto
authors.forename: Maciej J
authors.initials: MJ
authors.lastname: Nowak
mesh_terms: D044503:5-Methylcytosine / Q000737:chemistry* / Q000528:radiation effects; D003198:Computer Simulation; D005437:Flucytosine / Q000737:chemistry* / Q000528:radiation effects; D007259:Infrared Rays; D007536:Isomerism; D008968:Molecular Conformation; D015394:Molecular Structure; D013055:Spectrophotometry, Infrared; D014466:Ultraviolet Rays
chemical_list: D044503:5-Methylcytosine; D005437:Flucytosine