UV-induced generation of rare tautomers of allopurinol and 9-methylhypoxanthine -- a matrix isolation FTIR study.

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 (lambda>230 nm or lambda>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.

Radiation-induced hydroxyl addition to purine molecules: EPR and ENDOR study of hypoxanthine hydrochloride monohydrate single crystals.

Three radical species were detected in an EPR/ENDOR study of X-irradiated hypoxanthine.HCl.H2O single crystals at room temperature: RI was identified as the product of net H addition to C8, RII was identified as the product of net H addition to C2, and RIII was identified as the product of OH addition to C8. The observed set of radicals was the same for room-temperature irradiation as for irradiation at 10 K followed by warming the crystals to room temperature; however, the C2 H-addition and C8 OH-addition radicals were not detectable after storage of the crystals for about 2 months at room temperature. Use of selectively deuterated crystals permitted unique assignment of the observed hyperfine couplings, and results of density functional theory calculations on each of the radical structures were consistent with the experimental results. Comparison of these experimental results with others from previous crystal-based systems and model system computations provides insight into the mechanisms by which the biologically important purine C8 hydroxyl addition products are formed. The evidence from solid systems supports the mechanism of net water addition to one-electron oxidized purine bases and demonstrates the importance of a facial approach between the reactants.

Radical formation in single crystals of hypoxanthine.HCl.H2O, inosine, and the disodium salt of 5'-inosine-monophosphate.

Radical formation in single crystals of hypoxanthine.HCl.H2O, inosine and Na2-5'-IMP.(7.5 H2O) by X-irradiation has been studied using electron-spin-resonance spectroscopy at 9.5 and 35 GHz. In all crystals both H-addition radicals at position C2 and C8 of the purine ring are found. The coupling constants of these two radicals are different and depend strongly on the protonation state of the base. INDO-calculations indicate that the C8-radical is protonated at O6. In Na2-5'-IMP OH-addition radicals at position C2 of the purine ring are formed. Electron adduct radicals are found in the neutral and the N7-protonated base after X-irradiation at 77 K. In Na2-5'-IMP no electron adduct is formed but a radical which probably is the cation. In hypoxanthine.HCl.H2O a radical could be observed after X-irradiation at 77 K, which results from addition of a Cl- to the nitrogen N1.