Jurchen John C, Garcia David E, Williams Evan R
Department of Chemistry, University of California at Berkeley, 94720, Berkeley, CA, USA.
J Am Soc Mass Spectrom. 2004 Oct;15(10):1408-1415. doi: 10.1016/j.jasms.2004.06.006.
Dissociation of gas-phase protonated protein dimers into their constituent monomers can result in either symmetric or asymmetric charge partitioning. Dissociation of alpha-lactalbumin homodimers with 15+ charges results in a symmetric, but broad, distribution of protein monomers with charge states centered around 8+/7+. In contrast, dissociation of the 15+ heterodimer consisting of one molecule in the oxidized form and one in the reduced form results in highly asymmetric charge partitioning in which the reduced species carries away predominantly 11+ charges, and the oxidized molecule carries away 4+ charges. This result cannot be adequately explained by differential charging occurring either in solution or in the electrospray process, but appears to be best explained by the reduced species unfolding upon activation in the gas phase with subsequent separation and proton transfer to the unfolding species in the dissociation complex to minimize Coulomb repulsion. For dimers of cytochrome c formed directly from solution, the 17+ charge state undergoes symmetric charge partitioning whereas dissociation of the 13+ is asymmetric. Reduction of the charge state of dimers with 17+ charges to 13+ via gas-phase proton transfer and subsequent dissociation of the mass selected 13+ ions results in a symmetric charge partitioning. This result clearly shows that the structure of the dimer ions with 13+ charges depends on the method of ion formation and that the structural difference is responsible for the symmetric versus asymmetric charge partitioning observed. This indicates that the asymmetry observed when these ions are formed directly from solution must come about due either to differences in the monomer conformations in the dimer that exist in solution or that occur during the electrospray ionization process. These results provide additional evidence for the origin of charge asymmetry that occurs in the dissociation of multiply charged protein complexes and indicate that some solution-phase information can be obtained from these gas-phase dissociation experiments.
气相质子化蛋白质二聚体解离成其组成单体可导致对称或不对称的电荷分配。具有15 +电荷的α-乳白蛋白同二聚体解离会产生对称但较宽的蛋白质单体电荷分布，电荷状态集中在8 + / 7 +左右。相比之下，由一个氧化形式分子和一个还原形式分子组成的15 +异二聚体解离会导致高度不对称的电荷分配，其中还原态物种主要携带11 +电荷，氧化态分子携带4 +电荷。这个结果不能用溶液中或电喷雾过程中发生的差异充电来充分解释，但似乎最好的解释是还原态物种在气相中活化时展开，随后分离并将质子转移到解离复合物中展开的物种上，以最小化库仑排斥。对于直接从溶液中形成的细胞色素c二聚体，17 +电荷状态经历对称电荷分配，而13 +的解离是不对称的。通过气相质子转移将具有17 +电荷的二聚体电荷状态降低到13 +，随后对质量选择的13 +离子进行解离，会导致对称电荷分配。这一结果清楚地表明，具有13 +电荷的二聚体离子的结构取决于离子形成方法，并且结构差异是观察到的对称与不对称电荷分配的原因。这表明当这些离子直接从溶液中形成时观察到的不对称性必定是由于溶液中存在的二聚体中单体构象的差异，或者是在电喷雾电离过程中发生的差异所致。这些结果为多电荷蛋白质复合物解离中发生的电荷不对称起源提供了额外证据，并表明可以从这些气相解离实验中获得一些溶液相信息。
