Department of Mechanical Engineering, Yale University, New Haven, CT, USA.
J Am Soc Mass Spectrom. 2011 Jan;22(1):158-72. doi: 10.1007/s13361-010-0014-7. Epub 2011 Jan 28.
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.
采用平面式差分迁移率分析仪（DMA）结合飞行时间质谱仪（TOF-MS），在干燥空气中测量了分子量在 12.4 kDa（细胞色素 C 单体）至 154 kDa（非特异性伴刀豆球蛋白 A 六聚体）之间的电喷雾蛋白质和蛋白质多聚体的迁移率。DMA 在大气压下确定真实迁移率，而不会破坏电喷雾产生的离子结构。在非变性的 20 mM 三乙基铵甲酸缓冲液中，产生带低电荷状态的紧凑离子，从而减轻了离子迁移率的极化效应。将迁移率转换为截面涉及到实际迁移率相对于与光滑表面弹性镜面碰撞相关的迁移率的减小因子 ξ。从密立根的油滴实验可知，空气的 ξ 值为 1.36。在数值模拟中发现了与原子级表面粗糙度相关的类似增强效应。采用密立根的值 ξ=1.36 并假设球形几何形状，可得出我们所有蛋白质数据的气相蛋白质密度 ρ(p)=0.949±0.053 g cm(-3)。这明显高于最近对单电荷蛋白质进行的低分辨率 DMA 测量得出的 0.67 g cm(-3)。DMA-MS 可以区分在电喷雾过程中形成的非特异性蛋白质聚集体与在溶液中优先形成的聚集体。观察到的电荷与直径关系与基于电喷雾液滴中三乙基铵离子蒸发的蛋白质电荷还原机制兼容。
