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

Abstract

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.

Synthesis, spectroscopic and X-ray characterization of a copper(II) complex with the Schiff base derived from pyridoxal and aminoguanidine: NMR spectral studies of the ligand

“…The existence of the intramolecular hydrogen bonds between the phenolic hydroxyl oxygen and azomethine nitrogen was recently confirmed in a study of some pyridoxal Schiff adducts by combination of crystallographic and liquid state NMR techniques [13]. A downfield shift of the azomethine proton in PL-AG AE HCl AE H 2 O with respect to the corresponding PL-AG by 0.19 ppm, as well as slight downfield shifts of the proton at C-6, confirms the protonation of the hydrazine nitrogen.…”

Aspartate aminotransferase: An old dog teaches new tricks

“…[7, 56–60] Further experiments using N-15 NMR showed that protonation of the pyridine nitrogen promotes proton transfer from O3′ to the imine nitrogen when aldimines are formed from aliphatic amines but not from aromatic ones, and detailed the natures of the inter- and intramolecular O-H-N hydrogen bonds. [53, 54]…”

The zwitterion of N-(2-carboxyphenyl)-4-dimethylaminebenzylideneimine

“…Amino acid Schiff bases are used as a model compounds in the study of enzymatic transformations of amino acids (e.g., decarboxylation, transamination, and racemization), which require pyridoxal-5 0 -phosphate as a cofactor [17][18][19]. During this catalytic process, the proton migrates from the hydroxyl group to the imine nitrogen atom and in consequence the O-HÁÁÁN hydrogen bond becomes the N ?…”