Metastable decay of negatively charged oligodeoxynucleotides analyzed with ultraviolet matrix-assisted laser desorption/ionization post-source decay and deuterium exchange

In an effort to understand the initiating step in metastable-ion decay of UV matrix-assisted laser desorption/ionization (MALDI)-produced ions, we conducted experiments in which we exchanged all active protons for deuterons of tetrameric and hexameric oligodeoxynucleotides. We wish to address the controversial proposal that in the negative-ion mode, as in the positive-ion mode, fragmentation is driven by nucleobase protonation. The results show unambiguously that proton transfer, leading to zwitterion formation, charges a nucleobase prior to its elimination. The zwitterion formation directs fragmentation of both positive and negative oligodeoxynucleotide ions. Poly-T-rich oligodeoxynucleotide tetramers show surprising differences in the negative compared to the positive-ion mode, as thymine is preferentially expelled, instead of a nucleobase with higher proton affinity. For the exceptional case of negatively charged poly-T-rich oligodeoxynucleotide tetramers generated by MALDI, we propose that zwitterion formation with positive charging of a nucleobase leads to base stabilization in the negative-ion mode through an interaction of the positive nucleobase with the excess negative charge. Moreover, backbone cleavages (accompanied by H rearrangement) of a phosophodiester bond give first-generation products that can be traced back to this tripolar complex bearing one positive and two negative charges, all of which may be interacting. (C) 2001 American Society for Mass Spectrometry.