Mechanical Engineering Department, Yale University, New Haven, CT 06520-8286, USA.
J Am Soc Mass Spectrom. 2012 Dec;23(12):2115-21. doi: 10.1007/s13361-012-0454-3. Epub 2012 Oct 6.
The reasons for the existence of two gas phase conformers for electrosprayed ions of the large tetradecameric protein complex GroEL are considered. Key features are that: (1) both conformers extrapolate to very similar cross sections in the limit of zero charge; (2) both conformers supercharge above the maximum value permitted by the Rayleigh criterion; and (3) one of the conformers supercharges substantially more than the other. We hypothesize that the supercharging observed is associated to the approximately cylindrical shape of GroEL in aqueous solution, enabling accumulation of extra charge on either one or two of its bases when they intersect the evaporating drop surface. The two conformers would then correspond to cases when either one or the two bases of the cylinder carry extra charge. Apparently, the conformer symmetrically (doubly) supercharged on both ends is Coulombically stretched, therefore exhibiting a mobility different from the conformer asymmetrically supercharged only on one end. Several general consequences follow. First, non-spherical proteins may generally be charged above the usual Rayleigh limit. Second, we confirm the previously advanced but contentious notion that gas phase protein ions are readily compacted or stretched away from their crystal structure by capillary and Coulombic forces during the electrospraying process.
考虑到电喷雾离子化的大型十四聚体蛋白复合物 GroEL 存在两种气相构象的原因。关键特征是：(1) 两种构象在电荷为零时都外推到非常相似的截面；(2) 两种构象都在瑞利判据允许的最大值以上超荷；(3) 一种构象的超荷比另一种构象大得多。我们假设观察到的超荷与 GroEL 在水溶液中的近似圆柱形形状有关，当它与蒸发液滴表面相交时，允许在一个或两个基上积累额外的电荷。那么这两种构象将对应于圆柱的一个或两个基携带额外电荷的情况。显然，在两端对称（双重）超荷的构象是库仑拉伸的，因此表现出与仅在一端非对称超荷的构象不同的迁移率。以下是一些普遍的结论。首先，非球形蛋白质通常可以在通常的瑞利极限以上被荷电。其次，我们证实了之前提出的但有争议的观点，即在电喷雾过程中，蛋白质离子会受到毛细力和库仑力的作用，很容易从其晶体结构中被压缩或拉伸。
