Different tautomeric and zwitterionic forms of chelidamic acid (4‐hydroxypyridine‐2,6‐dicarboxylic acid) are present in the crystal structures of chelidamic acid methanol monosolvate, C₇H₅NO₅·CH₄O, (Ia), dimethylammonium chelidamate (dimethylammonium 6‐carboxy‐4‐hydroxypyridine‐2‐carboxylate), C₂H₈N⁺·C₇H₄NO₅⁻, (Ib), and chelidamic acid dimethyl sulfoxide monosolvate, C₇H₅NO₅·C₂H₆OS, (Ic). While the zwitterionic pyridinium carboxylate in (Ia) can be explained from the pK_a values, a (partially) deprotonated hydroxy group in the presence of a neutral carboxy group, as observed in (Ib) and (Ic), is unexpected. In (Ib), there are two formula units in the asymmetric unit with the chelidamic acid entities connected by a symmetric O—H...O hydrogen bond. Also, crystals of chelidamic acid dimethyl ester (dimethyl 4‐hydroxypyridine‐2,6‐dicarboxylate) were obtained as a monohydrate, C₉H₉NO₅·H₂O, (IIa), and as a solvent‐free modification, in which both ester molecules adopt the hydroxypyridine form. In (IIa), the solvent water molecule stabilizes the synperiplanar conformation of both carbonyl O atoms with respect to the pyridine N atom by two O—H...O hydrogen bonds, whereas an antiperiplanar arrangement is observed in the water‐free structure. A database study and ab initio energy calculations help to compare the stabilities of the various ester conformations.

中文翻译：

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螯胺酸及其二甲酯的假多晶型物

螯胺酸（C ₇ H ₅ NO ₅ ·CH ₄ O，（I a），螯合物二甲基铵）的晶体结构中存在不同的互变异构形式和两性离子形式的螯合物（4-羟基吡啶-2,6-二羧酸）（6-羧基-4-羟基吡啶-2-羧酸二甲基铵），C ₂ H ₈ N ⁺ ·C ₇ H ₄ NO ₅ ^-，（I b）和氯胺酸二甲亚砜单溶剂化物，C ₇ H ₅ NO ₅ ·C ₂ H ₆ OS，（Ic）。尽管可以从p K _a值解释（I a）中的两性离子吡啶鎓羧酸盐，但是在（I b）和（I c）中观察到，在存在中性羧基的情况下，a（部分）去质子化的羟基是意外。在（I b）中，在不对称单元中有两个分子式单元，其具有通过对称的OH ... O氢键连接的螯胺酸实体。同样，获得了作为一水合物C ₉ H ₉ NO ₅ ·H ₂ O的螯胺酸二甲酯（4-羟基吡啶-2-2，6-二羧酸二甲酯）晶体（II a），并且作为无溶剂的修饰，两个酯分子均采用羟基吡啶形式。在（II a）中，溶剂水分子通过两个OH-H ... O氢键稳定了两个羰基O原子相对于吡啶N原子的同上平面构象，而在无水结构中观察到了反同上平面排列。数据库研究和从头算能量的计算有助于比较各种酯构象的稳定性。