Analysis of Modified Oligonucleotides by Matrix-Assisted Laser Desorption/Ionization Fourier Transform Mass Spectrometry

Matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance mass spectrometry (FTMS) has been applied to the structural characterization of modified oligodeoxyribonucleotide 4-, 6-, and 11-mers. Each oligonucleotide contained one modified base, either an O⁶-methyl-substituted guanine, an N⁶-(10R)-transopened benzo[α]pyrenediol epoxide adduct of adenine, or an N²-(R)-styrene oxide adduct of guanine. 3-Hydroxypicolinic acid was used as the MALDI matrix for molecular weight and purity determinations, while either 2, 5-dihydroxybenzoic acid (DHBA) or an anthranilic/nicotinic acid (AA/NA) mixture was used to induce fragmentation for the production of structurally significant fragment ions. For the 4- and 6-mers, the oligonucleotide sequence could be obtained from the direct AA/NA or DHBA spectra. Sequence information was also obtained by inserting a time delay between the laser desorption event and ion detection to permit metastable decomposition. For the 11-mers, high-mass sequence ions were not detected. Although similar sequence ions were observed in both the positive and the negative ion mass spectra, more fragmentation was generally observed in the positive ion mode. In the positive ion mode, modified base fragment ions were observed when DHBA was used, and these fragments were examined using accurate mass measurements, collisionally induced dissociations, and ionmolecule reactions to characterize the modified base. MALDI-FIMS signals from one sample application can be used for the measurement of hundreds of spectra. The direct MALDI-FT mass spectra show matrix-dependent, structurally informative fragments, and CID experiments can be implemented using low-picomole sample quantities.