19F NMR chemical shifts induced by a helical peptide

F-19 NMR spectra of two neutral, organic-soluble helical peptide octamers, each labeled at its N terminus with either 4-fluorobenzamide or 4-trifluoromethylbenzamide, in solvents with widely varying dielectric constants have been observed. The peptides are oligomers of alpha-aminoisobutyric acid (Aib), which is a residue known to form stable 3(10) helices in organic solution. In relation to the F-19 NMR spectra of a control molecule, the peptide terminating in 4-fluorobenzamide shows a solvent-dependent down-field chemical shift of between similar to 1.5 and similar to 4 ppm, whilst the peptide terminating in 4-trifluoromethylbenzamide shows only an similar to 0.2 ppm chemical shift dependence on the solvent dielectric constant. The experimental observations were compared to calculated values of the electric field generated by the correlation of dipolar amide units through the peptides helical conformation. We find the chemical-shift response of the 4-fluorobenzamide group to the peptide's calculated electric field is consistent with the magnitude of F-19 chemical shift dispersion observed in proteins.