Sharif Shasad, Powell Douglas R, Schagen David, Steiner Thomas, Toney Michael D, Fogle Emily, Limbach Hans Heinrich
Institut für Chemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany.
Acta Crystallogr B. 2006 Jun;62(Pt 3):480-7. doi: 10.1107/S0108768105040590. Epub 2006 May 15.
Crystal structures of Schiff bases containing pyridoxal (PL), N-(pyridoxylidene)-tolylamine, C(15)H(16)N(2)O(2) (I), N-(pyridoxylidene)-methylamine, C(9)H(12)N(2)O(2) (III), and their 1:1 adduct with 2-nitrobenzoic acid, (I)(+) C(7)H(4)NO_4;- (II), and 4-nitrobenzoic acid, (III)(+) C(7)H(4)NO_4;- (IV), serve as models for the coenzyme pyridoxal-5'-phosphate (PLP) in its PLP-dependent enzymes. These models allow the study of the intramolecular OHN hydrogen bond of PL/PLP Schiff bases and the H-acceptor properties of their pyridine rings. The free base (I) forms hydrogen-bonded chains involving the hydroxyl side groups and the rings of adjacent molecules, whereas (III) forms related hydrogen-bonded cyclic dimers. The adducts (II)/(IV) consist of 1:1 hydrogen-bonded complexes, exhibiting strong intermolecular bonds between the carboxylic groups of the acids and the pyridine rings of (I)/(III). In conclusion, the proton in the intramolecular O-H...N hydrogen bond of (I)/(III) is located close to oxygen (enolamine form). The added acids protonate the pyridine ring in (II)/(IV), but only in the latter case does this protonation lead to a shift of the intramolecular proton towards the nitrogen (ketoimine form). All crystallographic structures were observed in the open form. In contrast, the formation of the pyridinium salt by dissolving (IV) leads to the cyclic aminal form.
含有吡哆醛（PL）、N -（吡啶叉） - 甲苯胺（C₁₅H₁₆N₂O₂，I）、N -（吡啶叉） - 甲胺（C₉H₁₂N₂O₂，III）及其与2 - 硝基苯甲酸的1:1加合物（I⁺C₇H₄NO₄⁻，II）和4 - 硝基苯甲酸（III⁺C₇H₄NO₄⁻，IV）的席夫碱的晶体结构，可作为依赖于磷酸吡哆醛（PLP）的酶中辅酶磷酸吡哆醛（PLP）的模型。这些模型有助于研究PL/PLP席夫碱的分子内OHN氢键及其吡啶环的H受体性质。游离碱（I）形成涉及羟基侧基和相邻分子环的氢键链，而（III）形成相关的氢键环状二聚体。加合物（II）/（IV）由1:1氢键复合物组成，在酸的羧基与（I）/（III）的吡啶环之间表现出强分子间键。总之，（I）/（III）分子内O - H...N氢键中的质子靠近氧（烯胺形式）。添加的酸使（II）/（IV）中的吡啶环质子化，但仅在后一种情况下，这种质子化导致分子内质子向氮的移动（酮亚胺形式）。所有晶体结构均以开放形式观察到。相比之下，通过溶解（IV）形成吡啶鎓盐会导致环状缩醛胺形式。
