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首页> 外文期刊>Biochemistry >NMR Studies of the Stability, Protonation States, and Tautomerism of C-13- and N-15-Labeled Aldimines of the Coenzyme Pyridoxal 5 '-Phosphate in Water
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NMR Studies of the Stability, Protonation States, and Tautomerism of C-13- and N-15-Labeled Aldimines of the Coenzyme Pyridoxal 5 '-Phosphate in Water

机译：水中辅酶Pyridoxal 5'-磷酸盐的C-13和N-15标签的亚胺的稳定性，质子化状态和互变异构的NMR研究

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摘要

We have measured the pH-dependent H-1, C-13, and N-15 NMR spectra of pyridoxal 5'-phosphate (C-13(2)-PLP) mixed with equal amounts of either doubly N-15-labeled diaminopropane, N-15(alpha)-labeled. L-lysine, or N-15(epsilon)-labeled L-lysine as model systems for various intermediates of the transimination reaction in PLP-dependent enzymes. At low pH, only the hydrate and aldehyde forms of PLP and the free protonated diamines are present. Above pH 4, the formation of single- and double-headed aldimines (Schiff bases) with the added diamines is observed, and their C-13 and N-15 NMR parameters have been characterized. For 1:1 mixtures the single-headed aldimines dominate. In a similar way, the NMR parameters of the geminal diamine formed with diaminopropane at high pH are measured. However, no geminal diamine is formed with L-lysine. In contrast to the aldimine formed with the epsilon-amino group of lysine, the aldimine formed with the alpha-amino group is unstable at moderately high pH but dominates slightly below pH 10. By analyzing the NMR data, both the mole fractions of the different PLP species and up to 6 different protonation states including their pK(a) values were obtained. Furthermore, the data show that all Schiff bases are subject to a proton tautomerism along the intramolecular OHN hydrogen bond, where the zwitterionic form is favored before deprotonation occurs at high pH. This observation, as well as the observation that around pH 7 the different PLP species are present in comparable amounts, sheds new light on the mechanism of the transimination reaction.
机译：我们已经测量了吡x醛5'-磷酸酯（C-13（2）-PLP）与等量的双N-15标记的二氨基丙烷混合而成的pH依赖的H-1，C-13和N-15 NMR光谱，N-15α标记。 L-赖氨酸或N-15（ε）标记的L-赖氨酸作为模型系统，用于PLP依赖性酶中转氨反应的各种中间体。在低pH下，仅存在PLP的水合物和醛形式以及游离的质子化二胺。在pH高于4的情况下，观察到形成了带有添加的二胺的单头和双头醛亚胺（席夫碱），并且已表征了它们的C-13和N-15 NMR参数。对于1：1混合物，单头的亚胺占主导地位。以类似的方式，在高pH下测量由二氨基丙烷形成的双链双胺的NMR参数。但是，L-赖氨酸不会形成双双胺。与由赖氨酸的ε-氨基形成的醛亚胺相反，由α-氨基形成的醛亚胺在中等较高的pH值下不稳定，但在pH 10以下略微占优势。通过分析NMR数据，两种分子的摩尔分数不同。获得了PLP种类和多达6种不同的质子化状态，包括其pK（a）值。此外，数据表明，所有席夫碱均沿分子内OHN氢键发生质子互变异构，在高pH下去质子化之前，两性离子形式是有利的。该观察结果以及在pH 7左右存在不同数量的PLP物种的观察结果为转氨反应的机理提供了新的思路。

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