Near-IR laser-induced and spontaneous conformational transformations in monomers of 9-methylhypoxanthine.

Monomers of 9-methylhypoxanthine isolated in low-temperature Ar and Ne matrices adopt nearly exclusively the oxo tautomeric form. Upon excitation with UV (λ = 278 nm) light, the oxo tautomer converts into the hydroxy form. There are two conformational structures of the hydroxy tautomer of 9-methylhypoxanthine. These conformers differ in the orientation of the OH group, with the hydrogen atom pointing either toward the N1 or toward the N7 atom of the purine frame. We have demonstrated that, upon irradiation with narrowband near-infrared light tuned to the spectral position of the 2νOH overtone absorption, the two hydroxy conformers can be selectively converted into each other. Exposure of the matrix to the broadband mid-infrared and near-infrared radiation also results in conformational transformations involving the two OH rotamers. Finally, when the matrix is kept in the dark and at low temperature, the higher-energy hydroxy conformer (with OH group oriented toward the N7 atom) spontaneously converts into the lower-energy counterpart (with OH group oriented toward the N1 atom). The last process must be governed by the H-atom tunneling. The above isomerizations enabled the reliable separation and assignment of the infrared spectra of the two hydroxy conformers. The presence of multiple isosbestic points is a strong evidence that the observed transformations involve only the two OH rotamers. Interpretation of the experimental observations was supported by theoretical prediction of infrared spectra and calculations of isomerization barriers.