Wu J, Serianni A S, Vuorinen T
Department of Chemistry and Biochemistry, University of Notre Dame, Indiana 46556.
Carbohydr Res. 1990 Sep 30;206(1):1-12. doi: 10.1016/0008-6215(90)84001-b.
The tautomeric compositions of D-erythro-2-pentulose (D-ribulose) and D-threo-2-pentulose (D-xylulose) in aqueous solution have been studied by 13C-n.m.r. spectroscopy at various temperatures using 2-13C-substituted compounds. The alpha-furanose, beta-furanose, and acyclic carbonyl (keto) forms were detected at all temperatures, whereas the acyclic hydrate (gem-diol) form was not observed. The percentage of keto form increased with increasing temperature, at the expense of the furanose forms. Thermodynamic (delta G0, delta H0, delta S0) and kinetic parameters for the interconversion of alpha- and beta-furanoses with the acyclic carbonyl form were determined and compared with those determined under similar conditions for the structurally-related aldotetrofuranoses. The ring-opening rate constant (kopen) measured by 13C saturation-transfer n.m.r. spectroscopy in 50mM sodium acetate (pH 4.0) at 55 degrees were as follows: beta-threofuranose (0.65 s-1) greater than alpha-erythrofuranose (0.51 s-1) greater than beta-erythrofuranose (0.37 s-1) approximately beta-threo-2-pentulofuranose (0.35 s-1) greater than alpha-threofuranose (0.25 s-1) greater than alpha-threo-2-pentulofuranose (0.20 s-1) approximately alpha-erythro-2-pentulofuranose (0.18 s-1) approximately beta-erythro-2-pentulofuranose (0.18 s-1). Within each structural type the pentulofuranose anomer having O-2 and O-3 cis (O-1 and O-2 cis in aldotetrofuranoses) opens faster than, or at a similar rate to, the alternative anomer having these oxygen atoms trans. Ring-closing rate constants (kclose), calculated from kopen and Keq, decrease in the order beta-erythrofuranose (15 s-1) greater than beta-threofuranose (12 s-1) greater than alpha-erythrofuranose (9.9 s-1) greater than alpha-threofuranose (6.2 s-1) greater than beta-threo-2-pentulofuranose (0.71 s-1) greater than alpha-erythro-2-pentulofuranose (0.38 s-1) greater than alpha-threo-2-pentulofuranose (0.13 s-1) approximately beta-erythro-2-pentulofuranose (0.13 s-1). Replacement of H-1 in aldotetrofuranoses by a hydroxymethyl group (i.e., conversion to 2-pentuloses) significantly decreases the ring-opening and ring-closing rate constants of furanose anomerization.
利用2-¹³C取代的化合物，通过¹³C核磁共振光谱在不同温度下研究了D-赤藓糖-2-戊酮糖（D-核糖）和D-苏阿糖-2-戊酮糖（D-木酮糖）在水溶液中的互变异构组成。在所有温度下均检测到了α-呋喃糖、β-呋喃糖和无环羰基（酮式）形式，而未观察到无环水合物（偕二醇）形式。酮式形式的百分比随温度升高而增加，呋喃糖形式则相应减少。测定了α-和β-呋喃糖与无环羰基形式相互转化的热力学（ΔG⁰、ΔH⁰、ΔS⁰）和动力学参数，并与在类似条件下为结构相关的醛糖四呋喃糖所测定的参数进行了比较。在55℃下于50mM醋酸钠（pH 4.0）中通过¹³C饱和转移核磁共振光谱测得的开环速率常数（kopen）如下：β-苏阿糖呋喃糖（0.65 s⁻¹）＞α-赤藓糖呋喃糖（0.51 s⁻¹）＞β-赤藓糖呋喃糖（0.37 s⁻¹）≈β-苏阿糖-2-戊酮呋喃糖（0.35 s⁻¹）＞α-苏阿糖呋喃糖（0.25 s⁻¹）＞α-苏阿糖-2-戊酮呋喃糖（0.20 s⁻¹）≈α-赤藓糖-2-戊酮呋喃糖（0.18 s⁻¹）≈β-赤藓糖-2-戊酮呋喃糖（0.18 s⁻¹）。在每种结构类型中，具有O-2和O-3顺式（醛糖四呋喃糖中为O-1和O-2顺式）的戊酮呋喃糖端基异构体的开环速度比具有这些氧原子反式的另一种端基异构体快，或与之相似。由kopen和Keq计算得出的闭环速率常数（kclose）按以下顺序递减：β-赤藓糖呋喃糖（15 s⁻¹）＞β-苏阿糖呋喃糖（12 s⁻¹）＞α-赤藓糖呋喃糖（9.9 s⁻¹）＞α-苏阿糖呋喃糖（6.2 s⁻¹）＞β-苏阿糖-2-戊酮呋喃糖（0.71 s⁻¹）＞α-赤藓糖-2-戊酮呋喃糖（0.38 s⁻¹）＞α-苏阿糖-2-戊酮呋喃糖（0.13 s⁻¹）≈β-赤藓糖-2-戊酮呋喃糖（0.13 s⁻¹）。醛糖四呋喃糖中的H-1被羟甲基取代（即转化为2-戊酮糖）会显著降低呋喃糖异构化的开环和闭环速率常数。
