COMPLEX ISOMERIZATION OF KETOSES - A C-13 NMR-STUDY OF THE BASE-CATALYZED RING-OPENING AND RING-CLOSING RATES OF D-FRUCTOSE ISOMERS IN AQUEOUS-SOLUTION
COMPLEX ISOMERIZATION OF KETOSES - A C-13 NMR-STUDY OF THE BASE-CATALYZED RING-OPENING AND RING-CLOSING RATES OF D-FRUCTOSE ISOMERS IN AQUEOUS-SOLUTION
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DOI：
10.1021/ja00300a041
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发表时间：
1985-01-01
影响因子：
15
通讯作者：
GOUX, WJ
中科院分区：
文献类型：
--
作者：
GOUX, WJ
在本研究中，D-[2-13C]果糖的异构化在碱性条件下用 13C NMR 光谱进行了表征。在 pH 7 以上和 17 至 50°C 之间获得的 13C NMR 光谱中，未观察到由开链水合物或 D-吡喃果糖引起的可检测共振。观察到，由 13C 富集碳的 呋喃糖和/3-呋喃糖以及 D-果糖的无环酮形式引起的光谱中的共振随着 pH 值或温度的增加而急剧变宽，而在相同的光谱中， 3-吡喃糖形式的异头碳共振线宽保持小于2Hz。通过将测量的线宽拟合到三位点交换模型来确定糖的α-和/3-呋喃糖形式的开环的表观一级速率常数（kaf，ac，j/jf.ac）。在研究的所有 pH 值和温度下，发现 ksfi „iC. 几乎是 ka! iH 0 的两倍。根据表观一级速率常数的 pH 依赖性确定了呋喃糖开环的二级速率常数，并发现 α-呋喃糖开环为 (2.88±0.5) X 106 7 和 (3.6±0.6) X 106 M" 1 s" 1和/3-呋喃糖形式，AH*、AG* 29i 和 AS* 29i 是根据 kaí C 和 &sf ac 在 pH 8.4 下的温度依赖性确定的。在这些条件下，发现 AS* 298 对于两种呋喃糖开环反应均呈阳性，这与根据 D-半乳糖的呋喃糖开环速率确定的大负值 AS* m 形成鲜明对比。酸性条件下的 D-苏糖、D-赤藓糖和 2-脱氧-D-核糖已根据涉及稳定 D-呋喃果糖阴离子周围极性溶剂组织的模型合理化。表征环状结构之间以及环状和无环形式之间相互转化的 AH0、AG29i 和 AS 298 值是根据 pH 8.4 下平衡常数的温度依赖性确定的。在所有情况下，AGm 的符号和大小均由 AH0 确定。D-呋喃果糖的表观一阶闭环速率由开环速率常数 /caf ac 和 k $f ac 以及相应的平衡常数 Á^} a c 和 A^} a c 确定。在所有研究温度下，发现 /3-呋喃果糖的闭环速率大约是 3-呋喃糖形式的相应速率的五倍。在 pH 8.4 和 27 C 下对邻[2-I3C]果糖进行 13C NMR 实验，以进一步表征相对较慢的/3-吡喃糖开环和闭环速率 (^ p ac 和 fcac>(3)。根据反转转移数据和平衡强度，确定 kpf% ís 和 20 s" 1 的上限为 0.1 s" 1对于 FCA，在相同条件下，α-和/3-呋喃糖形式的闭环速率常数约为 80 和 500 s”1。
In the present study, the isomerization of D-[2-13C] fructose has been characterized under alkaline conditions with 13C NMR spectroscopy. No detectible resonances arising from the open-chain hydrate or-D-fructopyranose could be observed in 13C NMR spectra acquired above pH 7 and between 17 and 50 C. Resonances in spectra arising from the 13C-enriched carbon of the-furanose and/3-furanose and the acyclic keto form of D-fructose were observed to broaden drastically with increasing pH or temperature while, in the same spectra, the linewidth of the anomeric carbon resonance of the/3-pyranose form remained less than 2 Hz. Apparent first-order rate constants for ring opening of the a-and/3-furanose forms of the sugar (kaf, ac,^/jf. ac) were determined by fitting measured line widths to a model for three-siteexchange. At all pH values and temperatures studied, it was found that ksfi „iC. was nearly twice as large as ka! iH 0 Second-order rate constants for furanose ring opening were determined from the pH dependence of the apparent first-order rate constants and were found to be (2.88±0.5) X 106 7and (3.6±0.6) X 106 M" 1 s" 1 for ringopening of the-furanose and/3-furanose forms, respectively. Thermodynamic activation parameters, AH*, AG* 29i, and AS* 29i, were determined from the temperature dependence of kaí. i C and &sf ac at pH 8.4. Under these conditions, AS* 298 was found to be positive for both furanose ring-opening reactions, in marked contrast to the large negative AS* m determined from furanose ring-opening rates of D-galactose, D-threose, D-erythrose, and 2-deoxy-D-ribose under acidic conditions. The positive AS* m values have been rationalized in terms of a model involving the organizationof polar solvent around the stabilized D-fructofuranose anions. Values of AH0, AG29i, and AS 298 characterizing interconversions between cyclic structures and between cyclic and acylic forms were determined from the temperature dependence of equilibrium constants at pH 8.4. In all cases, AGm is determined both in sign and magnitude by AH0. Apparent first-order ring-closing rates to D-fructofuranoses were determined from ring-opening rate constants,/caf ac and k $ f ac, and corresponding equilibrium constants, Á^} a c and A^} a cAt all temperatures studied, the ring-closing rate to/3-fructofuranose was found to be roughly five times that of the corresponding rate to the-furanose form. Inversion-transfer 13C NMR experiments were carried out on o-[2-I3C] fructose at pH 8.4 and 27 C in order to further characterizethe relatively slow/3-pyranose ring-opening and ring-closing rates (^ p ac and fcac>(3). From inversion-transfer data and equilbrium intensities, an upper limit of 0.1 s" 1 was determined for kpf% ís and 20 s" 1 for fca. Cij3p. Under identical conditions, ring-closing rateconstants to the a-and/3-furanose forms were found to be about 80 and 500 s" 1.