The separation of small molecule isomers has become a proving ground for ion mobility spectrometry (IMS). To date, successful IMS separation of steroid isomers has been accomplished largely by examination of alkali metal cationized dimers, with few examples of successful separations of monomeric steroid isomers in either positive or negative ion modes. Here, we report on the novel use of anion attachment to form negative ion monomeric adducts of steroid isomers that may be separated by IMS. Computational modeling shows that [prednisolone + Cl]⁻ adopts a conformation wherein the attaching chloride is chelated between two hydroxyl hydrogens. The electrostatic interaction between Cl⁻ and the two substituent electropositive hydroxyl hydrogens causes “conformation pinning” of prednisolone, thereby forcing the complex to adopt a constricted conformation. The tighter conformation of [prednisolone + Cl]⁻ permits baseline IMS separation from its isomer [cortisone + Cl]⁻ that exists only in elongated form. Although distinguishable as anionic adducts, these isomeric steroids were impossible to separate as either protonated (MH⁺, positive mode) or deprotonated ([M‐H]⁻, negative mode) analogs. Another pair of isomeric steroids 21‐deoxycortisol and corticosterone showed improved IMS separations as chloride adducts as compared to the analogous MH⁺ or [M‐H]⁻ pairs. Lastly, success was shown in separating protonated dimeric forms of isomeric steroid pairs by IMS, and we distinguish these separations from those of alkali metal cationized dimers that have been previously reported.

中文翻译：

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通过阴离子附着实现“构象固定”，可通过离子迁移谱法分离异构类固醇单体

小分子异构体的分离已成为离子迁移谱（IMS）的试验场。迄今为止，甾族化合物异构体的成功IMS分离很大程度上是通过碱金属阳离子化二聚体的检测完成的，很少有以正离子或负离子模式成功分离单体甾体异构体的例子。在这里，我们报道了阴离子附着在形成甾体异构体的负离子单体加合物上的新用途，该类固醇异构体可以被IMS分离。计算模型表明，[泼尼松龙+ Cl] ^-具有一种构象，其中连接的氯化物被螯合在两个羟基氢之间。氯之间的静电相互作用^-而两个取代基的正电羟基氢会导致泼尼松龙的“构象固定”，从而迫使配合物采用狭窄的构象。[泼尼松龙+ Cl] ^-的紧密构型允许基线IMS从其异构体[可的松+ Cl] ^-分离，后者仅以细长形式存在。尽管可以区分为阴离子加合物，但这些异构类固醇无法分离为质子化（MH ⁺，正模式）或去质子化（[ MH ] ^-，负模）类似物。与类似的MH ⁺或[ MH ] ^-相比，另一对异构体类固醇21-脱氧皮质醇和皮质酮显示出更好的IMS分离（作为氯化物加成物）对。最后，在通过IMS分离质子化的类固醇对的二聚体形式中显示出成功，并且我们将这些分离与先前已报道的碱金属阳离子化二聚体分离。