Ion mobility measurements of nondenatured 12-150 kDa proteins and protein multimers by tandem differential mobility analysis-mass spectrometry (DMA-MS)

Christopher J. Hogan, Juan Fernández De La Mora

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64 Scopus citations

Abstract

The mobilities of electrosprayed proteins and protein multimers with molecular weights ranging from 12.4 kDa (cytochrome C monomers) to 154 kDa (nonspecific concanavalin A hexamers) were measured in dry air by a planar differential mobility analyzer (DMA) coupled to a time-of-flight mass spectrometer (TOF-MS). The DMA determines true mobility at atmospheric pressure, without perturbing ion structure from that delivered by the electrospray. A nondenaturing aqueous 20 mM triethylammonium formate buffer yields compact ions with low charge states, moderating polarization effects on ion mobility. Conversion of mobilities into cross-sections involves a reduction factor ξ for the actual mobility relative to that associated with elastic specular collisions with smooth surfaces. ξ is known to be 1.36 in air from Millikan's oil drop experiments. A similar enhancement effect ascribed to atomic-scale surface roughness has been found in numerical simulations. Adopting Millikan's value ξ=1.36 and assuming a spherical geometry yields a gas-phase protein density ρ p=0.949±0.053 g cm -3 for all our protein data. This is substantially higher than the 0.67 g cm -3 found in recent low-resolution DMA measurements of singly charged proteins. DMA-MS can distinguish nonspecific protein aggregates formed during the electrospray process from those formed preferentially in solution. The observed charge versus diameter relation is compatible with a protein charge reduction mechanism based on the evaporation of triethylammonium ions from electrosprayed drops.

Original languageEnglish (US)
Pages (from-to)158-172
Number of pages15
JournalJournal of the American Society for Mass Spectrometry
Volume22
Issue number1
DOIs
StatePublished - Jan 1 2011

Keywords

  • Charge reduction
  • Differential mobility analysis
  • Gas-phase protein conformation
  • Ion mobility
  • Native-state protein MS

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