A 19F nuclear magnetic resonance study of the interaction of carbon dioxide with fluoro-amino acids

Abstract

The 19F nuclear magnetic resonance (NMR) spectra of 3-fiuorophenylalanine and 3-fluorotyrosine in bicarbonate buffer are characterized by a main resonance band and a well-resolved upfield resonance. We show that the upfield resonance is due to the formation of a carbamate between the amino group of the amino acid and bicarbonate. The intensity of the upfield resonance is dependent on the pH (pD) of the solution and reflects changes in the concentration of the free amino acid and carbamate. Thus the normalized spectral peaks can be used to quantitate the free amino acid and carbamate concentrations and these concentrations have been used to determine the equilibrium constant of the carbamate reaction. The value of the equilibrium constant obtained, using an equation which considered the various ionic states of the reactants, was 8.52 × 10−6 ± 0.27 × 10−6 for 3-fiuorophenylalanine and 8.84 × 10−6 ± 0.72 × 10−6 for 3-fluorotyrosine. These values are within the range expected for amino acids and indicate that the fluorine nucleus does not perturb the reaction significantly and that the 19F NMR can be used to quantitate the formation of carbamates in solutions of fluorine-labelled amino acids.