Wu J, Serianni A S
Department of Chemistry and Biochemistry, University of Notre Dame, Indiana 46556.
Carbohydr Res. 1991 Apr 24;211(2):207-17. doi: 10.1016/0008-6215(91)80092-2.
The ring-opening reactions of the furanose forms of the penturonic acids D-arabinuronic acid (1), D-lyxuronic acid (2), D-riburonic acid (3), and D-xyluronic acid (4) in aqueous solution have been studied as a function of temperature and solution pH by 13C saturation-transfer n.m.r. (s.t.-n.m.r.) spectroscopy using 1-13C-substituted compounds. Unidirectional rate constants of ring-opening (kopen) have been determined for the cyclic forms of 1-4 in their protonated (pH 1.5) and ionized (pH 4.5) forms, and have been compared to the k-values measured previously for structurally related furanose sugars. At 50 degrees and pH 1.5, kopen values decrease as follows: alpha-xyluronic acid (2.57 s-1) greater than alpha-riburonic (1.65 s-1) greater than beta-arabinuronic (1.52 s-1) greater than beta-xyluronic (1.09 s-1) greater than beta-riburonic (0.76 s-1) greater than beta-lyxuronic (0.55 s-1) greater than alpha-arabinuronic (0.46 s-1) greater than alpha-lyxuronic (0.40 s-1). At 50 degrees and pH 4.5, this order changes significantly (e.g., beta-arabinuronate is most reactive); in general kopen values for beta anomers appear to be enhanced relative to those for corresponding alpha anomers, suggesting the involvement of intramolecular catalysis in which the carboxylate anion assists in abstracting the hydroxyl proton from O-1. Activation energies of ring-opening, determined for the alpha and beta anomers of 1-4, were found to depend on ring configuration and solution pH.
利用1-¹³C取代化合物，通过¹³C饱和转移核磁共振（s.t.-n.m.r.）光谱研究了戊糖醛酸D-阿拉伯糖醛酸（1）、D-来苏糖醛酸（2）、D-核糖醛酸（3）和D-木糖醛酸（4）呋喃糖形式在水溶液中的开环反应，该反应是温度和溶液pH的函数。已测定了1-4的环式结构在质子化（pH 1.5）和离子化（pH 4.5）形式下的单向开环速率常数（kopen），并与先前测定的结构相关呋喃糖的k值进行了比较。在50℃和pH 1.5时，kopen值按以下顺序降低：α-木糖醛酸（2.57 s⁻¹）＞α-核糖醛酸（1.65 s⁻¹）＞β-阿拉伯糖醛酸（1.52 s⁻¹）＞β-木糖醛酸（1.09 s⁻¹）＞β-核糖醛酸（0.76 s⁻¹）＞β-来苏糖醛酸（0.55 s⁻¹）＞α-阿拉伯糖醛酸（0.46 s⁻¹）＞α-来苏糖醛酸（0.40 s⁻¹）。在50℃和pH 4.5时，该顺序发生显著变化（例如，β-阿拉伯糖醛酸根最具反应性）；一般来说，β异头物的kopen值相对于相应的α异头物似乎有所提高，这表明存在分子内催化作用，其中羧酸根阴离子有助于从O-1处夺取羟基质子。已发现1-4的α和β异头物的开环活化能取决于环构型和溶液pH。
