Characterizing DNA photo-oxidation reactions by high-resolution mass measurements with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

High‐resolution mass measurements by matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectrometry were employed to characterize laser‐induced oxidation of guanine in a small synthetic deoxyoligonucleotide. The oligonucleotide was exposed to high‐intensity UV radiation at 266 nm to produce modifications on the guanine base. The primary product showed a +16 Da mass shift relative to the original strand, whereas secondary products showed mass shifts of +32 and +34 Da. The mass shift of the primary product is consistent with an 8‐oxoguanine modification. However, the reactivity of the primary product with hot piperidine and other secondary oxidizing agents was different from that of a synthetic oligonucleotide containing 7,8‐dihydro‐8‐oxo‐2′‐deoxyguanine (8‐oxoG). Based upon the results, a new reaction scheme involving the formation of an epoxide ring across the C‐4 and C‐5 positions by UV laser‐induced oxidation is suggested. The results also illustrate the ability of MALDI to characterize chemical reactivity rapidly at the a low picomolar level. Copyright © 2000 John Wiley & Sons, Ltd.