UV-induced transformations of matrix-isolated 6-azacytosine

UV-induced transformations were studied for monomers of 6-azacytosine isolated in low-temperature Ar matrices. In contrast to cytosine, where the amino-hydroxy (AH) tautomer is the lowest-energy form, the amino-oxo (AO) and imino-oxo (IO) isomers of 6-azacytosine were found to be the most stable and most populated. Due to the high relative energy of the AH tautomer of 6-azacytosine, this form is not populated in low-temperature matrices after their formation and prior to any irradiation. Excitation of 6-azacytosine monomers with UV light from the 328-300 nm range led to structural transformations of AO and IO forms. The initially most populated AO tautomer was observed either to convert, in a phototautomeric reaction, into the AH product or to undergo photodecarbonylation to yield 4-amino-1,2,3-(2H)-triazole. The relative efficiencies of the two processes depend on the wavelength and on the pulsed or continuous-wave character of the UV light used for excitation. For the IO tautomer of 6-azacytosine, the excitation with UV 328-300 nm light induced the photoconversion of the initially more populated anti IO1 isomer into the syn IO2 form. This transformation was found to be partially photoreversible. Published by AIP Publishing.