Salem Nuha, Andreescu Silvana, Kulla Eliona, Zuman Petr
Department of Chemistry, Clarkson University, Potsdam, New York 13699-5810, USA.
J Phys Chem A. 2007 May 31;111(21):4658-70. doi: 10.1021/jp071151s. Epub 2007 May 9.
Orthophthalaldehyde (1,2-dicarboxaldehyde) (OPA) forms in the presence of a strong nucleophile with amino acids isoindole derivatives. The reaction is used in fluorometric determination of amino acids. The mechanism of these processes is not understood. OPA is present in aqueous solutions in three forms: unhydrated (I(a)), monohydrated acyclic (I(b)), and cyclic hemiacetal (I(c)). The absence of data for the molar absorptivities of these forms, together with overlap of their absorption bands, limits the application of spectrophotometry. Measurement of polarographic limiting currents of forms I(a) and I(b) enables determination of equilibrium constants K1 (formation of I(b)) and K2 (for the ring formation). The presence of these forms was supported by 1H NMR and 13C NMR. The rate of hydration of OPA is general-acid-base-catalyzed, but that of dehydration shows only specific-acid-base catalysis. The rate of hydration is controlled by general-acid-base-catalyzed addition of water to I(a). The rate of dehydration depends on the opening of the ring in I(c), which is specific-acid-base-catalyzed. At pH > 10 OPA undergoes a complex set of acid-base reactions (Scheme 3). The presence of polarographic anodic waves and oxidation on the gold electrode indicates the importance of the presence of a geminal diol form (II(a)). Establishment of equilibria among the three forms of OPA together with reactions at pH > 10 has to be considered in elucidating the reaction scheme of procedures using OPA as a reagent in the determination of amino acids.
邻苯二甲醛（1,2 - 二甲醛）（OPA）在强亲核试剂存在下与氨基酸形成异吲哚衍生物。该反应用于氨基酸的荧光测定。这些过程的机制尚不清楚。OPA在水溶液中有三种形式：未水合的（I(a)）、单水合无环的（I(b)）和环状半缩醛（I(c)）。这些形式的摩尔吸光系数数据缺失，加上它们吸收带的重叠，限制了分光光度法的应用。测量I(a)和I(b)形式的极谱极限电流能够确定平衡常数K1（I(b)的形成）和K2（环形成的平衡常数）。1H NMR和13C NMR证实了这些形式的存在。OPA的水合速率是一般酸碱催化的，但脱水速率仅表现出特定酸碱催化。水合速率由一般酸碱催化的水加到I(a)上控制。脱水速率取决于I(c)中环的打开，这是特定酸碱催化的。在pH > 10时，OPA会发生一系列复杂的酸碱反应（方案3）。极谱阳极波的存在以及在金电极上的氧化表明偕二醇形式（II(a)）存在的重要性。在阐明使用OPA作为试剂测定氨基酸的程序反应方案时，必须考虑OPA三种形式之间平衡的建立以及pH > 10时的反应。
