title: Ion mobility measurements of nondenatured 12-150 kDa proteins and protein multimers by tandem differential mobility analysis-mass spectrometry (DMA-MS).
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.
authors.affiliation: Department of Mechanical Engineering, Yale University, New Haven, CT, USA.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Hogan
authors.forename: Juan Fernández
authors.initials: JF
authors.lastname: de la Mora
mesh_terms: D000818:Animals; D002417:Cattle; D010316:Particle Size; D010940:Plant Proteins / Q000737:chemistry; D055503:Protein Multimerization; D011506:Proteins / Q000737:chemistry*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D010940:Plant Proteins; D011506:Proteins; D000644:Quaternary Ammonium Compounds; C066585:triethylammonium cation