Compaction of Duplex Nucleic Acids upon Native Electrospray Mass Spectrometry

Abstract

Native mass spectrometry coupled to ion mobility spectrometry is a promising tool for structural biology. Intact complexes can be transferred to the mass spectrometer and, if native conformations survive, collision cross sections give precious information on the structure of each species in solution. Based on several successful reports for proteins and their complexes, the conformation survival becomes more and more taken for granted. Here we report on the fate of nucleic acids conformation in the gas phase. Disturbingly, we found that DNA and RNA duplexes, at the electrospray charge states naturally obtained from native solution conditions ([≥] 100 mM aqueous NH4OAc), are significantly more compact in the gas phase compared to the canonical solution structures. The compaction is observed for short (12-bp) and long (36-bp) duplexes, and for DNA and RNA alike. Molecular modeling (density functional calculations on small helices, semi-empirical calculations on up to 12-bp, and molecular dynamics on up to 36-bp duplexes) demonstrates that the compaction is due to phosphate group self-solvation prevailing over Coulomb-driven expansion. Molecular dynamics simulations starting from solution structures do not reproduce the experimental compaction. To be experimentally relevant, molecular dynamics sampling should reflect the progressive structural rearrangements occurring during desolvation. For nucleic acid duplexes, the compaction observed for low charge states results from novel phosphate-phosphate hydrogen bonds formed across both grooves at the very late stages of electrospray.