Near-IR-Induced, UV-Induced, and Spontaneous Isomerizations in 5-Methylcytosine and 5-Fluorocytosine


Lapinski L., Reva I., Rostkowska H., Fausto R., Nowak M. J.

JOURNAL OF PHYSICAL CHEMISTRY B, vol.118, no.11, pp.2831-2841, 2014 (SCI-Expanded) identifier identifier identifier

  • Publication Type: Article / Article
  • Volume: 118 Issue: 11
  • Publication Date: 2014
  • Doi Number: 10.1021/jp411423c
  • Journal Name: JOURNAL OF PHYSICAL CHEMISTRY B
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.2831-2841
  • Istanbul Kültür University Affiliated: Yes

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 (lambda <= 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.