Position-Specific Isotope Analysis of Xanthines: A (13)C Nuclear Magnetic Resonance Method to Determine the (13)C Intramolecular Composition at Natural Abundance.
Position-Specific Isotope Analysis of Xanthines: A (13)C Nuclear Magnetic Resonance Method to Determine the (13)C Intramolecular Composition at Natural Abundance.
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DOI：
10.1021/acs.analchem.5b00559
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发表时间：
2015-06
影响因子：
7.4
通讯作者：
Didier Diomandé;Estelle Martineau;A. Gilbert;Pierrick Nun;Ariaki Murata;Keita Yamada;N. Watanabe;I. Tea;R. Robins;N. Yoshida;G. Remaud
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作者：
Didier Diomandé;Estelle Martineau;A. Gilbert;Pierrick Nun;Ariaki Murata;Keita Yamada;N. Watanabe;I. Tea;R. Robins;N. Yoshida;G. Remaud
天然黄嘌呤类咖啡因、可可碱和茶碱在咖啡、可可、茶和许多其他饮料中作为风味成分具有重要的商业价值。然而，它们对真实性的开发是有限的，这是这些基于原产地的价格范围很大的商品的一项要求，通过质谱仪监测同位素比率的标准方法。我们现在已经开发出一种克服这一缺陷的方法，它利用同位素定量(13)C核磁共振光谱的能力，结合黄嘌呤的化学修饰，能够高精度地确定分子内(13)C/(12)C比率(δ(13)Ci)。然而，只有咖啡因是可以分析的：可可碱和茶碱由于其较差的溶解性而存在很大的困难。然而，它们的N-甲基化为咖啡因使得获取光谱是可行的。该方法被证实是可靠的，咖啡因中的δ(13)Ci值具有良好的重复性，适用于自然丰度下的同位素分馏测量。结果表明，在化学N-甲基化过程中，同位素分馏可以忽略不计。因此，该方法保留了原始的位置δ(13)Ci值。该方法已用于测量咖啡因中~(13)C/~(12)C分布随位置的变化。不仅观察到来自不同来源的咖啡因之间的明显差异，而且发现来自可可的可可碱显示出与咖啡因不同的(13)C模式。
The natural xanthines caffeine, theobromine, and theophylline are of major commercial importance as flavor constituents in coffee, cocoa, tea, and a number of other beverages. However, their exploitation for authenticity, a requirement in these commodities that have a large origin-based price-range, by the standard method of isotope ratio monitoring by mass spectrometry (irm-MS) is limited. We have now developed a methodology that overcomes this deficit that exploits the power of isotopic quantitative (13)C nuclear magnetic resonance (NMR) spectrometry combined with chemical modification of the xanthines to enable the determination of positional intramolecular (13)C/(12)C ratios (δ(13)Ci) with high precision. However, only caffeine is amenable to analysis: theobromine and theophylline present substantial difficulties due to their poor solubility. However, their N-methylation to caffeine makes spectral acquisition feasible. The method is confirmed as robust, with good repeatability of the δ(13)Ci values in caffeine appropriate for isotope fractionation measurements at natural abundance. It is shown that there is negligible isotope fractionation during the chemical N-methylation procedure. Thus, the method preserves the original positional δ(13)Ci values. The method has been applied to measure the position-specific variation of the (13)C/(12)C distribution in caffeine. Not only is a clear difference between caffeine isolated from different sources observed, but theobromine from cocoa is found to show a (13)C pattern distinct from that of caffeine.