UV-induced generation of rare tautomers of allopurinol and 9-methylhypoxanthine — A matrix isolation FTIR study

• Published in: Biophysical chemistry 2006-07, Vol.122 (2), p.123-135
• Main Authors: Gerega, A., Lapinski, L., Reva, I., Rostkowska, H., Nowak, M.J.
• Format: Article
• Language: English
• Subjects: 4( 3H)-pyrimidinone; 9-methylhypoxanthine; Allopurinol; Allopurinol - chemistry; Allopurinol - radiation effects; Computer Simulation; DFT calculations; FTIR spectroscopy; Hypoxanthine; Hypoxanthines - chemistry; Hypoxanthines - radiation effects; Inosine; Isomerism; Matrix isolation; Models, Chemical; Molecular Structure; Photochemistry; Pyrimidinones - chemistry; Pyrimidinones - radiation effects; Quantum Theory; Sensitivity and Specificity; Spectroscopy, Fourier Transform Infrared - methods; Tautomerism; Ultraviolet Rays
• ISSN: 0301-4622; 1873-4200
• DOI: 10.1016/j.bpc.2006.03.002

Monomers of allopurinol and 9-methylhypoxanthine were studied using the matrix isolation technique combined with Fourier transform infrared spectroscopy. The oxo tautomeric forms of both compounds were found to dominate in freshly deposited low-temperature argon matrices. For 9-methylhypoxanthine, a small amount of the hydroxy tautomer was also detected in an Ar matrix before any irradiation. Upon exposure of the matrices to the UV ( λ > 230 nm or λ > 270 nm) light, a proton transfer photoreaction converting the oxo forms of both compounds into the corresponding hydroxy tautomers occurred. Generation of conjugated ketenes as minor photoproducts was also observed. For 4( 3H)-pyrimidinone (a model compound for both allopurinol and 9-methylhypoxanthine), photoreversibility of the UV-induced oxo → hydroxy transformation was experimentally proven by direct observation of the back hydroxy → oxo photoreaction. The substrates (oxo tautomers) and products (hydroxy tautomers) of the observed phototransformations were identified by comparison of their IR spectra with the spectra theoretically predicted at the DFT(B3LYP)/6-31++G(d,p) level. The IR bands in the experimental spectra were assigned to the calculated normal modes.