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 NH 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.