15N/14N isotope ratios are widely used to study processes and systems involving amino acids. Nitrogen isotope fractionation in biological processes occurs primarily at sites of bond-breaking and formation; the finest discrimination for “isotopic fingerprinting” and studies of isotopic fluxes is thus obtained at the position-specific level. While there are numerous reports of natural intramolecular carbon isotope variability, there are no literature reports of 15N/14N position-specific isotopic analysis (N-PSIA) of biologically relevant molecules. We report a methodology for high-precision N-PSIA of four polynitrogenous α-amino acids (asparagine, glutamine, lysine, histidine) and the first survey of natural intramolecular 15N/14N in these biomolecules. Selective liberation of N-atoms from multiple commercial standards of each parent amino acid was achieved by an appropriate enzymatic reaction or by acid hydrolysis. 15N/14N measurements were performed on N-ethoxycarbonyl ethyl ester derivatives of the parent amino acids and their analogues by gas chromatography combustion isotope ratio mass spectrometry, and the average precision for replicate injections was found to be SD(δ15N) = 0.3‰. Position-specific δ15N values of the parent amino acid were directly observed or indirectly calculated using mass balance. The average precision obtained for directly measured positions was SD(δ15N) = 0.2−0.4‰. The average precision for indirectly obtained positions was SD(δ15N) = 0.6−1.3‰ as a result of propagation of errors. Enrichment in the side chain-N with respect to the peptide-N was observed in nearly all of the amino acid sources, most notably in asparagine (average Δδside-peptide = +11‰), which may be indicative of its method of production. In some cases, it was possible to distinguish commercial sources by N-PSIA that could not be distinguished at the compound-specific level.
15N/14N同位素比率广泛用于研究涉及
氨基酸的过程和系统。
生物过程中的氮同位素分馏主要发生在断裂和形成的键位置;因此,同位素指纹鉴别和同位素通量研究的最好分辨率是在特定位置的
水平上获得的。虽然有许多关于天然分子内碳同位素变化的报道,但关于与
生物相关的分子进行15N/14N特定位置同位素分析(N-PSIA)的文献报道尚无报道。我们报道了一种高精度的四元多氮
α-氨基酸(天冬酰胺、谷
氨酰胺、赖
氨酸、组
氨酸)N-PSIA方法,并对这些
生物分子中的天然分子内15N/14N进行了首次调查。通过适当的酶反应或酸
水解,实现了从每个母体
氨基酸的多个商业标准中有选择性地释放氮原子。通过气相色谱燃烧同位素比质谱法对母体
氨基酸及其衍生物的N-乙氧基羰基
乙酯进行15N/14N测量,重复注射的平均精度为
SD(δ15N)= 0.3‰。通过质量平衡直接观察或间接计算母体
氨基酸的特定位置的δ15N值。直接测量位置的平均精度为
SD(δ15N)= 0.2−0.4‰。间接获得位置的平均精度为
SD(δ15N)= 0.6−1.3‰,这是由于误差的传播。在几乎所有的
氨基酸来源中,侧链N相对于肽N的富集最为显著地出现在天冬酰胺(平均Δδ侧链-肽键= +11‰),这可能是其生产方式的指示。在某些情况下,可以通过N-PSIA区分商业来源,而无法在化合物特定
水平上进行区分。