X-ray crystallographic structures of enamine and amine Schiff bases of pyridoxal and its 1:1 hydrogen-bonded complexes with benzoic acid derivatives: evidence for coupled inter- and intramolecular proton transfer.

摘要: 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), 4-nitrobenzoic (III)(+) (IV), serve as models for the coenzyme pyridoxal-5'-phosphate (PLP) in its PLP-dependent enzymes. These allow study intramolecular OHN hydrogen bond PL/PLP H-acceptor properties pyridine rings. The free base (I) forms hydrogen-bonded chains involving hydroxyl side groups rings adjacent molecules, whereas (III) related cyclic dimers. adducts (II)/(IV) consist complexes, exhibiting strong intermolecular bonds between carboxylic acids (I)/(III). In conclusion, proton O-H...N (I)/(III) is located close to oxygen (enolamine form). added protonate ring (II)/(IV), but only latter case does this protonation lead a shift towards nitrogen (ketoimine All crystallographic were observed open form. contrast, formation pyridinium salt by dissolving (IV) leads aminal