Ion/molecule reactions of cation radicals formed from protonated polypeptides via gas-phase ion/ion electron transfer

Cation radicals formed via gas-phase electron transfer to multiply protonated polypeptides have been found to react with molecular oxygen. Such cation radicals are of interest within the context of electron transfer dissociation, a phenomenon with high utility for the characterization of peptide and protein primary structures. Most of the cation radicals show the attachment of O-2 under room temperature storage conditions in an electrodynamic ion trap. At higher temperatures and under conditions of collisional activation, the oxygen adduct species lose O-2, HO circle, or HO2 circle, depending upon the identity of the side chain at the radical site. The fragments containing the C-terminus, the so-called z-ions, which are predominantly radical species, engage in reactions with molecular oxygen. This allows for the facile distinction between z-ions and their complementary even-electron c-ion counterparts. Such a capability has utility in protein identification and characterization via mass spectrometry. Intact electron transfer products also show oxygen attachment. Subsequent activation of such adducts show dissociation behavior very similar to that noted for z-ion adducts. These observations indicate that ion/radical reactions can be used to probe the locations of radical sites in the undissociated electron transfer products as well as distinguish between c- and z-type ions.

通过气相电子转移形成的阳离子自由基与多质子化多肽结合后，已被发现会与分子氧发生反应。这种阳离子自由基在电子转移解离的背景下具有研究价值，电子转移解离是一种对肽和蛋白质一级结构的表征非常有用的现象。大多数阳离子自由基在室温下于电动离子阱中储存时会与O₂结合。在较高温度及碰撞活化条件下，氧加合物会根据自由基位点侧链的特性失去O₂、·OH或·HO₂。包含C末端的片段，即所谓的z离子（主要为自由基物种）会与分子氧发生反应。这使得z离子与其互补的偶电子c离子能够很容易地区分开来。这种能力在通过质谱法进行蛋白质鉴定和表征方面具有实用性。完整的电子转移产物也会显示出与氧的结合。对这些加合物随后进行活化，其解离行为与z离子加合物所观察到的非常相似。这些观察结果表明，离子/自由基反应可用于探测未解离电子转移产物中自由基位点的位置，以及区分c型和z型离子。