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19F and 1H NMR of aldimines of fluoroamino acids with pyridoxal-5′-phosphate

✍ Scribed by Claude Beguin; Sylvain Hamman

Publisher
John Wiley and Sons
Year
1981
Tongue
English
Weight
444 KB
Volume
16
Category
Article
ISSN
0749-1581

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✦ Synopsis

Abstract

^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.

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