19F and 1H NMR of Aldimines of Fluoroamino Acids with Pyridoxal-5'-phosphate

Article abstract of DOI:10.1002/mrc.1270160214

1H and 19F NMR spectra were obtained for six Schiff bases (aldimines)formed by pyridoxal-5'-phosphate (PLP) with four fluorinated or their two parent non-fluorinated α-amino acids (phenylalanine and α-aminobutyric acid). pK_A Values were derived from 1H and 19F titration curves.The imine nitrogen of the aldimines is very basic (ca. 13) and sensitive to the electron withdrawing effect of fluorine.The pyridine nitrogen is less basic in the aldimines (ca. 7) than in PLP (8.12) and is less sensitive to the electron withdrawing effect of the fluorine than is the imine nitrogen.The phosphate group has a pK in the same range (ca. 6) and the chemical shifts of some nuclei are sensitive to both pK values.Protonation of the aldimine causes the 1H signal to shift downfield at the methyl protons of the pyridine ring and at the aldehydic proton of the aldimine for the high pK value (except for the aldimines prepared from the β-fluorophenylalanine), but upfield at the aromatic proton and at the aldehydic proton of the aldimine for the low pK.Protonation of the aldimine causes the 19F signal of an aryl fluorine to shift downfield but gives an upfield shift at a β-fluorine.These data are related to the highly conjugated electronic structure of the Schiff bases.