19F and 1H nuclear magnetic resonance studies of ring-fluorinated imidazoles and histidines

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Abstract
1 H Titration curves (in D2O) and 19F titration curves (in H2O) have been obtained for 2- and 4-fluoro-imidazole and -histidine. By use of computer-assisted curve fitting, pK values were derived for the several dissociation steps in these compounds. Fluoroimidazoles are weaker bases and stronger acids than bromoimidazoles, indicating that the inductive effect of the halogen greatly overshadows its resonance effect. Introduction of an alkyl group at C-5 displaces the F-4 signal upfield (as in o-fluorotoluene) but that of F-2 downfield, suggesting further that –4 and –5 are coupled both by induction and resonance while C-2 and –5 are coupled only by induction. Introduction of fluorine displaces the H-2 and –4 signals upfield, in parallel with the behaviour of fluorobenzene. The shielding effect of the magnetic anisotropy of 19F evidently counteracts the deshielding effect of its electronegativity. Protonation of the imidazole ring causes the 19F signal to shift downfield at F-2 but upfield at F-4; conversely, the formation of the imidazole anion produces an upfield shift at F-4 but downfield at F-2. In the fluorohistidines, there is some evidence for field perturbation of the 19F signal by the charge on the amino-acid side chain; however, inconsistencies in the direction of signal displacement due to σ, π, and charge variation within the imidazole ring render difficult the analysis of possible field effects. These data are intended as a basis for extensive 19F n.m.r. studies of fluorohistidines in polypeptides and proteins.

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