First observation of infrared and UV–visible absorption spectra of adenine radical in low-temperature argon matrices

A UV-light induced photoreaction product of adenine in low-temperature argon matrices was identified as an adenine radical, where the hydrogen atom in the N H group was detached from adenine, by analyses of IR and UV–visible absorption spectra with aids of the density-functional-theory (DFT) and time-dependent DFT calculations. The adenine radical returned to adenine by recombination with the detached hydrogen atom upon longer-wavelength UV irradiation. A reversible isomerization to another adenine isomer via the adenine radical was also observed when the wavelength of UV irradiation was changed. The photoreaction mechanism of adenine in low-temperature argon matrices is discussed, including UV-light induced amine–imine tautomerism.