Department of Chemistry, University of New Orleans, 2000 Lakeshore Dr., New Orleans, LA 70148, USA.
J Am Soc Mass Spectrom. 2011 Dec;22(12):2125-36. doi: 10.1007/s13361-011-0255-0. Epub 2011 Oct 14.
A new model has been developed to account for adduct formation on multiply charged peptides observed in negative ion electrospray mass spectrometry. To obtain a stable adduct, the model necessitates an approximate matching of apparent gas-phase basicity (GB(app)) of a given proton bearing site on the peptide with the gas-phase basicity (GB) of the anion attaching at that site. Evidence supporting the model is derived from the fact that for [Glu] Fibrinopeptide B, higher GB anions dominated in adducts observed at higher negative charge states, whereas lower GB anions appeared predominately in lower charge state adducts. Singly charged adducts were only observed for lower GB anions: HSO(4)(-), I(-), CF(3)COO(-). Ions that have medium GBs (NO(3) (-), Br(-), H(2)PO(4)(-)) only form adducts having -2 charge states, whereas Cl(-) (higher GB) can form adducts having -3 charge states. The model portends that (1) carboxylate groups are much more basic than available amino groups; (2) apparent GBs of the various carboxylate groups on peptides do not vary substantially from one another; and (3) apparent GBs of the individual carboxylate and amino sites do not behave independently. This model was developed for negative ion attachment but an analogous mechanism is also proposed for the positive ion mode wherein (1) binding of a neutral at an amino site polarizes this amino group, but hardly affects apparent GBs of other sites; (2) proton addition (charge state augmentation) at one site can decrease the instrinsic GBs of other potential protonation sites and lower their apparent GBs.
已开发出一种新模型来解释负离子电喷雾质谱中观察到的多电荷肽的加合物形成。为了获得稳定的加合物，该模型需要在给定的肽上带有质子的特定部位的表观气相碱度 (GB(app)) 与在该部位附着的阴离子的气相碱度 (GB) 进行近似匹配。支持该模型的证据源自这样一个事实，即对于 [Glu] Fibrinopeptide B，在更高的负电荷状态下观察到的加合物中，较高 GB 的阴离子占主导地位，而较低 GB 的阴离子主要出现在较低电荷状态的加合物中。仅观察到较低 GB 的阴离子形成单价加合物：HSO(4)(-)、I(-)、CF(3)COO(-)。具有中等 GB 的离子（NO(3) (-)、Br(-)、H(2)PO(4)(-)) 仅形成 -2 电荷状态的加合物，而 Cl(-)（较高 GB）可以形成 -3 电荷状态的加合物。该模型预示着：（1）羧酸根比可用的氨基基团碱性强得多；（2）肽上各种羧酸根的表观 GB 彼此之间没有显著差异；（3）各个羧酸根和氨基部位的表观 GB 不会独立起作用。该模型是为负离子附着而开发的，但也提出了类似的正离子模式机制，其中：（1）中性物质在氨基部位的结合使该氨基基团极化，但几乎不影响其他部位的表观 GB；（2）一个部位的质子添加（电荷状态增加）可以降低其他潜在质子化部位的固有 GB 并降低它们的表观 GB。
