Recently, we developed a novel non-fragmenting quaternary ammonium ionization tag for the mass spectrometric sensitive sequencing of peptides, based on the N-spiro proline residue (5-azoniaspiro[4.4.]nonyl-carbonyl). Herein, we present an unexpected racemization and the hydrogen–deuterium exchange (HDX) at the α-C atom of the proline derivative under basic aqueous conditions (1% water solution of triethylamine). The deuterium atom, substituted for the α-C atom, does not undergo back-exchange under acidic aqueous conditions which makes the deuterated isotopologue a promising stabile isotope-coded internal standard for quantitative analysis by mass spectrometry. The applicability of the prepared isotopologues of the quaternary ammonium salt labeled peptides for quantification experiments using the isotopic dilution method was also examined.

中文翻译：

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含有5-azoniaspiro [4.4]壬基的脯氨酸类似物的α-碳处氢的意外消旋作用和氢-氘交换† ‡

最近，我们开发了一种新型的非片段化季铵离子化标签，用于基于N的肽的质谱敏感测序-螺螺脯氨酸残基（5-氮杂螺[4.4。]壬基-羰基）。在此，我们提出了在碱性水溶液（1％的三乙胺水溶液）下脯氨酸衍生物的α-C原子发生意外的消旋作用和氢-氘交换（HDX）。取代了α-C原子的氘原子在酸性水溶液条件下不进行反向交换，这使氘化的同位素体成为通过质谱进行定量分析的有希望的稳定同位素编码的内标。还检查了制备的季铵盐标记的肽的同位素异构体在使用同位素稀释法进行定量实验中的适用性。