High-Precision Position-Specific Isotope Analysis of <sup>13</sup>C/<sup>12</sup>C in Leucine and Methionine Analogues
作者:Gavin L. Sacks、J. Thomas Brenna
DOI:10.1021/ac0344889
日期:2003.10.15
We report an automated method for high-precision position-specific isotope analysis (PSIA) of carbon in amino acid analogues. Carbon isotope ratios are measured for gas-phase pyrolysis fragments from multiple sources of 3-methylthiopropylamine (3MTP) and isoamylamine (IAA), the decarboxylated analogues of methionine and leucine, using a home-built gas chromatography (GC)-pyrolysis-GC preparation system coupled to a combustion-isotope ratio mass spectrometry system. Over a temperature range of 620−900 °C, the characteristic pyrolysis products for 3MTP were CH4, C2H6, HCN, and CH3CN and for IAA products were propylene, isobutylene, HCN, and CH3CN. Fragment origin was confirmed by 13C-labeling, and fragments used for isotope analysis were generated from unique moieties with >95% structural fidelity. Isotope ratios for the fragments were determined with an average precision of SD(δ13C) < 0.3‰, and relative isotope ratios of fragments from different sources were determined with an average precision of SD(Δδ13C) < 0.5‰. Δδ13C values of fragments were invariant over a range of pyrolysis temperatures. The Δδ13C of complementary fragments in IAA was within 0.8‰ of the Δδ13C of the parent compounds, indicating that pyrolysis-induced isotopic fractionation is effectively taken into account with this calibration procedure. Using Δδ13C values of fragments, Δδ13C values were determined for all four carbon positions of 3MTP and for C1, C2, and the propyl moiety of IAA, either directly or indirectly by mass balance. Large variations in position-specific isotope ratios were observed in samples from different commercial sources. Most dramatically, two 3MTP sources differed by 16.30‰ at C1, 48.33‰ at C2, 0.37‰ at C3, and 5.36‰ at C(methyl). These PSIA techniques are suitable for studying subtle changes in intramolecular isotope ratios due to natural processes.
我们报告了一种对氨基酸类似物中的碳进行高精度位置特异性同位素分析(PSIA)的自动化方法。利用自制的气相色谱(GC)-热解-GC 制备系统和燃烧同位素比质谱系统,测量了多种来源的 3-甲硫基丙胺(3MTP)和异戊胺(IAA)(蛋氨酸和亮氨酸的脱羧类似物)的气相热解碎片的碳同位素比。在 620-900 °C 的温度范围内,3MTP 的特征热解产物为 CH4、C2H6、HCN 和 CH3CN,IAA 的特征热解产物为丙烯、异丁烯、HCN 和 CH3CN。通过 13C 标记确认了片段的来源,用于同位素分析的片段是由结构保真度大于 95% 的独特分子生成的。片段的同位素比值测定的平均精度为 SD(δ13C) < 0.3‰,不同来源片段的相对同位素比值测定的平均精度为 SD(Δδ13C) < 0.5‰。IAA 中互补片段的 Δδ13C 值与母体化合物的 Δδ13C 值相差在 0.8‰以内,这表明该校准程序有效地考虑了热解引起的同位素分馏。利用碎片的Δδ13C 值,直接或通过质量平衡间接地确定了 3MTP 所有四个碳位置以及 IAA 的 C1、C2 和丙基的Δδ13C 值。在来自不同商业来源的样品中,观察到特定位置同位素比值的巨大差异。最显著的是,两个 3MTP 来源的样品在 C1 位相差 16.30‰,在 C2 位相差 48.33‰,在 C3 位相差 0.37‰,在 C(甲基)位相差 5.36‰。这些 PSIA 技术适用于研究分子内同位素比值因自然过程而发生的微妙变化。