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19 F and 1 H NMR of aldimines of fluoroamino acids with pyridoxal‐5′‐phosphate
Author(s) -
Beguin Claude,
Hamman Sylvain
Publication year - 1981
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270160214
Subject(s) - aldimine , chemistry , imine , protonation , pyridoxal , pyridine , chemical shift , schiff base , medicinal chemistry , stereochemistry , organic chemistry , phosphate , catalysis , ion
1 H and 19 F 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). p K A Values were derived from 1 H and 19 F titration curves. The imine nitrogen of the aldimines is very basic (∼13) and sensitive to the electron withdrawing effect of fluorine. The pyridine nitrogen is less basic in the aldimines (∼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 p K in the same range (∼6) and the chemical shifts of some nuclei are sensitive to both p K values. Protonation of the aldimine causes the 1 H signal to shift downfield at the methyl protons of the pyridine ring and at the aldehydic proton of the aldimine for the high p K 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 p K . Protonation of the aldimine causes the 19 F 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.